Actually, it seems as if we’ve pretty much skipped spring and gone straight to summer. We had a week away, and got back in the evening on Easter Saturday to find the living room at almost 23 deg C, and the thermal store at 68 deg C! Upstairs, our max/ min thermometer showed the temperature had been up to 28, so obviously there had been some sunny days whilst we were away. We opened some windows and pretty soon the temperature was more comfortable, but it did make us wonder a bit about leaving the house for long periods in the summer – it isn’t really a serious problem, but it would be a pain to come back to melted candles!
Part of the issue may be that we had turned the MVHR down to its “unoccupied” setting, so there was less air circulation than normal. The MVHR has a “summer bypass” – which means that, when the extracted air exceeds a set temperature, it by-passes the heat exchanger, and just brings cold air into the house – so maybe, during the warmer seasons, we should just leave the MVHR running as usual and take advantage of its cooling potential. And maybe we should store the candles somewhere cooler.
And this has made me think a bit about the way we feel temperature. The air temperature in our caravan was frequently above 23 deg C, and it never really felt uncomfortable, in fact there were times when it still felt quite cold at that temperature. On the other hand, the house feels pleasant at around 19, which would certainly have been too cold to sit around in the caravan.
There are three factors at work here. Air temperature is important, and easy to measure, but the temperature of the surfaces, and the velocity of moving air, are also significant. By “surfaces” I mean mainly the walls, windows and floors, all of which radiate, so just as it can feel warm in sunshine on a cold day, it can feel cold in warm air if the house itself is cold. This is a big problem in solid-wall houses, and also a problem with big, cold, windows. The point about air movement is maybe more obvious, it’s the same effect as blowing on your soup to cool it down. But both the temperature of the surfaces and the air movement are major issues in Passive House construction, and as a result we are comfortable at slightly lower temperatures than we had expected. Maybe there’s also a lesson here for non-Passive houses (active houses?) – cheaper and better glazing has made big windows increasingly popular, but not only do they lose more heat relative to their area than the walls, the internal temperature is also lower. I’m beginning to think the normal standard for all new houses should be triple glazing – the extra cost is now quite modest, and the benefits are greater than might be expected.
Finally, a while back I touched on the embedded energy of construction. Traditionally, this wasn’t that big an issue, the operational energy needed to heat houses was much greater than the energy used to build them. But as we reduce the operational energy load, the construction energy becomes proportionally higher, and we felt there were probably things we could have done to use lower-energy materials. One example is the insulation materials – we used mainly mineral wool in the walls and roof, and polyisocyanurate (aka PIR, aka Celotex/ Kingspan) under the floor. These are all pretty good insulators, but some of the more natural products, especially those based on wood fibre or cellulose, have absorbed CO2 as they grew, so they help reduce greenhouse gases twice, once when they are growing and again by reducing heating needs.
So I was interested to hear about a Passive House in Fife which was built using Warmcell insulation in the walls and roof – see http://www.sescotland.co.uk/projects/passive-house-case.html. Warmcell is made from cellulose – mainly recycled paper, so it’s one of the materials that locks away CO2. The grade of Warmcell they used had a thermal conductivity of 0.04 Wm2/K, compared with 0.032 for our mineral wool – so I suspect it would only be realistic with a “blockier” shape of house than ours. Anyway, it just goes to show, there are always ways to do things better!