Thursday, 25 May 2017

Do modern homes overheat more than older ones?

Modern homes have a reputation for overheating more than old ones partly because they tend to be more airtight and partly because they are often more lightweight constructions that heat up quickly. However, overheating is not an inevitable consequence. For example features such as green roofs and external window shades can make a huge difference – but apparently ‘people are resistant to changes to the aesthetic of the homes and other buildings they occupy’ and these features ‘may be resisted by house buyers’ – so builders won’t build them [1]. Is it all our own fault then?




English style flat frontage in Chichester

Italian style deep set windows with external shading in Venice

Modern Cambridge home with external louvres above and deep shading on the ground floor
from Why are we still designing buildings for yesterday's climate?.


Winter excess deaths are many times summer excess deaths
Historically, we haven't had to worry about keeping cool so much as keeping warm, and it is still the case that excess winter deaths due to cold far exceed excess summer deaths due to heat. (43,900 excess winter deaths in 2015/2015 verses 2000 excess deaths in the summer heatwave of 2003 [1]). Climate change will even out these numbers to some extent, with less deaths due to cold and a lot more due to heatwaves. However our building regulations are still much more relaxed on overheating than keeping warm. They do require sufficient background ventilation to keep the air fresh, but not necessarily cool.

In any case, ventilation does not help to keep you cool unless the air outside is cooler than inside. When it gets really hot, you need alternative strategies - such as keeping windows shaded to prevent more heating and circulating fans to help you sweat.

Overheating caused by design flaws, cost cutting and residents' misconceptions
This post is based on some articles from a special issue of Building Research and Information on overheating. My favourite was about some retrofitted flats in northern England, because it demonstrates most of the issues in one case study. Overheating was caused by a combination of design flaws, last minute design changes to cut costs, and some misconceptions on the part of the residents. Some residents managed to keep their flats cool much more effectively than others.

To start with, the flats had windows on only one side so natural cross ventilation was impossible. The flats had extract fans which were supposed to be run all the time on low. However, most residents turned them off either because they found them too noisy, or because they thought they used too much electricity. (In one case a resident bought a portable air conditioning unit and ran that instead, even though it used a hundred times more power than the fan they considered too expensive).

There were windows that could be opened, but as is often the case there were obstacles to this strategy. Near ground level the problems are usually street noise and/or security issues but these flats were ten stories high and the main issue was safety. To stop people falling out there were restrictors on each window to limit the opening to 10cm. Plus, even with the restrictor the residents were not supposed to leave the windows open when they went out in case of high wind which could break the fixing; if it was very windy they had to close the windows completely. In practice it is rarely very windy and hot at the same time, so some people left the windows open when they went out even though this was against the rules.

Finally, some of the windows had blinds and trickle vents but not all, because of last minute cost cutting design changes. Even when the windows did have blinds some people did not use them because they preferred to have the extra light.

All of these problems have straightforward fixes
All of these problems have straightforward fixes. Venetian blinds are one type that allow control of the light/heat balance. If every window had adjustable blinds and more sturdy opening fixtures, if the extract fans were quieter and the residents advised properly on costs, everyone would have been more comfortable. Air tightness is not in itself a problem, as long as the rest of the design is right and the residents are savvy about how to use their ventilation and shading systems.

It takes a while for a background extractor to take effect
The importance of the extractor is quite hard for residents to learn for themselves - you just have to know or be told. The trouble is that it takes a while for a background extractor to take effect so it isn't very obvious that it is achieving anything at all. You can feel the breeze if you put it on boost but then it certainly would be too noisy. You get a similar problem with heat pumps or underfloor heating, which provide gentle, slow warmth quite unlike the immediate heat of a high temperature radiator or an electric fire.

We can learn to take vitamin pills, so why not leave the fan on?
However we are capable of learning to do things that aren't obviously and immediately good for us. I am thinking of the number of my friends and relatives who take vitamin pills every day to help prevent colds or other illnesses. A 10 W fan running continuously costs about 3p per day in electricity - less than most vitamin pills (or an extra serving of fresh vegetables). Of course whether you believe that the vitamin pills or the extract fan is good for you is another matter. My friends who take vitamin pills are firmly convinced of the benefit. Presumably this depends on how much you trust the source of your information.

Let's not reject deep eaves and external shades because they are un-English
Here in the UK until recently the building designs and practices for keeping cool in summer have been mostly irrelevant. We regard high ceilings (with potential for ceiling fans), deep eaves, external shutters and whitewashed walls as exotic and not very English. However, if I was buying a house now that is exactly what I would like to see. And even if we don't get these, we can at least have functioning ventilation systems and internal blinds. I want to be warm in winter and cool in summer too.



[1] Kevin J. Lomas & Stephen M. Porritt (2017) Overheating in
buildings: lessons from research, Building Research & Information, 45:1-2, DOI:
10.1080/09613218.2017.1256136
[2] Magdalena Baborska-Narożny, Fionn Stevenson, and Magdalena Grudzińska (2017) Overheating in retrofitted flats: occupant practices, learning and interventions, Building Research & Information 45:1-2 DOI: 10.1080/09613218.2016.1226671

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