Mark Brinkley on Thermal Mass

Are you familiar with the phrase ‘thermal mass’? It refers to the capacity of a material to store heat, and there is a debate around its importance which crops up every time low-energy building techniques get discussed. Thermal mass has its advocates who claim it helps keep homes warm in winter and cool in summer, but in order to do so you need lots of it. Lots of brick, lots of concrete.

Can you see where this is going? Masonry homes have thermal mass; lightweight timber frame ones don’t. Unsurprisingly, the thermal mass advocates tend to also like masonry construction — and so the thermal mass argument has opened up yet another front in the age-old debate between brick and block boys and timber framers.

So what is the case for thermal mass? As already mentioned, it boils down to two factors. Firstly, it helps keep homes warm in winter, and secondly, it also keeps them cool in summer. The question is, is this really true? Let’s look at winter first.

A heavyweight, masonry house can store enough heat in its structure to keep a house comfortable for several days. This can be both a plus and a minus, depending on how you live in the house. If you are there all the time, throughout the winter, you will enjoy glowing in the warmth around you. You won’t experience rapid temperature fluctuations and you will get a bit of free heat from the winter sunshine as it gets absorbed by all that thermal mass.

On the other hand, if you live a lifestyle where you are out during the day, then you really don’t gain very much by having a warmish house in the afternoon. And if you go away for a few days for a mid-winter break, it takes an age for the house to warm up again. The yuppie lifestyle fits best with a lightweight house and a rapid heat-delivery system. In many ways, the argument is very similar to the pros and cons of underfloor heating: it can help heat a home efficiently, but only if you are there to enjoy it.

What about summer cooling? Because much of the daytime solar radiation is absorbed by the thermal mass, temperatures stay noticeably lower than they do in a lightweight structure. The high massers like to point out that, in Mediterranean climates, almost all housing is built from masonry materials.

Whether this is to keep cool or simply because they haven’t got any trees isn’t immediately clear to me, but type ‘Arup Thermal Mass’ into Google and you can read for yourself the results of the consulting engineer’s research into this issue. Here is the nub of their argument: “The lightweight home (in a warming UK climate) was found to need air conditioning by the year 2021. This compared to 2061 for the medium-heavy and heavyweight homes.”

What tends to get glossed over is what happens in the middle of the night when the air temperature has dropped. If you’ve been living through a prolonged hot spell, the high-mass walls and floors will be hotter than the night-time air temperature and you’ll be reaching out for the air conditioning button in no time. So whilst the summer cooling argument holds good in the daytime, which makes it a no-brainer for schools and offices, in homes it’s not quite such a clear-cut winner.

As you can see, the whole subject of thermal mass is rife with claim and counterclaim and we can expect to hear more about it as the debate about how we should be building homes intensifies. My feeling is that for every household that saves energy by having a highmass home, there is another that loses just as much.

Mark Brinkley is an experienced builder and author of The Housebuilder’s Bible

Further Reading:

• A Stunning Home on a Sloping Site
• An Upside Down Eco House