I sometimes hear people say, as a criticism of very airtight homes, that a house has to “breathe”. Well, I agree completely – and our airtight home does breathe. I would just rather pass the air through a set of building lungs with filters that actually clean the air. In an old house that “breathes”, the air could very well be sucked in through walls full of asbestos and rat poop. It’s completely uncontrolled. And a leaky home also means heat pouring out (or in) when you’d prefer it didn’t.

Of course we can open the windows at will. But if we don’t, we have 100% filtered outdoor air being continuously supplied to our bedrooms and living spaces, and exhausted from the bathrooms and kitchen. We can turn it up or down if we want, but most of the time we set and forget – especially easy to do when the system is inaudible.

On top of the benefits of good ventilation, the heat recovery ventilator captures the heat from the exhaust air to preheat the incoming air, which is a key ingredient to getting the heating demand and cost so low.

Now that we agree that airtightness and ‘breathing’ are not mutually exclusive concepts, and that airtightness is a good thing, let’s talk numbers.

A certified Passive House must reach an airtightness of 0.6 Air Changes per Hour, tested at 60 pascals of induced pressure. A typical new home being built today is around 2.5 ACH (although this is starting to trend down in BC now that we have Energy Step Code requirements to test for airtightness). Older homes can be upwards of 10 ACH. Considered another way, this means that, under 60Pa test conditions, ten times the entire air volume of an old home leaks out of uncontrolled openings in the span of an hour. Well that’s a bit of a waste isn’t it.

So how did we do?

We did a mid-construction test way back in 2019, when the air barrier was still exposed and all the holes for the cladding attachments had been made:

This test is a useful exercise, although not required, to catch any big holes or systemic issues while the air barrier is still exposed and reparable.

In the mid-construction test, we hit 0.27 ACH. Woot! And actually not all that surprising. We have seen many new buildings that use a peel and stick membrane achieve a high degree of airtightness. This was an average of the pressurization (blowing air into the house) and depressurization (sucking air out) tests and the two individual tests were comparable.

Skip to the final test, which is the one that matters for Passive House certification. We did this one when the building was essentially done at the end of May 2020.

This time we tested to 0.27 ACH under pressurization and 0.39 under depressurization, for an average of 0.33. Still well below the threshold of 0.6 although I was puzzled that the final test was less airtight than the mid-construction test, and that the building was more leaky under depressurization than pressurization.

Given our enclosure approach, I would have expected an improvement if any change. We’d corrected at least one source of leakage since the mid-construction test. When we replaced the locking hardware on our entry doors (they had to be re-keyed to match the suites) we discovered that the lock hardware had not been fully caulked.

In a very airtight home these small things make a difference. During a few very cold days in January, our HRV went into unbalanced mode to prevent its core from freezing and we could feel the cold air being sucked through the locking hardware.

The fact that the depressurization test was higher suggests that air was more easily sucked through a hole than pushed out of it. We’re thinking it might be this open drain from the pressure relief valve of the hot water tank. From what we understand, it connects to the perimeter drainage system outside the house.

This doesn’t feel like a big deal to me. I don’t think there is any air leakage happening through this pipe during normal operating pressure. And regardless of my desire to achieve the best possible result , 0.33 ACH is still extremely airtight. At levels this low, further efforts to chase leaks will not have any real effect on energy performance and are not really worth the time and effort. I have other things to keep me up at night!