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Airtight Homes & Air Quality: Missing the Point

There have been a few articles in the media recently about modern airtight houses causing bad indoor air quality (IAQ). We find this misleading. The message is that the problem lies with building airtight homes – leading readers to assume that a draughty home is preferable and has better indoor air quality.

We’d say that bad indoor air quality is caused by inadequate ventilation, not the airtightness of a building.

Draughty homes can have poor air quality too

Poor ventilation can occur just as often in draughty homes as in airtight homes. A draughty house on a still day has too little ventilation and on a windy day has more ventilation (and associated heat loss) than is necessary.  A draughty house does not have any control over the direction the air moves in. Rather than moving air from dry to wet areas of the house, as is ideal, draughts can just as easily move air from moisture creating areas of the house (kitchens and bathrooms) to cooler, dry areas (bedrooms) creating condensation problems and mould on cold surfaces. This is a typical pattern as air tends to move up through a draughty house due to the stack effect and bedrooms are usually upstairs with kitchens down below.

Do windows help?

The strategy of opening windows to create adequate ventilation has similar problems to those in a draughty house, i.e. sometimes too much and sometimes too little ventilation and air often flowing the wrong way. Also, as people are not very sensitive to CO2 or VOC concentrations, we tend to open windows mainly to cool down rather than to ventilate so there is no reliable control for the ventilation rate. A “stuffy” feeling is to do with heat and humidity and very little to do with CO2 or other pollutants.

Airtight homes with poor ventilation

An airtight house without an adequate ventilation system can suffer from these same problems of too little ventilation, (although both on still and windy days), too much ventilation when windows are open and air moving in the wrong direction through the house.

What should happen?

A good ventilation system ensures that the right amount of air moves consistently through the house from dry areas to moisture creating areas and then outside, ensuring a good IAQ. However, badly functioning ventilation systems in both airtight and draughty houses will cause poor IAQ, condensation and mould.

New houses and airtightness regulations

Building regulations now sets a minimum airtightness standard for new houses of 10 m3/(h.m2) @50 Pa and a recommended level of 5. Passive house certified houses need to be below 0.6 ach (roughly the same units as m3/(h.m2). So all new homes have to be airtight, to some extent, to meet building regulations.

New houses and ventilation regulations

New homes are no longer built with “natural ventilation” (opening windows) as the only ventilation strategy as that is no longer compliant with building regulations. Trickle vents with intermittent extract or continuous extract (with or without heat recovery) are the three options available as ventilation strategies. All of these need to be used in combination with openable windows for situations where you need high ventilation rates, (painting, night purging with cool air). This is not to say that people shouldn’t usually open windows, they are very welcome to, just that people shouldn’t need to open windows in normal circumstances to keep their homes well ventilated. Ventilation relying on opening windows will fail due to people’s reluctance to do so when there are noise problems, security issues, heat loss and cold draughts or pollutants outside.

Performance of ventilation systems

Given that new homes should all be airtight and, just like draughty homes, they need good ventilation systems to ensure good IAQ, it is worth looking at how ventilation systems are performing.  Oxford Institute for Sustainable Development (OISD), the Mackintosh Environmental Architecture Research Unit (MEARU) and Ian Mawditt (Fourwalls Consultants) recently published a study on MVHR (mechanical ventilation with heat recovery) in airtight homes.

The focus of the study was the performance of MVHR systems in new housing in the UK. The study found that when MVHR systems were designed, specified, installed, commissioned, handed over to the users and maintained well, then the IAQ was extremely good, houses had very stable environmental conditions and overall energy use was lower. However, it found that for the vast majority of domestic MVHR systems in the UK there were multiple problems with the process at any of these stages. As a couple of examples: Out of 38 sets of commissioning data that was reviewed, only 6 (16%) systems have demonstrated that they have been satisfactorily commissioned. Regarding handover, half of users did not understand how to operate their MVHR system.

It must be pointed out that, even given the very poor state of MVHR systems generally installed in the UK, when the study compared MVHR systems with non MVHR (Mechanical Extract Ventilation or intermittent extract with trickle vents), it found that MVHR systems performed better in terms of IAQ than non-MVHR homes. As one example, peak CO2 levels in bedrooms were over 1500 ppm (too high) in about 90% of non MVHR houses compared to under 20% of MVHR houses.

MVHR done well, works

In summary, the study:
• Found that MVHR is very often not being carried out well in the UK.
• Found that even given the above point, IAQ in airtight homes with MVHR was significantly better than in airtight homes without MVHR.
• Found that where it is done correctly, MVHR provided extremely good IAQ and energy savings.

Good air quality is dependent on good ventilation

In conclusion, headlines blaming poor air quality on airtight buildings are seriously misleading and completely miss the point. Poor indoor air quality is caused by poor ventilation. Airtight homes are not the problem. Airtightness is part of the solution, providing an opportunity to prevent excess heat loss and ensure good, well regulated ventilation.

*MVHR – Mechanical Ventilation with Heat Recovery

Anna Marie Byrne, MVHR Design Engineer & Certified Passive House Designer, Green Building Store

Green Building Store’s MVHR Design Service specialises in Passivhaus and low energy projects, and designs, supplies and commissions heat recovery ventilation systems that are quiet, efficient and robust.

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