Talking to people about low-energy buildings, I’m often struck by how the conversation frequently swings to thermal mass as a way to reduce heating demand. To my mind this focus in the general public’s perception is out of all proportion to the ability of thermal mass to reduce heating demand; the key performance indicator for low-energy housing in the UK. I hope this blog post might go a little way to correcting that by bringing a bit of clarity to some often-misunderstood building physics.

Mass doesn't 'trap heat', it resists temperature change

When heat is added or taken away from a substance, that substance warms up (its temperature rises) or cools down (its temperature falls). How much the temperature changes for a given amount of heat added depends on the specific heat capacity and the mass of the substance. Put a pan of water on a hob for twenty seconds and it only warms a little, heat the same pan for the same length of time on the same hob with just air in (with the lid on, we’re interested in efficiency after all!) and the temperature will rise a lot more. Both the density (and thus the mass in the pan) and the specific heat capacity of water are higher, so the same amount of heat energy added to it causes its temperature to rise less than for air. We can say that water is more ‘thermally massive’ than air. Similarly a litre of warm water will cool slower than a litre of air starting at the same temperature.

The rate at which heat flows through walls, floors and roofs is determined by their U value, and taking this equation apart is informative. The U value describes the heat flow in Watts per square metre per degree Kelvin (or Celcius) difference between the internal and external temperature (W/m²-K). There is no mention of mass in this equation. For a wall with a U value of 0.10 W/m²-K (typical of a Passive House standard building in Scotland), if the temperature difference between inside and outside is 20°C, then heat loss through the walls will be 2 Watts per square metre of wall. A similar formula describes the heat lost through replacing the air in a building with fresh air. This depends on the temperature difference between the fresh and stale air and the rate of change. In both cases the thermal mass of the building makes no difference to this rate of heat loss.

If two buildings have the same U values and air-change rates but one is thermally massive (it has a lot of thermal mass within the insulated envelope of the building) and one is thermally lightweight, then the rate at which the temperature in the building changes if heat losses are not balanced by heat gains will be different (the thermally massive building will change temperature more slowly). For a given amount of heat loss or gain, the temperature of the high-mass building will change less than that of the low-mass building. Conversely it will take much more heat energy to warm a high-mass building up by the same amount as a low-mass building. So high-mass buildings don’t hold on to heat, but they do hold on to temperature.

To understand the impact this has on the heating demand of buildings let’s do a little thought experiment on two imaginary buildings to understand how thermal mass affects the heat demand in different scenarios. One building has high thermal mass and one low thermal mass, but with identical construction of the walls, roofs, floors (let’s say all the additional thermal mass is in intermediate floors and internal partitions), identical windows, shading, airtightness, MVHR, etc.

Scenario one - heat losses higher than gains, heating on

In the first scenario it is cold and cloudy, heat loss from the building exceeds heat gains from solar gains, occupants, appliances and hot water systems and both houses are being kept at the same constant internal temperature by the heating system. The heating required to keep a constant temperature will be determined by the rate at which the buildings lose heat, which will be determined by the U values and air change rates, both of which are identical for both buildings. In this first scenario the thermal mass makes no difference to the heat demand of the building.

Scenario two - heat losses higher than gains, heating off

In the second scenario the heating is off (let’s say the owners are away for the weekend) and the weather is, again, cold (0°C) and overcast. Even though the building is well insulated and airtight it is losing more heat than it is gaining from the sun and appliances (fridges, etc. running even when occupants are out). Both of our buildings start this second scenario at the same temperature of 20°C.

Because the internal temperatures are the same, and the U values and air change rates are the same, the two buildings lose heat at the same rate at the start of this period. However, the lightweight building will change temperature more quickly than the heavyweight building. Let’s say that after 24 hours the internal temperature of the heavyweight building has dropped to 19°C, while the lightweight building has dropped to 18°C. Because they are now at different temperatures, they are no longer losing heat at the same rate. Our heavyweight building is now losing heat at a rate of 19°C x 0.1 W/m²-K, or 1.9 W for every square metre of wall. Our lightweight building is now losing heat slightly slower, at 1.8 W/m². Similar differences will be present for the roof, floor, ventilation, infiltration etc. The longer this situation of heat loss exceeding heat gains persists, the greater the difference between the heat loss rates of the two buildings. Thus, after a short period of cold weather with the heating off, our lightweight building will be colder, but it will have lost less heat energy than our heavyweight building. It will therefore take less heat energy to warm it back up to a comfortable temperature. In this case thermal mass kept the heavyweight building warmer (often misunderstood as reducing heat loss) but actually increased heat loss and subsequent heating demand. In this second scenario the additional thermal mass has led to an increase in heating demand for the building.

Buildings built to very stringent thermal standards, such as Passive Houses, lose heat so slowly (regardless of how thermally massive they are) that they can be kept at comfortable temperatures all winter, 24 hours a day. Turning the heating off for a few hours a day doesn’t save much energy in buildings that lose heat so slowly, and having a consistent, small input of heat allows for smaller, simpler heating systems, and potentially more efficient operation of those systems. In buildings operated in this way the situation outlined in scenario two should be rare.

In a building that is never allowed to drop below comfortable temperatures, can adding thermal mass reduce heating demand? If so how, and by how much?

Thermal mass can reduce heating demand. But if mass doesn’t trap heat, how does it reduce heating demand? For scenario three let’s take a look at the way in which it does this.

Scenario three - heat gains higher than heat losses

Counter-intuitively, mass reduces heating demand by resisting temperature rise when gains are high. In a super-efficient house, on a sunny winter’s day, or when lots of people come to visit (each person adding about 100 W of heat) heat gains will be higher than heat losses and the temperature will rise above the heating setpoint (and the heating will switch off). For the same amount of net heat gains, a lightweight building will get warmer than an otherwise identical heavyweight one. Because it is warmer the difference between temperatures inside and outside will be greater and thus the rate of heat loss from the building will be higher. In an extreme case (maybe a really good party!) the lightweight building might get so warm that the occupants open the windows to maintain comfortable conditions, leading to even higher losses, whereas the heavyweight building can maintain comfort without opening windows (although with all those people it might be a good idea for air quality, the ventilation system in a Passive House is only sized for typical occupancy, or a bit more on ‘boost’). In this situation the heavyweight building has lost less of its gained heat, and will stay above the heating setpoint for longer once the sun has gone in/everyone has gone home.  In this situation the thermal mass has reduced heat demand.

So we've identified one way in which thermal mass can reduce heating demand, but how much difference does this make over the course of the year? Taking a sample of the five PHPP models I have for projects that are currently on site and which are all lightweight buildings, making them 'thermally massive' in PHPP reduces the annual heat demand by just 0.9 kWh/m². For a 100m² home that's a saving of just over five pounds a year on your heating bill (assuming a heat pump with a COP of 2.5 and an electricity cost of 15p/kWh). Small beer. "Ah", I hear you say, "but PHPP can't accurately take into account thermal mass because it is a static simulation". This is true, the effect of thermal mass in PHPP (as I understand it) is added in as an approximation gained from testing on dynamic simulations. But in my experience, when I've modelled buildings in both PHPP and dynamic tools (such as I did for this paper) if anything PHPP predicts slightly higher savings through thermal mass than dynamic simulation does for this part of the world.

What about inter-seasonal storage?

People get very excited about the idea that we can store some of the heat from summer and use it to ride out some of the cold of the autumn and winter. You can look into this question in a lot of detail and try and calculate how much mass would be needed, how much it would reduce heating demand and so on, but I think a simpler and quicker way to think about it is to flip this around: we know that it is possible to build lightweight buildings that require no more than 15 kWh/m² of heating each year (this is the Passivhaus standard and many lightweight Passivhaus buildings are in existence). Heated with a heat pump with a coefficient of performance of 2.5, and a UK grid carbon intensity of 200 gCO₂/kWh (higher than the rolling average for a year in the UK, see here) this equates to just 120 kg of CO₂ per year for a 100 m² house. In reality, since the carbon intensity of the grid is decreasing year on year, this will likely be an overestimate over the lifetime of the building. Let's be generous to mass and assume that by adding a lot of it we can reduce that heating demand to zero (we probably can't). Buildings I have seen that try to do this have done so with a lot of concrete - 50 tonnes in intermediate floors, dense concrete in the walls and so on. If we assume 200 tonnes of concrete for a 100m² house, and 180 kg of CO₂ emitted to make each tonne of concrete, that's 36 tonnes of upfront CO₂. Even with making generous assumptions for mass, that's a 300 year pay back on the up-front CO₂ for the additional concrete required.

On top of the nonsensical carbon maths of adding concrete to reduce heating demand, this approach will make other aspects of construction much more complicated: because of the increased mass the structural engineering required makes thermal-bridge-free construction harder to achieve, foundations have to be stronger (yet more embodied CO₂) and so on.

Wrapping up (warmly!)

In summary; If you want a building that loses heat very slowly, make it super-insulated, glazed optimally, super airtight and ventilated with efficient MVHR. Chasing thermal mass as a way to reduce your heating demand is inefficient in terms of embodied carbon, cost and complication.

I've only talked about winter performance here, because I feel it is this that is most often misunderstood when it comes to thermal mass (people see it as a good way to reduce winter heating demand). Some mass can help summer performance in some circumstances, but it's not a panacea. A topic for another blog post!