Build Tight...

Historically, buildings had ample ventilation with ventilation paths from top to bottom: From the ventilated solum, leaky floor boards, sash windows, transfer grilles above doors, up to the foul air chamber at the top. The fire places and chimney stacks also drew a lot of air out the dwelling, resulting in plenty of air infiltration. These buildings were well ventilated and they had to be, if you wanted to light a fire in your fire place. Fire needs lots of fresh air (Oxygen), without it just won’t burn. Additionally, hygroscopic internal surfaces absorbed and released moisture.

However, with modern methods of heating the need for ample air infiltration vanished and with the rise in energy costs, the need for a more energy efficiency arose. Both, insulation and air tightness can make a huge difference in the use of heating energy of a building. So, buildings were made better insulated and more air tight, much more air tight actually. Hygroscopic materials were replaced by plasterboard and modern paints, which cannot absorb moisture as well. – And we all thought that this didn’t matter – what crucial mistake!

The reason why we normally don’t think about our indoor climate, is that air is all around us. It is a commodity that is readily available and free of charge. So why should we be concerned about it?

Lack of Oxygen (O2) will make a candle or a fire go out. It will also affect us as we vitally depend on Oxygen. A lack of Oxygen indoors is indicated by a rise in Carbon dioxide (CO2) levels. As we hopefully all continue to breathe, we inhale Oxygen and exhale Carbon dioxide. Each breath contains about It is generally accepted that when CO2 levels reach 1400ppm the indoor air is poor. We should ideally keep below 1000ppm CO2*. Research shows that a rise in indoor CO2 levels coincides with the build-up of air pollutants.

The immediate effects of poor indoor air quality can be headaches, drowsiness and a lack of concentration. A 2008 study of the University of Reading revealed that lowering CO2 levels to 1000ppm in class rooms improved the student’s performance by up to 15%.
The long-term effects are more severe, especially as they affect children’s lung development.

Please see below the most important factors in indoor air quality (IAQ):

*) EN 13779 describes four levels of indoor air quality, depending on the difference between indoor and outdoor levels of CO2. The outdoor levels vary from 350ppm (parts per million) in rural areas to 450ppm in very urban areas – 400ppm on average. Unfortunately this EN has been withdrawn, but the science behind it still stands.

Indoor Air Quality has come under much more scrutiny lately, and rightly so.

As there are some overlapping phrases, let us briefly explain the difference:

Indoor Air Quality (IAQ) is mainly concerned about:

• 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)

Indoor Climate covers a wider field:

• Indoor Air Quality (IAQ)
• Humidity (not too dry and not too humid)
• Thermal comfort (heating and overheating prevention)

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)

The effect of building more air tight is, that we are left with comparatively little air infiltration, even if background ventilators (trickle vents) are used correctly. A study on the air quality in modern homes in Scotland with natural ventilation revealed, that in average the indoor air quality is extremely poor and far from generally accepted healthy levels of CO2 (400 – 1400ppm) (see graph below).

The abstract of the Scottish research paper reads: “Building more air-tight dwellings is having a deleterious impact on indoor air quality. In a range of recently completed dwellings CO2 concentrations were measured in occupied bedrooms at unacceptable concentrations (occupied mean peak of 2317 ppm and a time weighted average of 1834 ppm, range 480–4800 ppm). Such high levels confirm that air-tight dwellings with only trickle ventilators as the ‘planned’ ventilation strategy do not meet the standards demanded by Building Regulations. Reducing ventilation rates to improve energy efficiency and lower carbon emissions, without providing a planned and effective ventilation strategy is likely to result in a more toxic and hazardous indoor environment, with concurrent and significant negative long-term and insidious impacts on public health.” (SG Howieson, T Sharpe & P Farren, as published in Building Services Engineering Research & Technology, 2013)

A Scottish guidance paper (Building Standards Supporting Guidance Domestic Ventilation 2nd edition) states that CO2 concentrations of up to 5000ppm are quite safe. Unfortunately, we cannot find any scientific basis for this statement. All evidence points to safe CO2 levels of up to 1000ppm and that that short and long-term effects occur at concentrations over 1400ppm. Interestingly this figure of 5000ppm covers the maximum findings, as mentioned in the above mentioned research paper (4800ppm) and appears to be derived more out of the necessity to deny the problem than proper science.

Building Services Engineering Research Technology
0(0) 1–13
The Chartered Institution of Building Services Engineers 2013
DOI: 10.1177/0143624413510307
bse.sagepub.com

Read the full report:

As stated earlier, adequate ventilation is an important service for dwellings. Every dwelling should be evaluated for an adequate ventilation strategy. This needs to be done on a case by case basis, depending on the natural infiltration rate, the size of the dwelling, its number and function of individual rooms, the layout, its maximum and real occupancy.

The questions is, how much is the natural infiltration of a building. Can it be derived from the air tightness test result? Yes and no is the answer. The test applies controlled conditions: 50 Pascal pressure difference between inside and outside and it cannot be done at high wind speeds. In reality we have a number of different factors affecting the air infiltration: E.g.

– Layout and orientation of the dwelling
– Air tightness of the fabric
– Fire places or wood stoves installed
– Wind exposure, e.g. house on a hill
– Wind shielding, e.g. through trees, neighbouring houses, etc.
– Wind speeds
– External air temperatures
– Trickle vents open or closed
– Blinds or curtains drawn or not
– Internal doors open or closed

The air infiltration can be drastically different from one house to another, depending on their location and operation. It will also majorly fluctuate from day to day and hour to hour.

Therefore it is safe to say the tested air permeability will have an influence on the real-life air infiltration, but it is impossible to accurately predict it from the test result.

Following other factors will also affect the ventilation strategy:
– Typical external pollution levels, e.g. high pollution caused by traffic or industry.
– High external noise levels at night, e.g. busy road or airport nearby.
– Midges.
– Occupants seriously affected by hey-fever or asthma.
– Other reasons why occupants cannot open windows, e.g. disability or security concerns.

All of the above will make the regular opening of windows critical and may lead to the need of an alternative ventilation strategy.

In today’s context, we are basically left with three options for new-built properties:
A) Natural ventilation (Mechanically assisted).
B) Mechanical extract ventilation systems: MEV, dMEV
C) Mechanical ventilation with heat recovery (MVHR)

Scottish Building Standards have set forth a three-fold approach, depending on the air tightness of the building fabric, as shown in the scale of the following simplified schematic (scale = 0-10 m3/(h m2) at 50Pa).

As the ventilation strategy depends on the air tightness of the fabric, it potentially needs to be adjusted if the real/ tested air tightness deviates substantially from the target air tightness that the architect has set out in the planning stage. And when will you find this out? When the dwelling is finished, at the point when the air tightness test is carried out. We have come across a number of examples when the tradesmen have worked really well and the test revealed an air tightness of less than 3 m3/(h m2) at 50Pa, whereas the target air tightness was set forth at 5 m3/(h m2) at 50Pa. This meant that the planned ventilation strategy (natural ventilation) had to be changed to a ducted ventilation system (MVHR or MEV), which in most cases, unless it is a bungalow, is not possible without major disruption and cost.