Apparently in Germany, when they do an airtightness test on a house, they test both over pressure and under pressure. In other words, they shut all the windows and doors, and put a blower on one of them to blow air into the building until the pressure gets to be higher than outside, then they measure how quickly the air starts leaking back in again. Next they blow air out so the pressure is lower inside, then they measure how quickly the air starts leaking in again. They take the average of the two values to get the airtightness of the house, which is measured, at least for Passive House certification, in the number of times the air will change per hour. The passive house standard is 0.6.
In Japan, they usually only have the equipment to do the under pressure measurement, which apparently is usually a little better. So, in a sense, Japanese airtightness measurements suck.
They did another airtightness test in December, which I'm still waiting for the results for. I should have done this months ago, but I've just now started looking carefully at the results from August. The experts said that we needed a C value of 0.2, but we only got 0.3 which was not good enough. They said that this was a reverse calculation, making it sound really difficult.
Never trust experts, especially if they make things sound really difficult and complicated. If they do that, it's a sign that they don't know what they're talking about. If they do know what they're talking about it, they should be able to explain it and make it simple.
Anyway, as a result of this 0.3 that should have been 0.2, we became very sceptical of the Compriband's effectiveness, and added caulking around each window to improve airtightness. We had previously planned to add a layer of insulation around the inside of the window frame, on the few centimetres of wall perpendicular to the window. This insulation would have reduced the thermal bridge effect of the window from something like 0.04 W/mK to 0.03 W/mK. This doesn't look like a lot, but when you think of all the windows in the house, and measure around each frame, there are something like 80 metres, and there are 70,000 degree hours temperature difference over the part of the year that needs heating, so it amounts to about 50 kWh per year.
Anyway, it was basically presented to me as a choice between putting caulking around the window frames, to improve the airtightness, which wasn't good enough, or carrying on with the plan to insulate around the frames and improve the thermal bridges. The caulking was going to work out more expensive than the insulation, but the builders offered to cover the extra cost, so it would make no difference to my pocket.
The decision had to be made quickly as other parts of the wall were about to go up, and the frames would no longer be accessible. I agreed to them adding the caulking, which the airtightness and insulation people went ahead and did.
But, while waiting for the results of the latest airtightness test, I started looking a little more closely at the figures of the last one. I should have done this ages ago, and in fact I've been waiting for an opportunity to talk with them and find out more details of this devilishly difficult conversion between the C value and the number of air changes per hour.
According to the figures the airtightness experts emailed me 4th October, almost two months after the test, the result was 259 cubic metres per hour at 50 Pa pressure difference. The bit of the form where the number of changes per hour should have been was blank, I guessed because they didn't have the figure for the volume of the house. According to the Passive House database, the volume was 500 cubic metres. Obviously this is not the exact volume, but it's close and serves as a design volume. Taking this, and the 259 cubic metres per hour, that looked to me like 0.52 times per hour, which meets the PH standard.
So, I surmised that either 1) my calculations are incorrect and it's much less straightforward than [volume per hour / total volume]; 2) the design volume of the house (500 cubic metres) is a lot more than the actual volume; or 3) the architect or airtightness experts were too lazy or too incompetent to perform a straightforward calculation. My money was on 3.
I spoke to the architect on the phone, broken into two or three calls as he kept having to find information, or calls back because he had found more information. The figure he had was 0.542 exchanges per hour at 50 Pa. The actual volume of the house is 478.1 cubic metres. I'm beginning to wonder whether the airtightness people sent him a different copy of the results to the one they sent me... Why would they do that?
After first denying that the caulking had anything to do with the insulation and suggesting that the airtightness people had done it as an act of charity, he later came back and conceded that yes, the first airtightness test had met the standards, although they had told us that it had not, and that no there had been no need to add caulking on top of the Compriband, and yes, we could have had the extra insulation around the inside of the window frames and reduced the thermal bridge effects.
Another factor prejudicing them against extra insulation was that in some places the insulation would have stopped the windows from opening. As far as I was concerned though, it was a fairly straightforward choice.
Not sure exactly whether he's going to do anything about it, but he did at least say sorry, and not really related but he would get us a ventilation system with a bypass, and would cover the cost for it.
Maybe a complete coincidence but the airtightness and insulation people did the caulking work, and also did the airtightness tests.
There's something very satisfying about letting people know that they have done you wrong, but perhaps only relative to the much deeper dissatisfaction of feeling that you have been done wrong to.
