How far do we need to go on air-tightness?

  • Building Design Expert
  • 5 years ago

If you have missed it up to now, air-tightness is the major determining factor in new building design, and is even infiltrating into the realms of building retro-fit. If we cannot keep the air, we have paid through the nose to heat, inside our buildings at long last we have realised we will be losing money hand over fist. The government psychologists are still arguing on whether they try and sell it as ‘losing money if you don’t, Or ‘saving money if you do’? At the end of any day it’s what’s more important to you, as the outcome is the same.

The design limit on air permeability for new dwellings remained static in the last re-working of the Building Regulations (2010) Approved Document L1A @ 10m3/(hr.m2) @ 50Pa. It didn’t move from the value set in 2006. But then neither did thermal elemental U-values that much. Their increase in 2010 belied the apparent regime that was initiated in 2006, and we were left wondering to a large extent, what are they thinking? Where are we going with this?

The draft for 2014 provides a word on the street that will see a turn around in fortunes again, depending of course what side of the fence you are promoting your fortunes from?

The industry gurus have been banging on about codes for sustainable homes (4,5 & 6) and Passivhaus over the last year or two to such an extent that the minimum standards laid down in the current Building Regulations Approved Documents seem almost laughable, particularly when local planning authorities are flexing their muscles to insist on higher ‘sustainable code’ standards – just because they can. But because they can is good, – we must all do our part to transcend the current minimums, and do the best that is feasibly possible under given project constraints – costs and all.

But what difference does air tightness actually make? Those in the know about the standards ordained under Passivhaus at 0.6m3/hr.m2 and Enerphit @ 1.0m3/(hr.m2), with the code for sustainable homes coming in  at anywhere between the Building Regulations maximum of 10.0 and the recommended good practice levels of between 3.0 and 5.0m3/(hr.m2).

Of course the CfSH procedures for calculation are a moveable feast orbiting around the Standard Assessment Procedure (SAP) so although air permeability is important in determining the Target and Dwelling Emission Rates (TER and DER) it is a figure that can be off-set by the very nature of how SAP works. This approach has its pros and cons, although to achieve the higher code level 6 (zero carbon) the likelihood is a figure of 1.0m3/(h.m2) will be required. Still not Passivhaus though is it?

The ‘pro’ on Passivhaus is the immovable air permeability rate, and the high insulation levels and controlled ventilation will guarantee a warm stable environment. But Passivhaus is quite inflexible in design. The building orientation, strict window sizes / glazed areas – on specific building aspects will not be to everybody’s taste. The CfSH design criteria allows a far more traditional type appearance, and may appease Mr and Mrs average in terms of kerb appeal. The ‘con’ being that thermal performance and post completion operating costs will inevitably not match Passivhaus.

But super air tightness and Passivhaus are not joined at the hip. We are not technically precluded from achieving ultra low air permeability with a more traditional design approach that might suit the ‘volume’ house builders. The practicalities of achieving these levels may be another issue, and it is here that we must dig up the old chestnut concerning education of contractors in the importance of the smallest workmanship item that will never be seen usually concerning membranes, jointing tape and understanding cold bridging. – Perhaps another time.

Such a hybrid design may not provide Passivhaus thermal performance such that home owners will probably still require supplementary heating in winter. However, I would trade  a supremely air tight building, with controlled ventilation for a little less thermal performance any day (if that was the choice). Think about heat leaving the building instantly with the air. – Keep the air inside the building and the heat will hang around for so much longer until it leaches through the building fabric – No contest.

How far do we need to go on air tightness? – We need to buy the most expensive ticket we can afford, to go as far as possible.

Want to know more about air tight construction? Try the Passivhaus Handbook