We carried out a preliminary airtightness test last week. The idea of this is to pressurise the building with a big fan, and measure the airflow needed to maintain the pressure. That then gives a measure of how “leaky” the building is. We’ll need to do a final test when the building’s finished, but we wanted to get an initial idea at this stage, now that pretty much all the penetrations through the walls, floor and roof have been made, so as to make sure they’ve all been properly sealed – otherwise, if we wait until the building’s finished, it’ll be much more difficult to fix.
The general idea of checking for airtightness is pretty straighforward – most houses lose about a third of heat from the warm air just leaking out. Also, in our case, there’s no point in putting a heat exchanger on the ventilation system if all the air is going to leak out around the doors and windows. The more difficult question is, just how airtight does the building have to be? The Passivhaus Institut decrees that, to meet their standards, the building has to have less than 0.6 air changes per hour at a pressure difference between the inside and the outside of the house of 50 Pascals. For most people that needs a bit of context! A pressure of 50 Pascals is about that exerted by a 20 mph wind, so the test creates the sort of conditions of a moderately windy day, with the wind coming at the same speed from all directions. British building standards now require minimum levels of airtightness for new houses – the minimum (both in Scotland and in England) is 10 m3 of leakage per m2 of surface area per hour, at the same 50 Pascals pressure difference. How does that compare with the Passivhaus standard? Well, the conversion will depend on the shape of the house, but for us, the Passivhaus requirement is equivalent to 0.7 m3/ m2/ hour, or 14 times more stringent than the building regulations. So clearly we’re looking for something a lot better than the average house! One of the builders we approached when we were choosing contractors flatly told me the Passivhaus standard was impossible (shortly after telling me that his firm works to the highest standards!).
There’s one more minor complication, as the Passivhaus requirement applies to an average of pressurising and depressurising the building. The reason for this is that gaps around the doors, for instance, are likely to get bigger when the house is pressurised but smaller when it’s depressurised.
Getting things airtight has meant using an internal membrane covering all the walls and roof. Inevitably, this gets ripped during construction, so there was a fair amount of work patching it up. Some people advise driving home all staples and taping over them, but we only did this where there was a bigger problem. The window frames are all taped to the airtightness membrane. We minimised the number of holes through the fabric of the building (for services, the flue, ventilation system ducts, etc.) and they were all taped around. One area which is often a problem is the junction between the walls and the floor, and again this was taped up – and behind the foam, expanding foam has been sprayed into the space under the wall cassettes.
The windows and doors themselves are designed to be airtight, with three sets of gaskets around them.
So after all that, Colin got to work with his fan:
As the pressure difference increased, a few leaks were spotted (just by holding a hand up to the most likely places) and fixed. There were a few things we couldn’t do anything about though – most notably the gaps around the barrels of the locks!
There was also a bit of leakage around the doors themselves, which haven’t yet been finally adjusted in their frames.
The big question is, “what was the result?” and unfortunately I can’t give a definitive answer on this, we found it really difficult to get consistent readings. We’re pretty sure the reason for this was that the wind was blowing, not all that hard, but in gusts, making it impossible to get a consistent pressure difference all round the house. Saying that, the lowest readings were about 0.2 air changes per hour, and the highest about 0.8, and most of them were below the magic 0.6 level. So Colin’s away to plot them all on a graph and see if he can give us a figure, and meanwhile we’re feeling a bit cheated by the lack of a number, but encouraged by the generally low level and the fact that we should be able to improve things when the doors are adjusted. And also by Colin’s verdict: “You won’t have a draughty house – in fact, it’s the most airtight I’ve tested!”
Culbokie Passive House Diary 