Thermal mass: does it really save energy?

There is quite a significant body of opinion that holds that high thermal mass is one of the keys to low energy consumption. A house with high mass can absorb passive solar gain in winter and takes less energy to keep cool in summer. That is the theory. I think it’s questionable at best. Let’s examine the issues.

Firstly, what is meant by thermal mass? The mass bit refers to the heaviness or density of an object or a material. High mass building materials are concrete, brick, stone and tiles. In the low mass corner, we have not so much materials like timber and plasterboard, which are still relatively heavy, but the hollow, lightweight building systems such as timber or steel frame. Already, you can see that what we are lining up here is a re-run of the old timber frame versus brick and block argument.

The thermal bit refers to the capacity of a material to absorb heat. Broadly speaking, the heavier and denser an object or a material, the more heat is absorbs. A cubic metre of concrete can store around 80kWh of heat energy: in contrast, a cubic metre of air holds almost nothing. A whole house full of air, kept 20°C above external temperature, holds as little as 5kWh of heat energy. The structural fabric enclosing it holds anywhere between 50kWh — if it’s lightweight — up to 500kWh if it’s really heavyweight. Heavyweight doesn’t just mean masonry. Although all forms of masonry construction are heavier than framed techniques, concrete and dense blocks are much heavier than aerated concrete blocks, as made by Celcon and Thermalite.

Now, according to the theory of passive solar design, if you can capture lots of free solar energy (via large glazed walls or conservatories), you can store this heat inside the walls and floors of your heavyweight structure. Then during the night, instead of having to put the heating system on full whack, you can enjoy a free ride from the heat stored inside the structure.

But here is Problem No 1. You need lots of glazing to draw lots of heat during the day: at night time, this glazing will be leaking much of this stored heat back outside. In fact, even really good double glazed units leak six times more heat at night than walls or roofs. You can design this problem out by using insulated shutters, which cover the glazing during the night but, for many reasons, insulated shutters have never caught on and seem unlikely to do so. The fashion for large glazed areas doesn’t go with shutters or even heavy draped curtains. An awful lot of your passive solar gains will be given back through the glazing at night.


Problem No 2. In climates like the UK, you can’t get more than a proportion of your winter space heating from solar radiation. It’s often estimated to be in between 20% and 35% of the total space heating load. It’s difficult to increase this proportion because if you insulate the house massively, you reduce the overall heat load but, in so doing, you also reduce the useful contribution from solar gain. Why? Because a massively insulated house shortens the period for which you require space heating to the just the very coldest months of the year, precisely the time when passive solar has the least energy on offer.

Problem No 3 concerns winter holidays. If you were to go on holiday for a while and turn the heating off, then all the heat stored in your high mass building will leak away. This is something I have learned from bitter experience, having twice returned to a freezing cold house in the depths of winter to find that it takes 48 hours of continuous heating for it to become comfortable. And that’s in a house that has been built using heavy masonry materials only on the ground floor.

The phenomenon at work here is referred to as coolth. That is what happens when the surrounding surfaces are relatively cool and you yourself radiate a lot of your own body heat out towards them. This makes you feel colder than the air temperature suggests. As the surfaces warm up, you radiate less, which in turn makes you feel warmer.

In winter, coolth is bad news and takes a lot of energy to eliminate. But in summer, it’s a very different story. The physics at work in summer is no different to what happens in winter – you feel cooler than the air temperature because you are radiating large amounts of heat towards a cool surface. Because of this, the high massers contend that you need a lot less air conditioning in a high mass house. Not that we use a lot of air conditioning in the UK yet, but global warming is expected to change all that very soon. In the USA, the summer cooling demand is almost as large as that for winter heating so an ability to cope with this energy load is likely to become a significant part of our future fuel bills.

However, there is a snag here as well. Call it Problem No 4. During a prolonged hot spell, the structure eventually achieves equilibrium with the surrounding air temperature. Consequently, your body stops radiating heat away and the coolth effect vanishes. A massive structure will still tend to even out the difference between day and night time temperatures, this is true, but this means that whilst you will feel marginally cooler in the day time, you will feel just a little bit hotter during the night. Now whilst this is not a problem in offices and schools, it is with housing, an awful lot of which is empty during the day. Now, the high massers argue that air conditioning is less likely to be turned on in a heavy house because of the coolth effect, but methinks they are exaggerating the effect of the phenomenon. There is no coolth to be had in the middle of the night during a heatwave.

So, as regards low energy building strategies, I believe thermal mass is a classic case of the curate’s egg — i.e. it’s good in parts. It’s probably seen at its best in places which are occupied mostly during the daylight hours — schools, offices, workshops. It can be a useful technique to employ with buildings in constant use such as hospitals and rest homes and, indeed, some housing. But to get any benefit, these homes must be occupied throughout the year and throughout the day. Or, as we have learned to say, 24/7/365. If the occupation is going to be intermittent then there is every chance that high mass can end up being an energy drain.