What is Thermal Mass

Thermal mass is the capacity a material has to absorb, store and release heat, it is directly linked to the "specific heat capacity" of the material and its size and density.

This is not to be confused with insulation values, for example, a slab of concrete can be of high thermal mass but only provide a very average to low insulation value, whereas gram for gram, mineral fiber insulation the opposite would be true. So, How can thermal mass be utilised when heating your home?

Thermal mass can be used effectively to store excess heat in times of peak demand from sources of heat such as solar gain, machinery or even body heat to smooth the temperature spikes and troughs within a property, but is only efficient and effective in the correct circumstances.

Because high thermal mass buildings store more heat, they result in both a longer heat-up time and in turn, a longer cooldown due to the heat being released back into the cooling property, this is because when used effectively its slows the properties ‘thermal inertia’.

Thermal inertia is the speed at which a body of mass equalises temperature with its surroundings and is dependent on insulation (u value) of the outer surfaces, surface area, and thermal mass (y value) among other factors. This ‘low thermal inertia’ gives us the benefit of keeping cooler in summer whilst effectively storing heat to release at night time.

However, if your household is the type where you're barely home and only need a small amount of heat in the morning and a little in the evening the higher thermal mass will result in longer warm-up times and/or higher temperatures required from the boiler.

What Controls Best Suit High Thermal Mass Properties?

Because of the way high thermal mass properties work they are much more suited to weather-compensated control with a gentle warm-up and cool-down and little fluctuation in temperature during the day. This ensures minimal waste and no inefficient, overly high boiler temperatures.

However, a balance has to be drawn between the thermal mass of the building and its usage pattern, if the occupancy is low (either not used often or not used by many people) or the pattern of use is irregular a method using an internal reference may have to be used for a faster response and more instantaneous heat up. This is an inefficient approach, however, the intermittent use and erratic temperature swings of a building are inefficient in their own right, especially in buildings of high thermal mass.

On the other hand, if you have a property that has very good insulation yet low thermal mass control, utilising internal reference would be more suited, as this will counteract internal influences such as body heat or appliances, and weather-compensated control is less desirable. Usage and occupancy should again be considered when selecting control.

Emitters

Underfloor heating is a type of heating that, although extremely efficient, is also very slow reacting, as the floor itself is used as a thermal store. This makes it a perfect accompaniment for high thermal mass buildings. If the building is going to be used intermittently a different emitter type should most definitely be used, preferably ones that warm the air directly and don't use as much radiant heat, such as standard radiators or warm air heating.

In buildings of lower thermal mass, underfloor heating is less desirable as it is not quick enough to respond to changes in demand. This, unfortunately, seems to be the norm in today's new builds. Worth noting that these are simply ideals and no hard and fast rule, but again, the lower the system volume the faster the heat up and, in turn, responsiveness.

Using Thermal Mass To Your Advantage

The thermal mass of a property is usually something that is inherited, however, it can be slightly added to by the likes of hardwood furnishings and floor coverings. When selecting a control strategy and accounting for thermal mass, you should be aware that the first 50mm of the walls is the most effective, and after 100mm, the effects go beyond diminishing returns. It should also be in direct contact with the internal space and not separated by insulation.

It is common practice in passive design to use tiled floors and shades over windows externally. This has the effect of warming the floor during winter months when the sun sits lower in the skyline; this will slowly be released into the evening. Yet this shades the floor from solar gain during the summer when the sun sits higher, allowing buffer storage for heat from other sources and keeping the property cool.

This can be utilised effectively with weather-compensated underfloor heating as the circulating water can spread collected heat around the house.

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