I've been banging on about low energy homes on this blog for years, and promoting the concept to all who come here. Whether it's the full blown PassivHaus standard, or a less taxing version with maybe SIPS, MVHR and a wood burning stove, it seems to me to be a no-brainer.
But sometimes it's good to pause and reflect. And it has to be admitted that building a low energy house is not always as plain sailing as advocates would like to admit. They don't all work as planned, and sometimes it's hard to put a finger on exactly what has gone wrong.
Yesterday I took a phone call from a selfbuilder in Oxfordshire who has just built a massively insulated, near airtight timber frame house which is heated by underfloor heating on the ground floor (powered by a heat pump), a wood burning stove in the living room and an MVHR ventilation system to distribute the heat around the house. All indications were that the house was well built and it had achieved an airtightness score of 1.7a/c/hr@50p (which is extremely good). Yet whilst the heated downstairs of the house was basking in temperatures in the 20°Cs, the upstairs rooms were stuck at 14°C. All that acoustic insulation put in the floors to meet Part E of the building regs was stopping the heat rising into the bedrooms, and the MVHR system simply wasn't redistributing the heat as had been hoped for. The ventilation system even had a 1kW post-heater installed in the inlet ducting, and yet still the upstairs was still freezing.
And that wasn't all. The wood burning stove was also causing problems. Despite having an airflow ducted directly to the stove, it wasn't burning efficiently and was smoking through the vent hole. And when the door was opened to re-fuel, large amounts of smoke were escaping into the living room - the so-called spillage problem already discussed on this blog. And the heat generated from the wood stove wasn't finding it's way into the bedroom upstairs.
This wasn't as it was planned. The finger of suspicion would seem to point at the commissioning of the MVHR ventilation, and the suspicion must be that the system hasn't been balanced correctly. But the installers were adamant that it wasn't their fault and were proving difficult to deal with. "We told you it wasn't a heating system, " they said, "you are expecting far too much from it." Which leaves the selfbuilders here in a bit of a hole, because what they have designed is in effect quite a complex, interactive building which depends on several different elements working well with one another. And when the outcome doesn't meet expectations, then it's damn difficult to know what to do about it.
The fallback is always to install a central heating system throughout the house. But a lot of low energy designs sell themselves on the fact that they will cope with the severest of conditions without recourse to conventional heating, and it's difficult not to be tempted by this logic. I've spoken to many selfbuilders who have done away with conventional heating systems and been delighted by the performance of their homes - incidentally the Denby Dale PassivHaus is just such a case, performing really well during this prolonged cold snap - but it's good to bear in mind that not everyone who tries this route is delighted with the results, and that if things don't work out as expected, then it leaves the client in a hole.