Sunday, 26 February 2012

Coping with solar gain

Windows open at the top and with external shades
One of the reasons I am struggling to find time for this blog is that I am working on a very interesting piece of research for the Departments of Community and Local Government on 'Thermal management materials and systems'. It's about high performance insulation and heat storage systems for buildings, both residential and commercial. I have discovered there is a huge range of applications for heat storage, over hours (taking advantage of off peak electricity tariffs or optimising use of combined heat and power systems), days (making better use of your solar hot water panels on sunny days) to months - storing heat in summer for use in winter. One that seems to be immediately relevant to a lot of people (including myself) and also quite controversial, is managing solar gain. The trouble is that having insulated and draught proofed your house so it is cosy in winter, there is a tendency for it to get too hot in summer. In fact I'm sitting here now in February with the window open because it's a sunny day. However, there is no need for this to put you off insulating your house because there are effective solutions available.

There are at least four ways to manage solar gain.
  1. Adding thermal mass
  2. Using ventilation
  3. Using shading
  4. Using reflective coatings
Thermal mass is one of the subjects of my research.  If your house has lots of thermal mass it can absorb a lot of heat without much change in temperature, and also it cools down slowly. Stone buildings have huge thermal mass but brick and concrete aren't bad either. High thermal mass usually means it takes time to warm the house from cold. However, there is a new way to add thermal mass without this problem which is to add phase change materials (PCM) to the building construction. PCMs absorb heat when they melt. For example, ice is an excellent phase change material - you can say that ice 'stores' coolness. This is why ice is useful for keeping things cold for a long time. The key thing about PCM is that it absorbs or releases heat at a particular temperature  - the melting point. You can have a PCM with melting point, say, 24°C. When the room gets warmer than 24°C the PCM will start to melt. When it gets cooler, it solidifies again. My research has uncovered a lot of development going on in PCM for managing solar gain,  especially for hot climates where night time temperatures are cool. However, PCM is expensive and it is not the only way to manage solar gain.

I have been told there is no problem at all with solar gain if you have enough insulation, because the heat cannot penetrate the walls. From my experience, this is true up to a point - but there is a great deal of solar gain through windows. My office is in the converted roof space and I sit almost directly under a Velux window - maybe this is my first mistake! However, a combination of the other measures makes my office quite comfortable 99% of the time.

You can have organic shading - it's called a tree. We use this technique for the ground floor windows. It drops its leaves for the winter and lets more light in, while providing pleasant green-tinged shade in the summer.

We don't have any trees tall enough to shade the roof windows, so we use external blinds instead. This cuts out a large fraction of the sun's heat while still letting plenty of light through. The blinds are controlled by a cord inside, you can see it in the picture above, hooked on the wall.

If you are having a south facing extension built, I suggest you consider wide over-hanging eaves. They let sun shine in when it is low and weak, but shade it out when it is high and strong.

Also, ventilation is critical. The key point here is that warm air rises so to get rid of warm air you need windows open in the ceiling, or at least above head height. Windows that open at the bottom are useless. Our new velux windows hinge in the middle so they can open at the top. The difference is amazing.

You can also get reflective film coatings for your windows, but this will also cut down on desirable solar gain in the winter and does change the colour of the window slightly.

So there you have it: three low-tech solutions to solar gain and one high-tech for the future, in case global climate change brings us more extreme summers.

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