Summer 2013 - has the house kept its cool?

I have been asked this question throughout the summer. Last summer was so cold that we were not able to assess how well the building performs in hot weather.

Unfortunately, we don't have data loggers or similar equipment to provide lots of quantitative data. That said, July was our warmest month with temperatures in the high 20s (Centigrade) and from memory hitting 30C on one or two days. The house remained comfortable thoughout nearly all of that period within internal temperatures between 22C and 25C. We did have one day when the temperature in my home office (which has a PC with two monitors, one of which is quite old and gives off significant heat) reached 27.5C and it did feel a bit too warm. Other rooms in the house felt quite comfortable throughout.

Our cooling strategy

Site constraints meant that we needed more east and west glazing than one would have in an "ideal" Passivhaus. This meant needing to use external blinds or similar, not an overhang type shading device which is possible on a south facing window because the sun is lower in the sky in morning and evening. So....

1. We chose to fit Internorm windows with integral blinds - see below - this was a solution specific to our local circumstances, as the window is not a true triple glazed window, consisting of a sealed double glazed unit, and internal venetian blind and a third pane on the outside of the blind: 2 + 1. The blind is in a ventilated space. This means that they have a whole window U-value of only 0.93W/m2K, less than would be needed in many parts of the UK. Internorm do a 3 + 1 window, with a lower U-value but it is considerably more expensive, very heavy and they use Krypton, which is best avoided (more expensive/rare and - when it leaks out as it will after about a decade - results in a more greater drop in performance than Argon-filled units. All the windows on solar gain facades have high g-value glazing (60%), so they will capture 20% more heat than typical 50% g-value triple glazed units. This means that remembering to use the blinds is key to keeping the house cool. This can be solved by using automation but this seems like a step too far in complexity in a domestic situation; although perhaps it is a good idea in an office building.





2. We have to sliding pation doors, one on the east and on the west facade. These are "traditional" triple glazed. We planned two pegulas to provide seasonal shading on these two quite large glazed components. Unfortunately, we haven't got the pergulas in place yet, so we did get more solar gain than is ideal. I don't know how much impact this is having on the overall summer performance of the building. When I modelled it in the PHPP, it does affect the frequency of overheating and, had the pergulas been completed before this summer's not spell, we might have avoided the slight overheating on a day or two in July.
3. We do of course open our windows in the summer and I tried to keep the windows closed in the hot portions of the day (to help keep the heat out) and to make sure the windows were open at night. I think that if we were located in London (where night time temperatures don't drop as much), night time flushing would have been much less effective.

During the warm spell, I experimented with swiching off the MVHR at certain times of the day. Our intake (and exhaust) terminals are on the west facade wall. During the morning, when the west is in shade, I noticed that the MVHR seemed to help to maintain a lower temperature, as it was drawing in cooler air from outside. In the afternoon, it was drawing in warmer air and this possibly caused internal temperatures to warm more than might otherwise been the case. I tried switching off the MVHR in the afternoon but this seemed to make it feel less comfortable in the afternoon. I think that the reduced air change rate may have resulted in higher internal relative humidity levels, adding to thermal discomfort.

More generally, we have noticed that the house is more sensitive to internal heat loads than a typical UK home. Some of this is modelled in the PHPP but, in real world use, occupants may bring older electrical equipment, which will add to overheating risk. In my case, the old computer screen does give off significant heat and, although it is smaller than the newer one, uses more electricity. I don't think, however, that it makes economic or ecological sense to replace a perfectly good computer monitor.

Lessons learnt from this summer

1. Keep the MVHR running. Although it provides no cooling function (a common misconception amongst some Passivhaus sceptics), the regular air changes it delivers moderates internal relative humidity.
2. Use the blinds.
3. Consider the impact of internal gains - heat emitting devices will have a surprising impact.