26 May 2015

Are Eco Homes prone to Overheating?

Earlier this month, stories appeared in the Sunday Times and the Daily Mail about overheating in eco-homes. In particular, Passivhaus was mentioned.

Why exactly should an eco home overheat anymore than any other home? What the stories implied was that because these houses were so well insulated, they wouldn't be able to cool down in summer. Heavy insulation used to keep the buildings warm in winter also traps heat in summer — potentially putting vulnerable residents at risk warns Mail Online. A study at Coventry University found that Passivhaus flats built by Orbit housing association were overheating too often, although the definition of overheating seemed to be set pretty low at 25°C. Lots of people pay good money to go places where the temperatures never drop below 25°C.

But back to the point. Is there something special about eco-homes (and Passivhaus homes in particular) that causes them to overheat? I don't think so. In fact, the Passivhaus standard is rather unique in setting comfort standards for overheating. 25°C for more than 10% of the time is regarded as a Passivhaus failure but I don't know of any other housing standard where such a result would be deemed to fail. Indeed, the existence of a Passivhaus overheating hurdle was the very reason why this study was being undertaken in the first place.

It's difficult for any home to enjoy temperatures lower than the external air temperatures without introducing air conditioning. Without this, the best method of controlling summer overheating is to open lots of windows and cross-ventilate, so that there is a breeze running through the building. This is the standard house cooling method used in hot countries and there is no reason why it can't be employed in eco homes and Passivhauses. Passivhauses also benefit from having mechanical ventilation systems which, used correctly in heat waves, will contribute to night-time cooling. On its own, mechanical ventilation may not be adequate to dissipate all the heat build up but it's not designed to do this. Passivhauses also have windows!

If the building itself is to contribute to the problem, it won't be the insulation that causes the problem but the thermal mass of the structure. If a house is built largely of concrete or brick, these materials will gradually absorb background heat during a prolonged heat wave and radiate this heat back into the house during the relatively cool night time, precisely when the residents don't want it. Low mass materials like timber and insulation don't absorb significant amounts of heat and are therefore not going to contribute to night time overheating. Admittedly, they will also stop heat escaping from the structure at night but set against this is the fact that they will also stop heat absorption during the hottest part of the day. These two effects cancel each other out.

If there is a problem with overheating homes in the UK, it will almost certainly be caused by a combination of having large, unshaded, south-facing glazing (resulting in the conservatory effect of having spaces which are rendered almost uninhabitable during the day time), coupled with inadequate ventilation — probably caused by having too few opening windows. These are design issues which can affect any building during heat waves, regardless of building standards and insulation levels. The editorial decision by two right-wing papers to big this story up as an "Eco House Problem" is unwarranted and probably mendacious.


  1. I've actually wondered what the issues would be with 'perfect' insulation as even though we say that we need and want as little to escape the house we do actually need it as a vent.

  2. They will tend to overheat more than conventional houses. They are designed to keep heat in. Opening windows will only remove heat if there is sufficient air flow. Surface temperatures will also be higher increasing the operative temperature. I see no right-wing conspiracy here, on this occasion, though the papers' motivations may not be so pure.

    1. Strictly speaking Chris, the insulation is to stop heat passing through it in either direction. Remember fridges are insulated too. So there is no reason why the inside of an echouse in summer should be any hotter than the inside of any other house. You may be right about the right-wing conspiracy theory - but I bet the Mail and the Times instinctively prefer ordinary houses to eco homes.

  3. Insulated houses will be warmer internally ... this saves putting on the space heating in summer. Some old houses in this 'cool spell' have been seen to be emitting smoke from the chimney (I'm serious).

    The adverse comment about large unshaded south windows is too alarmist. I live in such a house, although it does have a high thermal capacity. This summer, it hasn't yet risen much above 22.5 degC internally downstairs and 23.3 degC upstairs.

  4. I have just completed reading the passive house designers manual. An interesting and revealing read: [1] No air circulation pump currently exists to cool a passive house if it becomes too warm in summer so windows are the way forward so bang goes the control around thermal comfort. [2] Flats with centralised heating and designed to meet building regulation were prone to becoming too hot if no a/c is fitted. From experience I have sat in sweltering heat in London in poorly designed flats like this. My conclusion is, after reading the final section of the book about returns on investing that I will be looking to vent first insulate second to cope with winter just close a few of the vents. Excuse my simple explanation.