Thermal Mass or Lightweight Build?

Timber framed houses are regarded as being low in thermal mass
Stone houses are thought of as high in thermal mass

Thermal mass is a simple idea but is actually complex in nature. The general idea runs thus:

Lots of high density materials (like stone, brick, concrete) can even out the temperature in buildings as they are able to absorb high daytime temperatures and then slowly release this stored heat back into the building during the cooler evenings. So buildings, made of these types of materials, will naturally and automatically create more constant diurnal temperatures for the occupants during the summer. In the winter these same materials will require heating from an external source (central heating for example). It will take them longer to heat up, but once warm they will remain warm and again help to regulate the heat through a 24 hour cycle.

Lots of low density materials will not have the capacity to ‘store’ excess heat and so light weight buildings tend to heat up and cool down quicker. So in the summer they cannot absorb the suns heat as well and so they are not so warm in the night time. In winter they will heat up quicker, but again cool quicker.

This all sounds quite straight forward. The larger the amount of heavy heat absorbing material the more constant the temperature in a house.

So in theory if you are planning to stay in a house for most of the time (working from home, retired, etc) then building or buying a thermal mass based house could be a good thing. If however you are out of the house for most of the day and only use the house in the evenings, then a light weight house seems more appropriate.

However there are some complicating factors, most notably:

Where is the thermal mass? – Does it get the passive solar gain from the sun, or is it shaded

Is the habitable area in touch with the thermal mass? – New buildings might be made of brick, but this might be separated from the internal environment by layers of insulation, so effectively the only internal mass might be some plasterboard.

How airtight is the building? A low thermal mass house might be very airtight and so able to keep some heat in, whereas a high thermal mass house might be very draughty and so lose much of its gained heat because of this.

How well insulated is the building? Again high thermal mass that is made from a good conductor of heat can take more heating in the winter due to a lack of insulation.

The picture is therefore more complicated. Life, eh!

When buying a house you don’t have a choice over what it is made from. But it is worth bearing in mind that older pre-1919 solid walled homes will have a higher thermal mass than a modern house. This is borne out by experience of many where older houses are cool in the summer and require constant trickle heat in the winter, whilst modern homes can be too hot in the summer and only require short periods of heating in the winter.

There are therefore a series of advantages and disadvantages to both systems. But could we create a house that has just the advantages?

In the UK we almost make our new homes inside out with regards to thermal mass. We tend to put the thermal mass on the outside (bricks) then insulate it from the living space and then line out our homes with a thin thermal mass product like plasterboard. We are also generally very poor at making airtight homes and also at managing ventilation. So imagine a house that has a light weight external wall, lots of insulation and solid brick or block internal walls. High thermal mass floors located in sun areas can also act as thermal collectors of heat, whilst those that are not could be lighter weight, well insulated and airtight. Large windows to the south will allow the sun in to heat the thermal mass and small windows to the north (that only allow heat to escape) can be smaller and better insulated (triple glazed).

Passiv Haus houses are good examples of where these types of ideas are used. They are also very airtight and use heat exchangers to minimise heat loss through ventilation. All very good. There are a few issues with them though that require a change in living practices in the UK – all rooms the same temperature, using the ventilation system for fresh air in the winter rather than opening windows etc.

So if designing a new home, it is worth thinking about what materials you use, where you use them, why you are using them and how to use them.

If you refurbishing an older house then be mindful to keep the thermal mass working for you. Higher thermal mass insulation like woodwool can be useful, but be careful not to isolate the thermal mass behind structures like insulated dry lining as can lead to a host of other problems. Never easy is it!