HEM Deep-Dive: Thermal Mass

HEM engine v2.00a2 (Python); FHS v1.00a7.

At time of writing, HEM is still in development and not yet approved for regulatory compliance.

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Where did the Home Energy Model rabbit-hole take me this week 🐇?

The exciting world of Thermal Mass, that's where! 😁

Is it better to build our homes with a low thermal mass and lightweight materials, or use dense materials to create a high thermal mass?

I always use the comparison of a shed to a cathedral... 

On a summers day, the shed will heat up really quickly, but get very cold at night. The internal temp in a cathedral is more consistent, but takes much longer to heat up or cool down.

So, I've built a HEM model to look into this. I've used a detached house with the (still work-in-progress) Future Homes Standard Wrapper and standard weather file.

The dwelling is based on U-Values to comply with current regs, but I've also created a version based on a 100 year old property.

These graphs show a snapshot, starting with 48 hours in summer.

Line graphs showing HEM results of internal temperatures and heating demand for new and old houses with extreme thermal mass (low and high)

The top graph shows the internal temperature (Celsius) of the newly built house. Orange is a low thermal mass (timber frame). Green shows a high thermal mass (concrete).

Background colours (yellow and black) indicate day and night through a 48 hour summer period.

On second graph is modelled on a 100 year old specification. This graph also shows a sneaky little red line at the bottom. This shows a small heating demand (kWh) based on the low thermal mass option only.

In the summer, when the building is heating and cooling naturally, the low thermal mass has bigger temperature spikes.

This is more prominent in the old dwelling, suggesting old homes with low thermal mass are more inclined to both overheat in the day, and require a small kick of heating overnight.

The small level of heating demand indicates the old house with low thermal mass cools down too much on a summers night, requiring a little pick-me-up while occupants are having their Weetabix.

(I apologise to any graph purists out there for displaying both Celsius and kWh on the same axis!) 😉

The next graph looks at the same data, but for 48 hours in winter:

Line graphs showing HEM results of internal temperatures and heating demand for new and old houses with extreme thermal mass (low and high)

In the winter, internal temperatures are controlled by the heating system, so temps are near identical (following HEM's pre-set heating pattern).

But the heating demand tells it's own story... The low thermal mass property needs more heating overnight because it loses heat quicker.

This is more noticeable on the older building which has a much higher heating demand overall (as you'd expect when comparing an old building to a new one).

Is there a winner? Inconclusive.

On the new build, the annual heat demand is the same for both low and high thermal mass options.

For the old property, the low thermal mass has a 2% higher heating demand.

Aside from energy performance, this info also shows high thermal mass is better for handling temperature spikes, so a better option for overheating mitigation.

Overheating assessors always look happier when they're modelling a high thermal mass building, as it's more likely to meet Part O's internal temperature targets. It's reassuring that HEM and TM59 are saying the same thing.

But, the embodied carbon lifecycle gang will tell you low thermal mass is better, because the more concrete you pour into a building the larger embodied carbon figures you have to deal with.

That's not a concern for Building Regulations today, but probably will be in the near future.

So the thermal mass debate is set to continue for years to come!