Exactly what is air tightness in a passive house?

  • Posted on: 16 January 2015
  • By: Jennifer


Last night I attended a talk and documentary presentation by Ben Adam-Smith (twitter:@BenAdamSmith) about the Future of Housing and how air tightness can help. The talk was hosted by the Australian Passive House Association. 

Ben is a podcast producer for House Planning Help (twitter: @Houseplanhelp) a UK based organisation that offers people all sorts of information to assist them in building a better home.  We all know this is something very close to my heart.

The documentary looked specifically at airtightness in residential buildings in the UK.  So what is airtightness and why does it matter?

Well in the UK it is mostly to do with heat loss from homes in winter.  As we all know, English winters are significantly different to those in most parts of Australia.

In Australia, though, it becomes more of an issue for cooling in summer rather than heating in winter.  The same principle applies.  If your building leaks air then it doesn’t matter whether you are heating it or cooling it, the warm or cool air ends up escaping, meaning that you spend more on energy costs trying to keep a constant temperature inside.

Air tightness means sealing up cracks and gaps that are often hidden within cupboards or walls or near windows and doors.  This isn’t high tech at all really, mainly completed with tape around pipes and conduits or silicon around windows and doors.  Simply doing this can make a big difference in how cool or warm your home feels.

As well, air tightness is different to ventilation.  When the weather outside is rather benign as it is mostly in Sydney, there is no problem at all with leaving a door or window open to let some breeze through.  It’s when the extremes of temperature hit, either hot or cold, that you want your house to be airtight.  When it is airtight, ventilation systems (different to and much more efficient than air-conditioning) such as Ventis systems or similar can provide fresh air at a suitable temperature to the interior of the home. 

So how do you know if your house is airtight?  Well I can pretty much guarantee that it isn’t.  The issue of airtightness has never really been addressed in Australia, not even by the Building Codes which have very vague wording when it comes to this issue.

Some testing that was done by the CSIRO measured how many air changes happen per hour.  ie. how many times the air is completely replaced in a home with all the doors and windows shut.  The best performing house (which I think happened to be the CSR house that I have mentioned elsewhere) had 8 air changes per hour some other houses were up to 20 or 30 air changes per hour.  The passive house standard is less than 0.6.  So a well designed, well sealed house in Australia is still 13 times more leaky than the passive house standard.

So what is passive house or passivhaus?  Passivhaus is a building standard that was developed in Germany to deal with the construction quality and performance of buildings.  The use of the Passivhaus standard has become far more popular in the UK in the last decade as people look to live in more efficient and comfortable homes.

At the end of the presentation I asked about moisture and mould in airtight environments.  This began a rather lengthy conversation with many of the people in the room.  I have seen some rather scary photos of mouldy insulation inside walls where insulation was not installed correctly.

In a nutshell the response was that wall construction needs to be handled correctly to avoid areas of condensation inside walls leading to the growth of mould.  There are also many new technological advances in membrane technology that can assist with keeping the right temperature in while letting the vapour out.

Airtightness is just one part of the Passivhaus standard, but is significant in the reduction of heating and cooling costs. So if you have a spare weekend and feel like getting your silicon gun out, then sealing up all those cracks, gaps and holes around pipes can go at least part of the way to helping keep the interior of your home comfortable.