Humidity Control

Dampness is one of the biggest factors that can deteriorate the structure of a building. Even if there is no water ingress through the exterior, e.g. roof or rising dampness through the foundations, dampness can occur through the use of the building. Damp surfaces can attract mould and fungus infestation, but also the proliferation of dust mites. These can have serious effects on the health of the occupants.

Our daily activities, such as cooking and using the shower or bath release moisture into the building. With every breath we also exhale a certain amount of moisture. On top of this water evaporates from every wet surface and indoor plants. The moisture content of the air is usually measured in relative humidity (rel.h.) with its peak at saturation point, which is 100% rel.h. At this point the moisture forms visible droplets (steam or droplets on surfaces). However in most cases dampness on walls is not visible. Often it is only discovered when black mould has formed.

How does condensation occur? 

Water’s capacity to retain moisture is dependant on its temperature. Warm air can physically hold more water than cold air, e.g. at saturation point, 20 deg. C warm air can hold 17.3 g of water per cubic meter; at zero degrees this drops to 4.8 g/m3. If warm air cools down, the absolute moisture content stays the same, while the relative humidity increases. In the example below we have 20 deg. C. warm air at 50% rel.h. containing 8.7 grams of water in each cubic meter. If this air cools down on a cold surface, it reaches its saturation point (100% rel.h.) at about 8 deg. C. If the air cools down even further moisture falls out on the cold surface as condensate. You will have seen this effect when water droplets form on the outside of glasses with cold beverage.

Dampness from internal humidity occurs typically on cold surfaces, e.g. windows, window reveals or external walls. The risk increases if there is little ventilation in a certain area, e.g. behind curtains or furniture.

How to counteract such dampness problem? 

Appropriate ventilation will help to expel excessive moisture.

MVHR systems will additionally reduce the indoor humidity, through the de-humidification effect of heat recovery. The more outside air temperatures are colder than inside air temperatures, the incoming air will increase in temperature and thereby decrease in relative humidity. E.g. Zero deg. cold air with 50% rel.h., raised up to 18 deg. C. will be introduced into the dwelling at less than 20% rel.h. As such it reverses the condensation effect mentioned above. Choosing the right amount of ventilation through the MVHR will help to keep the humidity levels in the right bracket.

Careful consideration of an appropriate ventilation strategy needs to be applied for both existing and new built housing stock. Existing properties that are being upgraded pose a special risk, as the building is being made more air tight and added insulation can often not be applied as consistently as in new buildings. Dampness can then easily form on the cold spots of the thermal envelope. According to Dr Sterling Howieson of the University of Strathclyde the risk of black mould triples for such renovated dwellings.

It is not unusual for modern dwellings to encounter dampness and mould issues, when there is no adequate ventilation strategy in place. Despite trickle vents and intermittent extract fans, humidity from respiration in habitable rooms is hardly being extracted and builds up. Any wall spaces or window reveals which have colder surfaces will then cause condensation with the mould problem. Also rooms which are colder than the rest of the dwelling will also more likely attract condensation than warmer rooms.

If there is smells appearing, this can be an indicator that mould infestation has started to settle in the home, even if there are no black spots visible. Existing mould needs to be treated independently from the humidity issue.

We would suggest following approach:

1. Monitor

Investigate the extent of your problem by measuring the air quality and humidity in your bedroom.
We can recommend the Wohler IAQ monitor: https://www.paulheatrecovery.co.uk/product/mobile-co2-temperature-data-logger/

You can also just buy a hygrometer and measure your relative humidity level in the master bedroom.

Your target humidity is 40 – 60% relative humidity and below 1000ppm of CO2.

2. Continuous ventilation

Exchange the intermittent extract fan in your en-suite to a continuously working extract fan (dMEV fan), e.g. CV2GIP
You would need  a continuous 230V power supply for the fan (not just the light circuit, which comes on and off). The CV2GIP fan will work 24h/ day. It can be placed into the same 100mm duct as the intermittent fan.

Adjust the fan to 10 l/s

3. Ensure air movement within the dwelling (cascade ventilation)

Close the trickle vent in your en-suite, but leave the trickle vent in your bedroom open. This will draw air from the bedroom into the en-suite, causing some cascade ventilation, which will help taking humidity out of your bedroom.

In order to allow air to move through your dwelling, door undercuts of 5-8mm are needed, unless there are other forms of transfer openings between rooms.

4. Keep monitoring

Keep monitoring your humidity and CO2 levels.

5. If possible install a well performing MVHR system

MVHR systems will address excessive humidity in three ways:

– Direct extraction of humidity at source, e.g. shower rooms, cooking, etc.

– Air movement throughout the dwelling, which will help to counter mould growth

– Passive de-humidification in the colder seasons. The more the ventilation rate and heat recovery rate and the lower the outdoor temperatures, the more the system will de-humidify the incoming air.

MVHR systems provide a powerful way of dealing with excess humidity in three ways:

1. Extraction of excess humidity where it is being produced.

Care needs to be taken that the extract terminals are located correctly. They should never be placed close to the bathroom or en-suite door, but be close to the shower area or above the bathtub.

Also special requirements for the extract air flow rates need to be considered, e.g. in social housing, when shower rooms are being used more than usual, especially in wet rooms without windows.

2. Air movement

The good experience of PIV systems with dealing with mould issues of existing dwellings is down to them creating some air movement throughout the building.

3. Passive de-humidification

MVHR will dehumidify the indoor air. The colder it is outside, the higher the ventilation rate and the higher the real efficiency of a MVHR system, the more it will dehumidify. In larger dwellings with higher ventilation rates this effect can get too strong that counter measures need to be taken.