Thermal Mass

We talked earlier about reverse brick veneer, high and low ‘R’ (Resistance to heat flow) values and the need for a well insulated external envelope to our building.

We want to control the heat coming into and out of our homes with high R value materials but there is much useful gain to be had from low thermal resistance materials inside the building.

A low R value material that lets heat in and out quickly will easily absorb the heat from the sun coming into our homes in the day and progressively heat up during the daylight hours. This absorption draws heat out of direct sunshine – on the floor – or the air in the room (which is a really good thing in the summer) and stores that heat till later. Bearing in mind that heat always flows from hot things to cold things, once the sun goes down and the temperature starts to drop in the room, heat will flow back out of the thermal mass into the room to keep it warm overnight. Fantastic! But there are some provisos! The heat will only flow back out of the mass if it has got to a higher temperature than the room drops to overnight. Now you can guess that there will be a correlation between the amount of heat gain and the amount of mass it has to heat up. If we take a reasonable autumn day and expect the sun to heat up St Georges Cathedral you can see it’s not going to get hot enough to be above the night time temperature you would like to achieve and will therefore just keep on absorbing heat when you want some back.

So too much mass is a definite impediment.  Too little heat will be absorbed and stored to get the mass hot enough for re-radiating over night.

Much the same happens in summer. Here the mass absorbs the heat in the house during the day helping to keep the temperature down and that same heat is flushed out overnight by allowing cool air to enter the house at low level and exit the house at high level via natural stack ventilation (hot air rising) through clerestory windows. The lower temperature then stored in the mass is referred to as coolth!

However too much mass in summer, whilst continuing to absorb heat in the day due to its greater heat holding capacity, will eventually reach a stable point higher than the amount that can be flushed overnight. This gives the same result as the heat islands that form in our cities and urban corridors where the density of high thermal mass is so great that the heat cannot be dissipated quickly enough to begin the cycle fresh again for the next day.

thermal mass

It is generally considered that no more than 100mm thickness of thermal mass should be created in any one panel as the solar radiation required to lift it’s temperature will be greater than that achievable over the daylight hours. It is this amount that can heat and cool effectively across the single day and night cycle.

An interesting potential benefit to using solid thermal mass is that the heat within it moves by conduction rather than convection. This means that, unlike the upward movement of heat in fluids and gasses by convection, the heat in solids moves by conduction and can move in any direction, including downwards – the opposite to warm convection currents. The heat stratified in air at high level can warm vertical panels of thermal mass (Walls) and transfer the heat down towards the ground and into the floor slab to repeat the cycle and increase the surface area available to flush the days heat gain during the cool hours of darkness.

We have seen that light weight materials generally have little thermal mass and that heavier i.e. denser materials have greater thermal mass. Solid masonry is clearly denser than any form of framed structure and concrete is denser than brick. The concrete in a solid floor is a very good thermal; store and is improved by earth coupling on solid ground further evening out the day/night temperature swings. Holding over 2,000 kj/m³/deg.k, the heat capacity of concrete is good in comparison with brick @ 1,360 but is poor when compared to water which has a volumetric capacity of almost 4,200. So for a framed structure on a steep slope where high mass is difficult to support, water can be a very effective solution – especially when its ability to be easily moved around is brought into the equation.

See more on liquid thermal mass.