It’s gotta be tight. Airtight!

The next passive house principle being realised in our house is airtightness. Say whaaaat? Or at least that is the reaction we get when we tell people we need to make the house airtight. People often think it is a given, but it’s not.

When we say airtight, we mean Tupperware-tight! Airtightness is important to avoid swapping heat or coolth (yes, that’s a word) from inside the house with the external environment.  Heat and coolth can escape from a house via gaps and cracks that you don’t even know are there; this is why architraves exist. Don’t worry – we can still open the windows; and even with them closed we will plenty of fresh air via a ventilation system – I’ll cover that in another post.  

To give you an indication of airtightness in a passive house versus a conventional house built to code; to achieve passive house certification the house must have a maximum of 0.6 air changes per hour (ACH) on a windy day (50 Pa), whereas a house built to code in Australia the average is 15 to 20 ACH! Say whaaaat? That is the total internal volume of air in the house turning over 15 to 20 times every hour! That’s why you need to keep the heater on to keep the house warm – the warm air is literally going out the window! Not through the window, but via the gaps around windows and other unseen or hidden cracks. 

A super airtight house provides control over the internal environment and in turn allows greater thermal comfort. Airtightness in a passive house is achieved by wrapping the entire house in an airtight membrane. A test is then conducted to test just how airtight it is.

We’ve decided to use Adhero from Pro-Clima for our membrane. Adhero is a two in one product providing both weather tightness and airtightness in the one wrap. It’s a perfect solution for the compact house, as if we’d used an internal membrane for airtightness (as many passive houses do) we’d lose internal space due to the need to create a service cavity between the membrane and the plasterboard.

Rolls of Adhero
Rolls ready to wrap

Adhero is supplied in rolls. They are  like a big rolls of ‘contact’ made out of Gortex that needs to be wrapped around the entire house envelope. Our builders are currently painstakingly attaching the Adhero to the Pavatex. If you’ve ever covered a school book in contact – it’s like that – but on the scale of a house where each roll is 1.25m high. Props to the builders who are wrestling it around our quirky shaped compact house – roof, walls, and everything in between. There are some very squeezy spots! We might need the kids with their slim agile bodies to help, although this will only be possible if we turn off the wifi.

Roof wrapped

Once the building is completely wrapped and the windows are put in, we test how airtight it is with a blower door test. A blower whaaaat?? Blower door tests simulate a strong wind and take measurements of the ACH to see how air leaky the house envelope is. We’re aiming for 0.6 ACH. Stay tuned!

Front of house with wrap almost complete and Gavin standing in window frame.
Where’s the Wally?

If you want to nerd out (like we do), check out this video form the Energy Matrix website where they do a blower door test and talk ACH.

This video from Canada is also worth a watch as it explains the two tests required to achieve the final ACH measurement and it’s great to see women doing the building science on this Canadian project. Enjoy!

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