Airtightness

One of the greatest challenges facing any PassivHaus projects - and the area where many such projects fail to meet the stringent criteria laid down by the PassivHaus institute in Germany - is airtightness.

WARNING, technical language: the airtightness of a building is measured in the rather archaic units of “cubic metres of air per square metre of surface area required every hour to keep the whole building pressurised to 50 Pascals above the outside atmosphere”, or m3/m2h@50Pa. (Another measure often used is air changes per hour at 50Pa). Current UK Building Regulations will allow a value of 10m3/m2h@50Pa on new buildings, while the maximum value allowed for a PassivHaus is around 1m3/m2h@50Pa. In English, this means that we must make this old, leaky Victorian terrace perform 10 times better than an average new building!

Our strategy is to create an airtight skin around the inside of the house. We do this using a combination of OSB and special tapes. The integrity of this layer is paramount -  absolutely no breaching of it is permitted without prior discussion. If a hole is deemed necessary (e.g. for solar pipe and wire runs), we carefully plan how to close it off. Anyone who breaks airtightness rules is thrown from the roof into the street below, then devoured by angry wolves. At least that is what our project manager would like to do.

Fortunately, our European neighbours have been wrestling with these problems for some time and there are a range of products available for ensuring the integrity of the airtight membrane. We have opted to use specially designed tapes around the corners, joints and holes in our airtight skin. When carefully applied, the tape seals all these holes off. Applying them is a fairly steep learning curve though on account of their incredible stickiness.

We can not be certain of how the house will perform until the whole building is sealed, which is particularly problematic at the moment because we are still in the process of designing our triple-glazed windows; one of a litany of challenges we face. Once sealed, we will test the airtightness with a “blower door” to find out whether we have met the highly demanding criteria…wish us luck!