How thermal mass works in a house

What does thermal mass do?

A material with high heat capacity takes a lot of energy to warm up and gives a lot back as it cools. A sunlit concrete floor warms during the day, then releases that heat into the room in the evening, which makes the house cooler at midday and warmer at night. In summer, a cool night followed by a hot day lets the mass absorb daytime heat and delay overheating.

The effect works in time, not in total energy. Mass does not create heat, and it does not stop heat leaving the house. It shifts and smooths. This is why it belongs inside a well-insulated envelope, a point set out in our passive solar design guide and developed in passive solar design rules.

How much heat can a concrete slab store?

A worked example with assumed figures. Concrete weighs roughly 2,300 kilograms per cubic metre and takes about 0.9 kilojoules to warm one kilogram by one degree C, so a cubic metre stores about 2,000 kilojoules per degree. A 100 millimetre slab over 40 square metres is 4 cubic metres. Letting it swing by 3 degrees C stores about 24,000 kilojoules, close to 7 kilowatt-hours.

That is useful for evening comfort, and small next to a cold-day heating load. Water holds about twice as much heat per cubic metre as concrete, which is why water tanks are the standard way to store heat at larger scale, as covered in thermal storage tanks for water. The general idea is in what is thermal energy storage.

Where should thermal mass go?

• Inside the insulation. Mass on the warm side of the insulation exchanges heat with the room. Mass outside the insulation just cools down.
• In the sun's path. For passive solar gain, a dark or mid-tone floor or wall that the winter sun reaches directly works best.
• Exposed. Heavy carpet, thick underlay or furniture block heat flow between the room and the mass.
• Moderate thickness. Roughly the first 100 millimetres or so does most of the daily work, and added depth gives diminishing returns.

Radiant slabs combine mass with heating pipes and use the slab as a storage buffer, discussed in radiant floor heating.

When does thermal mass not help?

In a cold Canadian winter, a house that is heated continuously has already got a stable temperature, so mass adds less. A heavy house also reacts slowly, which is a problem for a cabin used on weekends, because warming the mass takes time and fuel. In a lightweight house with large south windows and no mass, overheating is the usual symptom.

Some building scientists argue that the benefits of thermal mass are overstated compared with insulation and airtightness. Their view is reasonable: spend on the envelope first, in the order given in insulation upgrade priorities, then use mass where the design already collects sun. Lightweight alternatives such as phase change wallboard are discussed in phase change materials for homes.

What did Drake Landing do with mass?

Drake Landing Solar Community in Okotoks, Alberta, used ground as its thermal mass. The earth around 144 boreholes, each about 35 metres deep, acted as a very large store that was charged with solar heat over summer. The centre of the field approached 80 degrees C in late summer, as described in borehole storage and borehole thermal energy storage. The idea is the same as a slab, scaled up and insulated by the surrounding earth and a cap.

The project was wound down from about 2020, with decommissioning from 2024. For a broader look at how storage choices compare, see energy storage types for homes.

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