Thermal Mass in Houses – A Lesson from the Past

It has often been said that primitive architecture is an exhibition of man’s intelligent behaviour in adverse and difficult circumstances.  As prehistoric man developed and began to seek places for more permanent shelter, he frequently chose caves as the best places to settle.  The exposed mass of the floors of the caves helped to stabilise the internal temperature of the dwellings, so in making this choice they discovered some of the advantages of thermal mass.  One might even go as far as to say that these ‘cavemen’ became (very!) early pioneers of the environmental movement!  So what can we learn from this today?

Basic Principles

The term ‘thermal capacity’ (or specific heat capacity) refers to the amount of energy required to raise the temperature of a set amount of a material by a certain temperature interval.  So, to heat up a material with a high thermal capacity (like concrete) by a certain amount requires far more energy than if the material had a low thermal capacity (such as copper).  However, to make this more relevant to buildings (and to complicate things further!) we must take into account how much of each material is being used.  This is where our phrase ‘thermal mass’ comes into play.  This refers to a whole object, rather than a general material property, therefore is much more useful for us.  So, you and I both have a thermal mass, as does a ten-storey housing block, and clearly it will take more energy to heat up the housing block than it will to heat us up!

How is this all Relevant to Ecological Housing Design?

So, finally we’re getting around to the issue at hand.  With this knowledge of thermal mass, we can now see that a light timber-framed wall in a house will heat up (and cool down) a lot quicker and easier than a solid poured concrete wall would – in other words the concrete wall is said to have a greater thermal mass than the wooden framed one.  As our friends in the caves discovered, a building or dwelling with greater thermal mass has a more stable internal temperature.

The ability to reduce extreme peaks and troughs in the internal temperature of a home is a precious one, especially in the variable seasonal climates of Europe.  Let’s take a look at how thermal mass can be used at different times of the year.

Thermal Mass at Work – Winter

In a well considered, environmentally designed home one would expect to find areas of high thermal mass placed strategically throughout the building.  The diagram below shows how large South-facing openings can take advantage of low-angled winter sun, using it to slowly heat the fabric of the building.  After a significant time delay and once the sun has set, this stored energy is then slowly released, providing warmth for the internal spaces of the structure throughout the night.  Heavy insulation on the exterior faces of the thermal mass ensures that the majority of this released heat goes to the interior of the building, rather than escaping to the outside world.

Thermal Mass at Work – Summer

With sufficient shading on its South-facing openings, thermal mass can be equally as effective a tool during the hot summer months.  As the diagram below shows, the shading blocks direct exposure of the mass to the energy of the sun’s rays.  Most of the rest of the energy generated within the building comes from its occupants and their machinery (we are constantly giving out heat, as are our computers, televisions appliances etc.).  This, and any heat from outside the building, can be largely absorbed by our thermal mass.

How Can I Benefit from this?

So, we can now see that using the same principles applied to reducing troughs in winter temperatures, we can help to avoid extreme peaks in summer interior temperatures.  The main benefit to be derived from this is the reduced requirement for artificial heating and cooling systems, which are big energy consumers and therefore bad for your pocket and the environment.

Perhaps these ‘cavemen’ weren’t so primitive after all!  If you’re planning a new build project, a house extension or even a renovation or refurbishment, it might be worth keeping the ‘thermal mass’ phrase in mind – something which saves you money and makes you more comfortable can’t be ignored!

This article was written by Simon Thorpe. Simon Thorpe is a third year architecture undergraduate at Magdalene Cambridge. At the time of writing he is on an internship with Eco Vida. simoncthorpe@hotmail.co.uk