Size thermal mass from an energy balance: the volume you need is the heat to be stored divided by the material’s heat capacity per cubic metre per degree and by the temperature swing you will accept. Place it where winter sun strikes it directly and keep it 10-15 cm thick for daily cycling, because surface area counts for more than depth.
Part of Passive Solar Building Design
Working out how much thermal mass a passive solar room needs, and putting it where the sun and the room can actually use it.
Volume Comes From an Energy Balance
Heat stored Q equals volume V times volumetric heat capacity C (joules per cubic metre per degree) times the temperature swing dT: Q = V x C x dT. Rearranged, V = Q / (C x dT). The three decisions are how much heat you want to store, which material you use and how much the mass temperature may swing. A bigger allowed swing means less mass, but a wider indoor temperature swing.
The carrier’s placement rules constrain the shape: place mass where sun hits it directly; keep dense elements 10-15 cm thick for daily cycles, because thicker mass responds too slowly; and let surface area beat depth: a 10 cm wall with 20 m2 of surface stores and releases more useful heat than a 40 cm wall with 5 m2.
What You Need
- An estimate of the daily heat gain — measure it if you can; the example below uses a stated assumption.
- A volumetric heat capacity — from Mass Materials: Stone, Earth, Water.
- Floor and wall surfaces that the sun reaches — from Window Placement & Sizing.
Method: Size, Then Place
Step 1: Estimate the heat to store Estimate the winter solar heat that comes in through the glass per day. Not all of it goes into the mass; some warms the air directly. Use the whole amount for a conservative size.
Step 2: Choose the swing Choose the allowed swing of the mass temperature, for example 5 K or 10 K.
Step 3: Compute volume and surface V = Q / (C x dT). Surface area = V / thickness, with thickness 0.10 to 0.15 m for dense solids.
Step 4: Place it in the sun The first target is floor and wall surface struck by direct sun. Mass the sun never reaches does almost nothing. Mass walls opposite the south windows, mass floors and water containers in the direct sun path are the carrier’s effective placements.
Worked Example
Assumed inputs, for illustration only: 10 m2 of sun-facing glass transmitting 3 kWh per square metre on a clear winter day, so Q = 30 kWh = 108 MJ, all of it to be stored in the mass.
| Material | C (MJ/m3 K) | Volume at 5 K swing | Volume at 10 K swing |
|---|---|---|---|
| Water | 4.19 | 108 / (4.19 x 5) = 5.2 m3 | 2.6 m3 |
| Concrete | 2.11 | 108 / (2.11 x 5) = 10.2 m3 | 5.1 m3 |
| Adobe | 1.53 | 108 / (1.53 x 5) = 14.1 m3 | 7.1 m3 |
- Water at 5 K: 5.2 m3 is 26 drums of 200 litres.
- Concrete at 5 K and 0.10 m thick: 10.2 / 0.10 = 102 m2 of exposed surface; at 0.15 m, 68 m2. At a 10 K swing it halves to 51 m2 and 34 m2.
That is a large area for 10 m2 of glass. It shows that the mass surface should be several times the glass area, and that glass without enough mass overheats. Two caveats: much of the daytime gain warms the air and leaves, so the whole 30 kWh need not be stored; and the 3 kWh figure is assumed. Measure your own glass. Treat the result as the upper end of the mass needed to hold all the gain with that swing.
Failure Directions to Check
- Too much mass, too little sun — the mass never charges and drags the house toward the outdoor average: cold.
- Too much glass, too little mass — sunny days overheat, nights are cold.
- Mass outside the insulation — it heats the outdoors. See Where to Insulate vs Where to Mass.
- Mass hidden under rugs and furniture — the sun and the room cannot reach it.
Common Mistakes
- Making mass thicker to add capacity — beyond about the first 10-15 cm it joins the daily cycle slowly. See Heat Absorption & Release Cycles.
- Painting mass light-coloured — it reflects instead of absorbing.
- Using the arithmetic as a specification — it sizes an order of magnitude; adjust by measuring indoor swings in the first winter.
Safety
Mass is heavy
Floors, joists and shelves must carry water drums (about 200 kg each) and masonry. Check loads before loading a floor, and keep mass walls braced against falling during construction.
What’s Next
- Learn next: Heat Absorption & Release Cycles — why thickness has a limit
- Combine with: Where to Insulate vs Where to Mass — wrapping the mass