There are a lot of choices to make when building a new home, and when upgrading or extending an existing home. Besides working towards a certain energy standard, choosing a heating & DHW system, a suitable ventilation strategy should always be considered.
Think about the lungs of your home.
Historic buildingsHistorically, houses were mostly well ventilated, were designed for good humidity control and didn’t overheat:
As such these houses provided a healthy indoor environment, but – as we all know – have become very expensive to heat.
In comparison, modern houses in general have none of these properties. In the endeavour to reduce heating costs and make homes more comfortable to live in, air tightness levels have come down to such a degree, that all new homes are now build relatively air tight.
A 2013 study of Scottish academia on the indoor air quality in modern homes has revealed the full extent of this change in the building fabric: The average modern Scottish home has more than twice the recommended CO2 levels in occupied bedrooms. The study gives a clear warning: Living in such conditions long-term is a endangering people’s health & wellbeing.
Trickle vents and intermittant extract fans are no longer a safe ventilation strategy – unless the occupants co-operate by keeping their bedroom window open all night and cross-ventilating every couple of hours during the day.
But this affects not only new builds. Whenever existing homes are being upgraded by the exchange of windows and doors, large-scale insulation measures, draft-proofing, blocking up of fire places, extensions or new internal lining/ paints, the natural air infiltration rate is probably greatly reduced. Many home owners have found out that their now cosy home has also become a cosy breeding ground for house dust mites and mould, besides the accumulation of other pollutants. Upgrading existing properties should always be accompanied with an assessment of the ventilation strategy.
We have seen in the Scottish research paper on IAQ in modern homes, that the air quality in modern homes with natural ventilation is in most cases not adequate. The paper adds: “Furthermore, the methodology underpinning the current regulations cannot be considered as creditable. While the complexity around numerical modelling often leads to conclusions based on simplistic and unrealistic assumptions around all doors in a dwelling being open and trickle ventilators being unobstructed, this paper demonstrates that in ‘real life’ situations, this is not the case and could lead to significant risks of under ventilation. This is particularly the case when standards and guidance are based upon theoretically modelled scenarios that are not representative of real-life operation. The consequences of this are important in terms of the likely negative impacts on occupant health.”
In the majority of cases the occupants must co-operate by opening of windows, in order to achieve the necessary air quality, e.g. by keeping occupied bedroom windows opened all night and cross-ventilating during the day every couple of hours.
This should be communicated and discussed by all architects and designers with their clients if they want to implement this form of ventilation: Natural ventilation without cross-ventilation – although theoretically legal – is in reality not an adequate ventilation strategy as it does not result in the air quality demanded by Building Regulations.
The theory of such systems sounds good, but the reality shows often a very different picture:
A) The infiltration air follows the path of least resistance. E.g. if an occupied bedroom door is closed with curtains drawn (as it is often the case), most of the infiltration air might bypass this room and come from other rooms, e.g. with open internal doors or unobstructed trickle vents. There is a very high probability that very little infiltration in this occupied bedroom occurs, resulting in very poor air quality.
B) Even if a clear ventilation path is established, e.g. through a bedroom with an en-suite bathroom, the trickle extract ventilation rates are in almost all cases not enough to provide good air quality in an occupied double bedroom. As MEV systems are only boosted based on humidity and not CO2 readings, the system will keep operating on trickle mode throughout the night. E.g. it will extract with 15 m3/h instead of the necessary 40 m3/h needed to maintain a good IAQ level.
De-central mechanical extract ventilation (dMEV) has become a very popular choice for the mass house builders. It is endorsed by Scottish Building Standards for the mid-range of air-tight dwellings.
The Scottish Government issued a study into the effect of these systems: “Ability of decentralised mechanical ventilation to act as ‘whole-house’ ventilation systems in new-build dwellings”. The report revealed a shocking picture:
“The monitoring found that over 50% of homes appeared to have poor ventilation overnight (where carbon dioxide levels exceeded 1,000ppm for the majority of the time), and that bedrooms were a particular cause for concern. There were a number of variables that affected this. These included the nature of the trickle vents, the window coverings, the path between the room and the dMEV (including the door opening or undercut, and the arrangement of the home) and the installation and performance of the system. Essentially homes with shorter, more open paths for air movement performed better, but rooms which relied on more remote dMEV systems frequently had poor ventilation.”
If you want to know more, please read the full report.
A variation of cMEV systems are those who connect their exhaust with an air source heat pump.
As air source heat pumps, operated with external air suffer from reduced efficiencies in frosty weather conditions, such set up will prevent this loss of efficiencies. it seems to combine the best of both worlds: extract ventilation and heat pump technology for domestic hot water and space heating.
However there is one big disadvantage:
cMEV systems, like dMEV and other extract only systems (PSV), have the same disadvantges as mentioned for dMEV: The air quality in habitable rooms is a matter of hit and miss.
Exhaust air heat pumps are often not accepted by end-users due to the large background ventilators, which cause unpleasant drafts.
Having said that demand controlled cMEV systems, which measure CO2 in bedrooms and have extract points in bedrooms will make a difference though.
1 – Wet rooms and kitchens are extracted on a continual, but slow basis.
2 – Before the extracted air is exhausted out of the building, the warmth in the air is transferred by means of a heat exchanger into the fresh air, which is introduced into the building.
3 – The pre-warmed fresh air is introduced into all habitable rooms on a continual basis. Thereby the need to completely heat the fresh air as it enters the building is eliminated. Typically cold outside air at 0 ºC can be warmed to 18 ºC by warm extract air at 20 ºC when an efficient system is used.
MVHR systems are primarily ventilation systems, which reuse and distribute warmth from internal + solar gains and space heating systems. In some cases supplementary space heating or cooling can be provided by an MVHR system.
Energy efficient domestic ventilation systems, like MVHR have been installed since the 1980s and refined to Passive House standards in the 1990s. We have accompanied Passive House MVHR from its infancy in Scotland and gained valuable insight into what works, what works better and what doesn’t work. Coming from the well researched, monitored and refined Passive House approach, we tackled Heat Recovery Ventilation differently than most other UK based companies.
We have learned from the best of their field in Europe and the UK and constantly strove to improve our own standards and product portfolios. We see such a need in the Scottish and UK construction industry to improve and even to define best practice, that we see it as our duty to share our experience and knowledge with others.
Here we are, please feel free to dig in and digest the wealth of information. If you want to know more about specific topics, please feel free to contact us. We do offer CPD’s for design professionals free of charge and offer other forms of training, e.g. professional installer training.
Utilisation of passive design criteria is becoming increasingly more common as peoples awareness and understanding of it’s advantages rise. Whether it’s a certified Passive House (Passivhaus), a Passive Home without certification or a low energy building.
The MVHR (Heat Recovery Ventilation) system is an essential part of a Passivhaus building to provide sufficient ventilation without the heat losses that are associated with any other form of ventilation. An efficient MVHR system will reduce the heating demand from 35kWh/(m2a) to 15kWh/(m2a) at equal ventilation rates – compared with natural ventilation.
From this it can be seen that the process of recovering heat from stale, used air and transferring it to fresh yet cold air, has a major influence on energy and emission savings.
MVHR is the only form of ventilation that cuts out almost all ventilation heat losses, which can be up to 30% of the heating demand of a dwelling. Besides a well designed and specified demand controlled MEV system, it is also the only system that is fully controllable, leaving no room with too little ventilation.
Cascade MVHR systems are the small sister of MVHR, based on very little ducting and cascade fans, that keep the air moving within a dwelling.
Please note that the ventilation rates for continuous ventilation systems, such as MVHR, MEV, dMEV are lower than for intermittent ones, such as extractor fans and cooker hoods.
If MVHR is done right and operates quietly, then it will make a real difference in the life of the occupants.
Air Movement in the dwelling
All ventilation strategies, including natural ventilation only function if air movement throughout the dwelling can be established, e.g. an extractor fan will just make noise without moving much air, if the bathroom door is closed and has not transfer gap or grille. Typically, 10mm undercuts of doors are sufficient for continuous operating ventilation systems. For intermittent ones this might not be enough.
To summarise: A suitable and effective mechanical ventilation strategy needs proper planning and should be considered in the light of the building type, the surrounding environment, the occupant aspirations and budget. However, besides all financial aspects, the health, comfort and wellbeing of the occupants should not be forgotten.
PIV systems – because of their limited effect and reach into the dwelling – are only suitable as a retrofit solution.
Like extract only systems, the air quality in habitable rooms is hit and miss and the occupants would need to co-operate by opening windows in occupied bedrooms all night and cross-ventilating every couple of hours during the day.
From past experience, these systems seem to have made a difference in a number of projects that were suffering from dampness and mould.
| Jargon | Brief explanation | info |
| A/C – Air conditioning | Air conditioning is a device to adjust the air temperature in a room. These operate with active heating or cooling elements that increase or decrease the air temperature whilst they re-circulate the air. Therefore it is important to close all windows and doors, for the effective operation of the system. If at all, fresh air is introduced only in smaller quantities. MVHR systems operate in a complete different way as these usually not actively heat or cool, nor re-circulate air. | |
| Ach – Air change rate | The air exchange rate is the amount of controlled supply of fresh air or extraction of stale air in relation to the internal volume of a room or the whole building. Often the volume of double height spaces is capped off at 2.5m or 3m. It is expressed in m3 (of air supplied or extracted)/ (hr*m3 (of space)). | |
| AECB | Sustainable Building Association www.aecb.net | |
| Adiabatic cooling | It is the cooling effect through the evaporation of water. A means of temperature reduction which operates on the principle that water absorbs latent heat from the surrounding air when it evaporates. |  |
| Air barrier/ Airtightness membrane | Membranes that prevent the flow of air from the inside to the outside of a building (see ‘airtightness layer’ below). They are available in different materials according to their function. In its basic form, an air barrier / airtightness membrane is nothing more sophisticated than polythene sheeting that can also double as a vapour barrier; A more sophisticated membrane is one that permits the flow of moisture through the material which is allowed to diffuse through the structure to the exterior (‘breathing wall’). Air / vapour barriers can also include materials such as OSB and plasterboard. (see also: Airtightness membranes) | |
| Air infiltration rate | The air infiltration rate is varying with the weather conditions (mainly wind speeds and temperature). It depends on the air tightness of the building, its wind exposure and shielding, internal segmentation, curtains or blinds, etc. It is difficult to calculate the real infiltration rate of a building. | |
| Air tightness | Is the extent of which infiltration happens through gaps and cracks of the building envelope. It excludes any purpose-built ventilation openings, e.g. trickle vents and extract fans. | |
| Air tightness layer | Essential to the energy performance of a building, and almost equal to the insulation of the building envelope in terms of energy efficiency. It is the method by which the flow of air through the building structure is controlled. The layer represents an unbroken / un-penetrated envelope encompassing the interior of the building which prevents warm air ‘leaking’ to the exterior. In timber frame buildings the airtightness layer usually comprises of a membrane integrated into the structural element (located more often on the ‘warm’ side of the insulation), whereas in masonry construction plaster usually acts as an effective barrier. | |
| Air tightness test / Air permeability test | Since the 2010 Building Regulations all new built homes in Scotland have to be tested for their air tightness (also called air permeability test). The test verifies the target air tightness and is to be carried out when the building is finished. It excludes all controlled ventilation openings (e.g. background ventilators, extract fans, chimney flues). It is expressed in how much air escapes the building per hours (under 50Pa pressurisation) in relation to the surface envelope area (q50). See q50 and n50. | |
| ADF – Approved document F | English and Welsh Building Standards for domestic ventilation. The ADF is not applicable in Scotland. | |
| Approved Document O | English and Welsh Building Standards for overheating prevention in domestic properties. The ADF is not applicable in Scotland. | |
| ASHP – Air source heat pump | …is a heat pump that generates hot water from the heat energy in either external air or internal air. | |
| Background ventilators | Most common form of background ventilators are trickle vents, mainly used in the upper window frame help to provide natural ventilation for a dwelling. In modern buildings these are not sufficient to maintain a good indoor air quality and need to be used in conjunction with manually operated cross-ventilation through regular window opening, e.g. bedroom windows tilted at night and during the day every 2-3 hours for 5-10min. | |
| BIM – Building Information Modelling | BIM is a very broad term that describes the process of creating and managing digital information about a built asset such as a building, bridge, highway or tunnel. The range of BIM ‘maturity levels’ are categorized as: Level 0: Unmanaged CAD (Computer Aided Design). Level 1: Managed CAD in 2D or 3D. Level 2: Managed 3D environment with data attached but created in separate discipline models. Level 3: Single, online, project model with construction sequencing, cost and life-cycle management information. |
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| BESA – Building Engineering Services Association | The UK’s leading trade organisation for building engineering services contractors – representing the interests of firms active in the design, installation, commissioning, maintenance, control and management of engineering systems and services in buildings. | |
| BMS – Building Management System | is a control system that can be used to monitor and manage the mechanical, electrical and electromechanical systems and services in a facility. | |
| BPEC – British Plumbing Employers Council | A UK charity that aims to raise the knowledge and skills of those who work in the UK plumbing and heating industry. The BPEC and NICEIC accredited training for domestic ventilation installers is mandatory in England and Wales, but not in Scotland. | |
| BREEAM | Building Research Establishment’s Environmental Assessment Method (BREEAM) is the world’s longest established method of assessing, rating and certifying the sustainability of buildings. BREEAM was first published by the Building Research Establishment (BRE), UK, in 1990. | |
| BODX – Expanded Basis of Design | is a new building design method, focusing on improving the quality and cost effectiveness of the developed design throughout the full life cycle. It encompasses Construction, Operations and Maintenance considerations. | |
| Boost Ventilation | Is the temporary actuated high level of ventilation, in order to take out excessive moisture more quickly. | |
| Cascade fan | Internal demand controlled fan, which creates and air flow through a room without ducting. As such it links into the fresh air supply of a cascade ventilation system. These are often installed in bedrooms and exhaust the air e.g. into a hallway, thus forcing a fresh air supply into the room through the door undercut. | |
| CEPH – Certified European Passivhaus Designer Course | A stringent course for experienced professionals to be certified PH Designers. The course includes all aspects of the PH methodology and knowledge to use the PHPP. | |
| CHP – Combined heat and power | A replacement for boilers with economic and environmental benefits, simultaneously producing heat and power. | |
| CIBSE – Chartered Institute of Building Service Engineers | CIBSE have published several guides for ventilation systems. | |
| Closed cell insulation | A form of insulation that cannot absorb water. It is useful for insulating cold pipes in heated spaces, when condensation is to be avoided. | |
| Code for Sustainable Homes | Is an environmental impact rating system for housing in England. The Code replaces Ecohomes and sets new standards for energy efficiency above those in current building regulations and sustainability. | |
| Commissioning Certificate | The commissioning certificate of a mechanical ventilation system shows the measured ventilation rates at nominal level and should also show if all components have been installed and are working correctly. | |
| CO2 – Carbondioxide | Despite the negative press in relation to the greenhouse effect, it is a relatively harmless gas that is found in our atmosphere to the degree of 350-450 ppm (parts per million). Flames will extinguish with levels above 100,000 ppm and no life is possible above 200,000 ppm. Human and animal respiration turns O2 (Oxygen) into CO2 and releases it into the atmosphere. Exhaled breath contains about 40,000 ppm of CO2. Indoors increased CO2 levels flag up decreased Oxygen levels and low air exchange rates to the atmosphere. It is easily measured and therefor widely used as a tracer gas to test the indoor air quality. | |
| CO2 Monitor | A device that measures CO2 levels and records these in certain intervals. CO2 monitors are required for new build dwellings in all master bedrooms. | |
| CO2 Sensor | CO2 sensors measures CO2 levels without recording the values. | |
| COP – Coefficient of performance | It shows the efficiency of heat pumps, etc. E.g. a COP of 3.5 means that (at e.g. at 10ºC outside temperature and DHW temperature of 40ºC) the device (e.g. heat pump) will use 1kW for every 3.5kW put out. | |
| Cold bridging | Now called thermal bridging: A thermally conductive material which penetrates or bypasses an insulation system; such as a wall tie, metal fastener, concrete beam, slab or column. Thermal bridging lowers the overall thermal insulation of the structure by creating areas where heat loss is greater in one area than it is for another. The effect is to reduce the overall u-value of the construction element. The heat loss per unit length of thermal bridge is known as the Ψ-(psi) value and is measure in W/mK. | |
| Cold spots | Areas either on the inside or outside of the building envelope, whose temperatures are significantly lower than the surrounding area. Usually only apparent through infrared thermographic survey they can have a number of causes including areas that have simply been shaded from the sun or, more importantly, areas of differential heat loss occurring through the building fabric. Often a cold spot on an internal surface will be matched by a ‘hot spot’ on the exterior – representing a thermal bridge where opposite, or air leakage by a more complex path through the structure when not opposite. | |
| Counter-flow principle | By sending two media that exchange heat over a certain distance in opposite direction, up to 100% of heat can be transferred from the one to the other medium (e.g. water or air). | |
| Cradle-to-crade or cradle-to-grave | Life Cycle Analysis (LCA) is often broken down into phases of lesser ambition. Where recyclable / reusable products are the subject, the entire analysis is referred to as ‘cradle-to-cradle’. For non-recyclable materials that are destined to be disposed of, the complete analysis is referred to as ‘cradle-to-grave’. Expressions such as ‘cradle-to-gate’ or ‘cradle-to-site’ refer to production from extraction of raw material and factory production; and extraction, factory production and delivery to site, respectively – though these LCAs are useful and more common, they tend not to tell the whole story. | |
| Cross ventilation | Cross ventilation is caused by wind which causes pressure differences between one side of a building and the other. | |
| DCV – Demand Controlled Ventilation | Adjusts the ventilation rate according to real time air quality measurements, e.g. CO2 and humidity levels as well as temperature. Please be aware that some companies offer DCV based only on humidity. This is not truely demand control, as the nominal ventilation rate is not based on occupancy (CO2). Treu demand controlled ventilation achieves good air quality whist the energy consumption is minimized. | |
| Defrost pre-heater | A heating element on the intake leg of MVHR systems, used for frost -protection. It ensures that the system operates on maximum efficiency even in the coldest time of the year. | |
| DER – Dwelling emmission rate | (annual CO2 per sqm) due to space heating, water heating, ventilation and internal lighting minus any CO2 emissons saved by any generation of electricity. | |
| dMEV – decentral Mechanical Extract Ventilation | A form of continuous ventilation for dwellings, which extracts from wet rooms, the kitchen and toilets. The extract ventilation is achieved through individual fans for each room with minimal ducting. Infiltration air is provided through background ventilators in habitable rooms. | |
| EPE/ EPP/ EPS | Closed cell insulation materials, used for foam ducting or ventilation components that need to be vapour-proof. EPE (Extruded Polyethylene) is relatively soft. EPP (Extruded Polypropylene) is more rigid than EPE. EPS (Expanded Polystyrene) is a cheaper product, which is also rigid, but more brittle, compared to EPP. | |
| Embodied energy | All the energy required to grow, harvest, extract, manufacture, refine, process, package, transport, install and dispose of a particular product or building material. | |
| Energy efficiency | Using less energy to provide the same level of energy service. Along with renewable energy, energy efficiency is own of the twin pillars of sustainable energy. | |
| Equivalent Area | is the performance measurement of background ventilators (trickle vents), compared to the air flow performance of a rectangular opening. It takes the performance reduction of a more contorted ventilation path through the trickle vent into consideration. | |
| ERV – Energy Recovery Ventilation | A heat exchanger for MVHR systems which transfers heat and moisture. With a moisture recovery rate of up to 75%, it prevents the over- de-humidification of dwellings, when the internal humidity gains are low compared to the necessary ventilation rates. The EHX increases slightly the thermal performance of the dwelling through latent gains by reducing evaporation heat losses. | |
| EAHP – Exhaust Air Heat Pumps | EAHPs combine a central MEV system with an air source heat pump. The heat pump is not utilizinf external air but the extract air from wet rooms, thus optimising their COP. Larger background ventilators are needed in habitable rooms to make up the infiltration air. | |
| Evaporative cooling | A means of temperature reduction which operates on the principle that water absorbs latent heat from the surrounding air when it evaporates. | |
| Filtration levels | The size of particles that are being filtered out from the air to a certain percentage. ISO16890 regulates the filter types. | |
| Fire damper / fire & smoke damper | A fire damper is a closure mechanism within a ventilation duct in between a fire compartment wall or ceiling. It is often mechanically or intumescently activated by heat or fire. Intumescent fire valves are used in ceiling or wall applications. Fire dampers can typically not be used against the circulation of cold smoke. Fire/ smoke dampers are motorised valves that are actuated by direct fire & heat and are additionally triggered by the fire & smoke alarm. | |
| Free Area | The free area of a ventilation component equates to the available space that air can travel through. It is usually expressed in mm2 or cm2. (1 cm2 is 100 mm2) | |
| G-value | The g-value of windows indicates how much solar energy can travel through the glazing. The higher the value the more solar gains can enter into the dwelling. | |
| GHX – Ground heat exchanger | Ground heat exchangers on the intake leg of MVHR systems are used for frost protection and gentle additional summer cooling. There are two types in use: A) Ground-to-air GHX: These were the first of their kind. Typically a 30-40m long tube of 200-250mm dia., buried 1-1.5m in the ground. Ideally these are silver lined on the inside to prevent growth. B) Ground-to-brine GHX with a brine-to-air heat exchanger. These are pump driven devices, similar to a ground source heat pump. | |
| GSHP – Ground Source heat Pump | A system that extracts heat from the ground, upgrades it to a higher temperature and releases it where required for space and water heating. | |
| HX – Heat exchanger | Heat exchangers are elements that transfer heat from one medium to another in a passive way. In MVHR systems these recovery the heat from the extract air into the fresh incoming air. The first HX were plate heat exchangers with recovery rates of 65-75%. Modern cross-counterflow heat exchangers can recovery 85-95% of heat. | |
| Heat Pumps | A device for generating hot water by extracting heat from a source of a lower level of temperature (e.g. outside air or ground) and ”pumping it up” to a higher, more useful level for space heating and DHW. With some, the process can be reversed to extract heat from the building (air conditioning). Typical applications are air source heat pumps and ground source heat pumps. | |
| HRV – Heat Recovery Ventilation | Another term for MVHR. A form of balanced ventilation (supply and extract air) that cuts out most of the ventilation heat losses through a heat exchanger. | |
| Heat transfer | The transition of thermal energy from a hotter object to a cooler object, e.g. from heated to unheated rooms. | |
| Humidity | The amount of water vapour in the air. Relative humidity is defined as the ratio of the partial pressure of water vapour in a parcel of air to the saturated vapour pressure of water vapour at a prescribed temperature. | |
| HVAC – Heating, Ventilation & Air Conditioning | The technology of indoor and vehicular environmental comfort. Its goal is to provide thermal comfort and acceptable indoor air quality. | |
| Hydroscopic materials | Building materials that can buffer (absorb and release) moisture. E.g. Lime render, which was used internally in traditional dwellings had hygroscopic qualities, which regulated the indoor humidity. | |
| IAQ – Indoor Air Quality | The indoor air quality depends mainly on: Oxygen levels / CO2 balance Pollution from outside (NOx, ozone, micro-particles PM1 and PM2.5) Pollution from inside (VOCs, e.g. formaldehyde, micro-particles PM) Pathogens (viruses, bacteria, spores and other biological agents) Radioactive gases from the ground (radon) |
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| IEQ – Indoor Environmental Quality | Indoor Environmental Quality (IEQ) is covering all elements of the internal conditions: Indoor Air Quality (IAQ) Humidity (not too dry and not too humid) Thermal comfort (heating and overheating prevention) Air velocity (drafts) Sound environment (noise levels) Light Electromagnetic smog (Wifi and other radio waves) |
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| Indoor Climate | Indoor Climate covers a wider field than IAQ: Indoor Air Quality (IAQ) Humidity (not too dry and not too humid) Thermal comfort (heating and overheating prevention) |
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| Intermittant Ventilation | On demand ventilation, e.g. with natural ventilation. | |
| Internal heat gains | All emitted heat from technical equipment, e.g. fridges, cooker, TV, heat from baths and showers, as well as body heat from occupants and pets. Passivhaus buildings mainly use internal heat gains, solar gains to provide space heating. For periods of cold weather without solar gains, a backup heater or central heating system is needed. | |
| Interstitual condensation | Occurs when relatively warm moisture-laden air diffuses into a vapour-permeable material or structure such as fibrous insulation or a porous brick wall. If it is relatively warm on one side and below the dew point temperature on the other; this can result in the moisture-laden air reaching ‘dew point’ within the material and depositing liquid water at this point. Interstitial condensation presents a problem when it remains undetected, threatening structural damage such as timber decay, or degrading the effectiveness of insulation. | |
| ISO 16890 | International regulation that sets out the classification and testing of filters. | |
| Lambda value (λ) | The k-value, or otherwise knows as thermal conductivity or Lambda value is the intrinsic figure for the thermal conductivity of a material. The unit is W/(mK). Is the figure high, e.g. 220W/(mK) for Aluminium, the material is a good conductor of heat. Is it low, e.g. 0.035W/(mK) for silk, the material makes a good thermal insulation. | |
| Latent gains | The physical effect of evaporation of water is that a small amount of heat energy is lost in the process. Reduced humidity levels will increase the evaporation rate. Out of this reason enthalpy heat exchangers will prevent lower internal humidity levels and therefore decrease the evaporation heat losses. | |
| Latent heat cells | High-performance thermal stores. Ideal for small units or where there is not much space for a heat store (can e.g. be placed in the corner of a roof). | |
| LCA – Life-Cycle Assessment | also known as life-cycle analysis, ecobalance, and cradle-to-grave analysis) is a technique to evaluate a product’s full costs – starting from raw material to final disposal – in terms of consumption of resources, energy, and waste. To improve, you have to measure first. Life cycle assessment (LCA) is a tool to assess the environmental impact of products and solutions. Rigorous and complete, this tool is the most widely used on the sustainable construction market today. | |
| LEV – Local Exhaust Ventilation | It is an extract ventilation system that takes airborne contaminants such as dusts, mists, gases, vapour or fumes out of the workplace air so that they can’t be breathed in. Properly designed LEV will: collect the air that contains the contaminants. | |
| K-value | The k-value, or otherwise knows as thermal conductivity or Lambda value is the intrinsic figure for the thermal conductivity of a material. The unit is W/(mK). Is the figure high, e.g. 220W/(mK) for Aluminium, the material is a good conductor of heat. Is it low, e.g. 0.035W/(mK) for silk, the material makes a good thermal insulation. | |
| Mass flow constant operation | Like volume flow constant operation, this feature prevents the dis-balancing of MVHR systems or a drop in ventilation rates through increased resistance, e.g. soiled filters. In comparison to volume flow constant operation it is 7-8% more accurate, due to the seasonal differences in air volumes (cold air has less volume than warm air). | |
| MEV | A form of continuous ventilation for dwellings, which extracts from wet rooms, the kitchen and toilets. The central extract fan connects to the extract rooms vis ducting. Infiltration air is provided through background ventilators in habitable rooms. | |
| MVHR – Mechanical Ventilation Heat Recovery | Another term for HRV. A form of balanced ventilation (supply and extract air) that cuts out most of the ventilation heat losses through a heat exchanger. | |
| Moisture buffering | Moisture absorption and desorption of materials in contact with indoor air of buildings is believed to be a possible way to moderate indoor humidity passively. Research is being undertaken to explore the potential of moisture buffering as a means of improving indoor air quality as well as saving energy. | |
| n50 (air tightness) | The European standard for measuring the air permeability of buildings is in relation to the internal volume of the building (in contrast to q50). It is expressed in m3/(hr m3) at 50 Pa. In Europe a pressurisation and de-pressurisation test is needed, as well for the Passive House certification. | |
| Natural infiltration rate | The air (natural) infiltration rate is varying with the weather conditions (mainly wind speeds and temperature). It depends on the air tightness of the building, its wind exposure and shielding, internal segmentation, curtains or blinds, etc. It is difficult to calculate the real infiltration rate of a building. | |
| Natural Ventilation | The supply and removal of air inside a building through natural means. There are two types of natural ventilation occurring in buildings: wind driven ventilation and stack ventilation. | |
| NHBC – National House Building Council | It’s aim is to raise the construction standards of new homes in the UK, and providing consumer protection for homebuyers through its 10-year Buildmark warranty. Established in 1936, NHBC is the UK’s largest provider of new home warranties of controlled dwellings. The NHBC set also standards for MVHR systems. | |
| NICEIC – National Inspection Council for Electrical Installation Contracting | The NICEIC created a training course for Domestic Ventilation Installers, similar to BPEC | |
| Night-time ventilation | Involves cooling the building structure overnight in order to provide a heat sink during the daytime when air temperatures peak. | |
| Nominal ventilation rate | The ventilation level set forth which is needed for the design occupancy of a building. A mechanical ventilation system is commissioned at this level. E.g. MVHR level 2. In real life this level may be used only at certain times when the house is fully occupied or at night. | |
| NOx – Nitrogen oxides | Nitrogen Oxide and Nitrogen Dioxide are collectively know as Nitrogen Oxides. Nitrogen Oxides are primarily produced as a result of the combustion process, typically from motor vehicles and power stations. They are one of the precursors for photochemical ozone formation as well as being injurious to human health. | |
| nZEBs – Nearly Zero Energy Buildings | The European Energy Performance of Buildings Directive (EPBD, 2010/31/EC)1 introduced the definition of nZEB as a building with very high energy performance where the nearly zero or very low amount of energy required should be extensively covered by renewable sources produced on-site or nearby. | |
| ODA – Outdoor Air Quality Levels | There are three outdoor air quality categories with different concentration levels of CO2, NO2, SO2 and PM10: ODA 1: Rural areas ODA 2: Small towns ODA 3: City centres | |
| Off-gasing | The release of chemicals from various substances under normal conditions of temperature and pressure. Offgassing can take a variety of forms, and is an issue of concern for some people, since some of the chemicals released during the offgassing process are potentially harmful. | |
| Overheating prevention | Structural measures for preventing overheating through solar gains are the most effective ones: e.g. shading, overhangs, thermal mass. Secondly cross-ventilation is needed if there is a risk of overheating. Although most ventilation systems will help to bring in cooler air to a certain degree, these provide cooling to a smaller degree. Active cooling through air conditioning or cooling modules in the MVHR system should only be used as a last means. | |
| O3 – Ozone | An unstable (reactive), and water soluble gas having chlorine-like odour, and formed in the upper atmosphere by the action of solar radiation on oxygen. Its presence as a layer in stratosphere serves as a screen (called the ‘ozone layer’) to block harmful ultraviolet radiation from reaching the earth’s surface. At ground level it is formed by the combination of hydrocarbons and nitrogen oxides in the presence of sunlight and is the main ingredient of smog. | |
| Ozon depletion potential | The relative amount of degradation a chemical compound can cause to the ozone layer. Often used in conjunction with the transmittor gas within heat pumps. | |
| Pa – Pascal | A unit of measuring air pressure. | |
| Passive cooling | Technologies or design features used to cool buildings without power consumption. | |
| Passive House / Passivhaus | A building methodology to minimise the heating consumption of buildings (domestic and non-domestic), as developed by Dr. Feist from Germany. It optimises the building physics by incorporating high levels of insulation and air tightness, as well as MVHR. The basis is a planning tool called PHPP (Passivhaus Planning Package), which accurately predicts the heat energy consumption of a building. In the UK the Passivhaus Trust oversees Passivhaus building projects | |
| Passive House Certifier | A building consultant that has been trained in certifying Passive Houses. | |
| Passive House Designer (CEPH) | Certified European Passivhaus Designer. Architects, consultants and building services engineers that have been trained to Passive House Standards | |
| Passive House Standard | Is a construction standard for all buildings which emphasises high levels of insulation and airtightness, minimal thermal bridging, use of solar and internal heat gains and tightly controlled ventilation. | |
| Passive House Tradesperson | A trades person that has been trained to Passive House Standards. There are two variants: Building Envelope training and Building Services training | |
| Passive House Plus | A Passivhaus with renewable technology with a renewable prime energy demand of 45 kWh or less. | |
| Passive House Premium | A Passivhaus with renewable technology with a renewable prime energy demand of 30 kWh or less. | |
| PHT – Passivhaus Trust | The UK agency that oversees the Passivhaus standard. | |
| PSD – Passive solar design | A design strategy that optimises a building’s form, fabric and orientation to maximise solar gain from autumn to spring, whilst minimising it during the warmer part of the summer. At the same time, daylighting is maximised at all times. Passive solar design has long been the key element in developing low-energy building solutions in the UK – often characterised by large integrated conservatories. An alternative Passivhaus approach. | |
| Payback period | The number of years it takes to recoup an initial investment. | |
| PCDB – Product Characteristic Database | Or SAP Appedix Q database is the performance data of building technology, which is used in the SAP calculations. | |
| PHPP – Passivhaus Planning Package | Very accurate but also quite time consuming energy assessment, based on a very comprehensive Excel program. Used to certify passive houses. | |
| PIV – Positive Input Ventilation | A ventilation system for retrofitting, which pumps filtered air from a roof space into a dwelling. | |
| PM1, PM2.5, PM10 | ||
| Pollen filter (F7) | A finer grade of filtration than the standard G4 dust filters. It is recommended to be used for the intake of MVHR systems to prevent finer dust to enter the ducting system and dwelling. | |
| Post heater | A post heater is used in some MVHR systems to provide or supplement space heating through the air handling system. There are warm water based or electrically operated post heaters. | |
| Pressure constant operation | Control of the fan speed adjustment to keep the supply and extract air output at a constant pressure. Is the opposite of volume flow constant operation. Mostly used in commercial applications. | |
| Primary Energy | The amount of energy mined or extracted at source; e.g., from coal, oil, natural gas, uranium or wood. Includes losses within processes such as electricity generation and transmission. | |
| Psi value (Ψ) | The heat loss per unit length of thermal bridge, measured in W/mK. The Psi-value is related to the lambda-value – the unit is also W/(mK) -, but in this case it denotes a thermal conductivity of an assembly of materials, e.g. the edge of a window glazing panel, where glass sheets, one or more spacers and the window frame with one or more different materials come together. It describes therfore the thermal bridging of that detail. The higher the figure, the higher the heat loss through that detail. Figures of 0.01W/(mK) and smaller are considered “thermal-bridge-free” and this is what detailed planning of a Passivhaus is aiming for. In some special cases (e.g. for detais where a well-insulated wall meets a well-insulated floor slab), the value might go even below 0, i.e. negative. | |
| PSV – Passive Stack Ventilation | Ventilation systems based on the ‘Stack Effect’. This is the movement of planned air paths through the dwelling as a result of internal and external temperature differences and wind induced pressure differences. | |
| PTC heating element | The Positive Temperature Co-efficient heating element is a self-limiting heating element with high power density, used for quality defrost pre-heaters or electrical post heaters. | |
| Purge ventilation | Rapid ventilation achieved by opening windows and doors in order to remove moisture and odours. Can also refer to overnight ventilation of hot internal air. It typically is 4 air exchanges or more. Mechanical purge ventilation is needed when there are no or not enough openable windows and doors. | |
| q50 (air tightness) | The UK standard for measuring the air permeability of buildings is in relation to the surface area of the building (in contrast to n50). It is expressed in m3/(hr m2) at 50 Pa. In the UK only a pressurisation test is needed. | |
| Ra – Radon | Is a gas, emitted from rock, mostly granite. It permeates floor slabs and can accumulte within a building. It has negative health-effects, e.g. an increased risk of cancer. | |
| rh – Relative Humidity | The relative humidity of air depends on its moisture content (absolute humidity) and air temperature. Ideally internal humidity levels should be between 40 and 60% rh at 20 deg. C. For limited times the levels can range between 30 and 70 % rh. Regular humidity levels above 70% rh can promote mould growth. | |
| SAP – Standard Assessment Procedure | for new homes. Independed of location (using a ”standard location”) and including lights, heating and hot water but not cooking and appliances. Rating scale is from 1 to 100+ with the higher the number the lower the ful running costs. 100 corresponds to zero running costs, so the rating can be over 100 if surplus energy is exported. SAP rating provides the basis for the ECPs, which place the rating on an A-G scale (similat to what you would find on white goods). SAP energy use predictions for dwellings are to be seen with a pinch of salt as they can vary from the real performance by up to 40%. | |
| SAP Appendix Q | Or PCDB database is the performance data of building technology, which is used in the SAP calculations. | |
| Scottish Building Regulations | In terms of ventilation the Scottish Building Standards – Technical Handbook domestic and non-domestic applies. Additionally there is the Supporting Guidance Domestic Ventilation. | |
| Sensor control | Form of automated operation of a MVHR system with the help of CO2 and/ or humidity sensors. In contrast to time control. See also demand controlled ventilation. | |
| SBS – Sick Building Syndrom | It is a medical condition where people in a building suffer from symptoms of illness or feel unwell for no apparent reason. Symptoms can be headaches, eye, nose, and throat irritation, fatigue, and dizziness and nausea. These symptoms appear to be linked to time spent in a building, though no specific illness or cause can be identified. A 1984 World Health Organization (WHO) report suggested up to 30% of new and remodeled buildings worldwide may be subject of complaints related to poor indoor air quality. (Source: Wikipedia) | |
| Solar gains | Solar gains is heat energy that is introduced into a building by sunshine, mainly through windows. Depending on the size and orientation of windows solar gains can be very powerful. Typically 1kW of heat can be gained through one normal sized window with full sun exposure. Modern dwellings with large glazing and light weight building structures (timber frame) with hardly any thermal mass suffer especially from overheating through solar gains. | |
| Solar shading | Devices that control heat gain as well as controlling light level, particularly in summer. | |
| SRHRV – Single room heat recovery ventilator | Smaller MVHR systems without ducting. These can be continuous systems or push-pull units with oszillating ventilation. | |
| Stack effect | The flow of air that results from warm air rising, creating a positive pressure area at the top of a building and a negative pressure area at the bottom of a building. | |
| SBEM – Standard Building Energy Method | A software package developed by the BRE for calculating the carbon emissions of building other than dwellings. The user inputs data relating to the building design and the software compares the actual design with a notional building of the same design built to 2002 standards. | |
| SO2 – Sulfur dioxide | A gas formed when fuel containing sulfur, such as coal or oil, is burned. S02 dissolves in water vapour to form acid, and interacts with other gases and particles in the air to form sulfates and other products that can be harmful to people and their environment. | |
| Summer bypass | The summer bypass helps through passive cooling to a certain degree against overheating. It is typically an automatically operated flap that opens when the internal temperatures (measured with the extract air) rise above a certain threshold. This threshold can be adjusted according to the needs of the occupants. Some systems have a summer bypass simulation, which switches the extract fan off when the internal temperatures rise above a certain threshold. | |
| Target Air Tightness | The target air tightness of a dwelling is set forth by the designer at Building Warrant stage and is to be verified through an air tightness test at the end of the build. | |
| TCO – Total Cost of Ownership | is the total cost of owning an asset over a period of time. In the real estate and construction industry this usually means the total cost of designing, constructing, operating, and maintaining a project throughout its useful life. | |
| TER – Target Emmission Rate | The DER must not exceed the Target Emission Rate (CO2) for a notional dwelling of the same size and shape. | |
| Thermal bridge | A thermally conductive material which penetrates or bypasses an insulation system; such as a wall tie, metal fastener, concrete beam, slab or column. Thermal bridging lowers the overall thermal insulation of the structure by creating areas where heat loss is greater in one area than it is for another. The effect is to reduce the overall u-value of the construction element. The heat loss per unit length of thermal bridge is known as the Ψ-(psi) value and is measure in W/mK. | |
| Thermal bypass | Heat transfer enabled by the uncontrolled air movement within and through walls. A recently identified example of thermal bypass occurs within cavity walls acting as separating walls (party walls) between adjoining houses or flats. Cavities in these instances are not normally insulated thus allowing warm air to enter the cavity and by means of convection to rise through the space and escape into an attic or through the roof covering. | |
| Thermal envelope | The insulated external fabric of the building. It often co-incides with the air tightness envelope. | |
| Thermal mass | The heat that is contained in the structure of the building, e.g. in internally exposed stone or concrete floors or walls. By absorbing solar gains, it helps to level out in-door temperatures and reduces the risk of overheating in summer. | |
| Thermal resistance (R-value) | Thermal resistance is the measure of a component’s ability to restrict the passage of heat across its thickness. The R-value is calculated by combining the lamda value (thermal conductivity, or ‘k-value’) and the thickness of the material. Hence R=t/λ, where ‘t’ is the thickness. Units are measured in m2W/K. Used in connection with insulation, the higher the R-value, the more effective the insulation. The R-value is also used to calculate the U-value (see below) | |
| Thermal store | Used to store heat for hot water and/or space heating. | |
| Thermal transmittance (U-value) | Thermal transmittance is a measure of the overall rate of heat transfer, by all mechanisms under standard conditions, through a particular section of construction. This measure takes into account the thickness of each material involved and is calculated from R-values of each material as well as constants accounting for surface transmittance (Rsi and Rso, inner and outer surfaces respectively) and also for a small standard air gap (Rso). Thermal transmittance is measured in W/m2K | |
| Time Control | Way of controlling a ventilation system via different ventilation levels, that are being used at different times. | |
| TM52 | CIBSE guidelines for thermal comfort in naturally ventilated buildings. | |
| TM59 | TM59 Dynamic Thermal Modelling is a standardised approach to predicting overheating risk for residential building designs (new-build or major refurbishment). | |
| Transfer gaps | These are typically internal door undercuts of 10-15mm to allow air circulation within the dwelling. Noise attenuated transfer units in internal walls are available, too. The size of these gaps or the free area of transfer openings depend on the design transfer air flow. For MVHR systems transfergaps should be specified to perform to less than 1Pa pressure drop. | |
| TVOCs – Total Volatile Organic Compounds | The sum of all different VOCs. It reflects the health impact of the mix of volatile organic compounds. | |
| U-value | A measure of the rate of heat transfer of a building element. Units = W/(m2K). When comparing U-values, the whole building element U-value should be considered, not just the value of a individual component, e.g. the U-value of a window (frame, glass & spacers) is less than the value for the glass alone. | |
| VAV – Variable Air Volume | Ventilation system with variable speed settings, which can be activated according to the use and need of a building. | |
| Vapour barrier | A vapour control layer material that retards the transfer of the warm, potentially moisture-laden air from an internal space, penetrating the external fabric and coming into contact with cooler elements within the construction. The different membranes used as VCLs are distinguished by their abilities to retard moisture under different conditions defined by BS5350 as a series of ‘Classes’. The term ‘vapour barrier’ (rather than ‘vapour check’) usually refers to a membrane providing the most resistance to water vapour. | |
| VCL – Vapour control layer | A feature of lightweight construction, a VCL prevents moisture vapour from penetrating and condensing amidst cooler, water-vulnerable, components such as insulation and timber structure. Usually in the form of a water-vapour proof sheet or membrane, the VCL is located on the ‘warm’ or room side of the construction element. | |
| Ventilation | Ventilation is the air movement in and out of a building, both through uncontrolled infiltration and through controlled ventilation | |
| Ventilation heat losses | Heat energy that is lost to the atmosphere through natural infiltration, manual cross-ventilation and mechanical ventilation. Typically this is 25-35% of the overall heat demand of a dwelling, if it is correctly ventilated. This can be drastically reduced through heat recovery ventilation (MVHR). | |
| Ventilation rate | The ventilation rate is the amout of air movement, either expressed in ach (air changes per hour) or in m3/h, l/s or m3/s (industrial ventilation). | |
| Ventilation strategy | Part of the design of a house, which sets forth the ventilation demand and the system used to meet that demand. | |
| VOCs – Volatile Organic Compounds | Organic chemicals that easily vaporize at room temperature. VOCs include a variety of chemicals, some of which may have short- and long-term adverse health effects. The most prominent one is Flormadehyde. | |
| Volume flow | The volume flow of a ventilation system is either expressed in m3/h or l/s or m3/sec. 1 m3/h = 0.278 l/s | 1 l/s = 3.6 m3/h | 1 m3/sec = 3,600 m3/h | |
| Volume flow constant operation | The volume flow constant operation of MVHR systems will adjust the fan power to keep the air flow rates at the same level. Increased resistance, e.g. from soiled filters will not lead to dropped ventilation rates or a dis-balanced system. This feature helps to keep the system at peak efficiency at all times. See also mass flow constant operation. | |
| Whole Dwelling Ventilation Rate | is the nominal ventilation rate for dwellings, derived either from the floor area or from the number of bedrooms (England & Wales) or number of habitable rooms (Scotland). | |
| Zero Carbon Buildings | One where, annually, there are no net carbon emissions resulting from the operation of the building. In practice, different views are taken as to the degree of autonomy the definition of ‘Zero Carbon’ refers to. For example, the Code for Sustainable Homes requires much of the energy consumed to be generated from on-site renewable technologies, whereas other definitions would include off-site generation to various degrees and remoteness of source. No official definitions currently include the Carbon generated in the construction of a building. |
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Information on domestic ventilation strategies, ventilation rates, Passive House ventilation and specification.
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