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Why is air tight & breathable construction important?

Why build air tight buildings?

As we’re moving towards better and better insulated buildings, the simplistic use of U-values to measure the performance of the building envelope is becoming less and less adequate.  If warm air is leaking out of the building at junctions between materials and at poorly finished service penetrations it makes the insulation less effective, and the relative impact this has increases as the levels of insulation grow.

The current regulations in England for air tightness are still very poor. At a seminar I attended a while ago it was likened to having a hole the size of a 20 pence piece (or was that 50p?) in every square metre of the building envelope.  That’s hardly keeping warm air in and cold air out is it?  The regulations do encourage increasing the air tightness of your buildings in heat loss calculations, but as this is now tested following completion it relies heavily on good detailing and, more importantly, effective workmanship on site.

Some of these ideas are very simply put in this summary of air tightness from NBT.  The graph clearly shows air tightness increasing in importance as insulation gets better.  The article also touches on the issues of moisture control and ventilation to ensure air quality for the occupants.  Passipedia also discusses the issue of air tightness and, again, raise the issues of air quality in air tight buildings.

I think that generally it is now agreed that air tight construction is good.  However, there is some way to go to get, not only the regulations up to standard, but also the detailing, construction sequencing and controls on site to ensure that this can be achieved on site. briefly summarise the main construction issues.  I also find the approach that NBT take with their construction details a very clever one, making it much easier to assess the air tight performance of the envelope before it’s too late.

The current standards, however, do mean that the internal air quality is kept quite reasonable just through the exfiltration of stale air.  But with this stale air escaping, heat is being lost and cold air is entering the building.  It’s also possible that moist air is getting into the envelope construction and causing condensation damage.

If, however, as seems to be the agreed consensus, we continue moving towards completely air tight construction, this mechanism for keeping a healthy building with fresh air and no condensation will disappear.  We need to allow for other mechanisms for this to happen.

The most obvious of these is natural ventilation.  I am a big supporter of buildings that have natural ventilation (generally through openable windows).  In the summer a well designed arrangement of windows will keep the building and occupants cool without resorting to air conditioning.  Combined with well positioned exposed thermal mass, the fabric of the building can do much to prevent summer overheating.  However, the hot summer isn’t the most critical or difficult time.  You can’t open windows wide to purge stale air in the winter without letting in cold fresh air and increasing the heating requirements.  This starts to defeat the object of the air tight envelope we’ve created.  The same criticisms also apply to exhaust systems (passive or mechanical) like the traditional bathroom fan.

A way to overcome these issues is to use mechanical ventilation with heat recovery.  This preheats the incoming air supply with the heat of the exhaust air.  You are expelling the stale, moist air, but not losing the heat from the exhaust air, or introducing cold air from outside.

These various options for ventilation are well discussed and explained in more detail by Passipedia.

Why build breathable buildings?

One mechanism for helping improve air quality that Passipedia don’t touch on is the concept of breathability.  As I have mentioned before, I believe that the building construction and envelope should do as much as it can to passively improve the building performance before other active technologies are considered.  Breathable construction can go a long way to improving building health and performance at little or no initial extra cost.

This paper by Neil May from the NBT website sets out in a great deal of detail the mechanisms of and the main reasons for breathability in buildings.  Breathable construction essentially has 3 mechanisms: vapour permeability (the passage of water vapour through the construction), Hygroscopicity (the absorption and release of water vapour by materials in the construction), Capillarity (the absorption / desorption of water as a liquid).  The properties to enable these mechanisms are present to a greater or lesser degree in most building materials, but are negligible in many of the common materials as shown in May’s tables.

He explains how these various mechanisms can help with the building performance in terms of rain penetration, thermal performance and inner surface condensation.

What it can also do is improve indoor air quality.  By using vapour permeable and hygroscopic materials, it is possible to keep relative humidity at a level that is less conducive to mould growth, dust mites and allergies.  The construction can also absorb nasties such as VOCs and odours.  May goes into some depth about how this is achieved, and his reasoning for their use is much like mine.  Although active ventilation systems can work, they require maintenance and have a finite life span. In every situation I think there is an argument that the passive mechanisms should be enabled first and supported by active ones, where necessary.  In fact, in kitchens and bathrooms the humidity load is far too great for the building fabric alone to deal with and mechanical ventilation, ideally with heat recovery, will be required.

Air tightness and breathability are often seen as mutually exclusive.  In the typical construction we see in England, this is probably the case.  The materials used generally represent a vapour closed construction that can’t deal with any water beyond its external water tight layers.  We would end up with interstitial condensation, reduced thermal performance, rot and damage to the structure (it’s interesting to see that manufacturers such as Kingspan who’s products are not breathable in any way are quick to refute the claims made about the benefits of breathable construction, again from the NBT website).  However, we can achieve a fully air tight construction with the use of breathable materials and careful detailing.  By not having a vapour check layer, the construction not only keeps warm air in and cold air out, but allows water vapour to pass.  The nature of the insulation and cladding materials will quite happily deal with any potential interstitial condensation that may occur, with no loss in performance and no damage or rot.

I don’t claim this to be an exhaustive article on these subjects, just some thoughts that try to draw together things I’ve been reading recently.  You can probably tell that I think both of these things, air tightness and breathability, hand in hand with good insulation, thermal mass and a good mix of natural and mechanical ventilation are definitely the way forward.  I also don’t think they need to add particularly to the cost of construction and will definitely make the building cheaper to run and more enjoyable and healthy to use.  What’s not to like?