Building Tight


Providing well installed insulation is only one part of the insulation puzzle.  The other vital component is making sure that unwanted ventilation is stopped from blowing through the fabric of the structure.  A well insulated house won't perform very well on a windy day if it is a very leaky structure.

But how tight should you build?

There is no doubt that building tighter and stopping unwanted air leaks has benefits thermally, but this stratgey can also be at odds with indoor air quality and healthy outcomes.  Without an adequate exchange of fresh outside air, moisture, C02, VOCs and other indoor contaminants will build up inside.  This in turn can lead to mould creation and what has been called sick building syndrome.  We need to build tight enough to have good thermal outcomes, but not so tight that health is compromised.  So how tight is that?

The National Construction Code (NCC) does not specifically quantify an air leakage rate, but has a performance requirement that states:

“A building must have, to the degree necessary, a level of thermal performance to facilitate the efficient use of energy for artificial heating and cooling appropriate to the sealing of the building envelope against air leakage.” NCC Volume 2 (2015) – clause P2.6.1

But what does that mean in practice?  What should builders be aiming for? 

A good minimum guide is to at least comply with the air change assumptions of the NatHers house energy rating calculation.  This is around 10 air changes per hour at 50 pascals of pressure (10 ACH@50Pa).  It is reasonable to assume this is the minimum the industry should aim for.  In 2015 the CSIRO used blower door testing (pictured above), to test the tightness of 129 new homes across Australia, in their House Energy Efficiency Inspections Project report.  They found that of the 129 new homes they tested across Australia, the average tightness was 15.4 ACH@50Pa, with only 1/3 of the houses achieving below 10 air changes.  This means that technically 2/3 were out of compliance with the house energy rating.  There is no doubt that building tighter homes needs to be a focus across the industry.

But the question remains, how tight should we be aiming for?  The German Passiv Haus standard, which has been gaining a fair bit of traction here in Australia, advocates a super tight 0.6 ACH@50PA.  This is great thermally, but is so tight that mechanical heat recovery ventilation systems need to be installed to remove water vapour and contaminant buildup, and to stop possible asphyxiation.  The super tightness is technically quite difficult to achieve and usually requires specialized membrane products.  The Australian Building Codes Board, in their 2016 Scoping Study of Condensation in Residential Buildings p.43, noted that, "the mechanical ventilation approach is adopted in countries where envelope air-tightness regulations require less than 5 ACH@50Pa".  This is of course an arbitary line in the sand number, and will be mitigated somewhat by climate and how often people are able and willing to open their windows.  But a line needs to be drawn somewhere, and as a rule of thumb 5 ACH@50Pa and below seems a reasonable place to start to include mechanical heat recovery ventilation in the building. 

Due to this recommendation for mechanical ventilation when going tighter than 5 ACH@50Pa, any extra thermal gains beyond this level of tightness are likely to be offset by the need to run the mechanical plant 24 hours a day.  So unless you are aiming for Passiv Haus certification, or want mechanical ventilation and filtration for health reasons, there is not much to be gained (operational energy-wise) going tighter.

To answer the question, in my opinion, builders should at least build to below 10 ACH@50Pa, and 5-6 ACH@50Pa is probably the sweet spot to aim for. 

The good news is that past blower door testing confirms that this sweet spot can be achieved with conventional materials, and without much extra expense.      It does however take a focussed team effort to achieve. 

Here at Positive Footprints, we have worked hard over the years on our sealing and caulking details.  We also use a thermal imaging camera, and lessons learnt from past blower door tests to inform our construction and installation.  We have also looked at Passiv Haus techniques and incorporated those where applicable.  And lastly we have built a team of carpenters and trades people who understand that building tightly is everyone's job ("if you make a hole, or see a weak point you fix it").  That is how Positive Footprints has been able to build around that 'sweet spot' tightness level, to make sure that the house is built to perform as designed.  We also offer a blower door tests, as an extra charge, for those who want further piece of mind.

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