Rank thermal mass materials by how much heat a cubic metre holds per degree of temperature change: water about 4.2 megajoules, concrete and dense stone about 2.1-2.2, brick and adobe or rammed earth about 1.5-1.7. Water therefore stores roughly twice what stone does, but it needs sealed containers, while stone, earth and brick can be built into the walls and floors.
Part of Passive Solar Building Design
Comparing the heat-storage capacity, availability and drawbacks of water, stone, earth, brick and salvaged concrete as building mass.
Volumetric Heat Capacity Is Density Times Specific Heat
How much energy a material stores per degree is its density times its specific heat capacity. For a building, per volume is the useful measure. The values below are typical handbook figures for density and specific heat, multiplied here; they vary by 10-20% with the exact stone, moisture content and compaction, so treat them as comparisons, not specifications.
| Material | Density (kg/m3) | Specific heat (J/kg K) | Heat per m3 per K (MJ) |
|---|---|---|---|
| Water | 1,000 | 4,186 | 4.19 |
| Granite | 2,700 | 800 | 2.16 |
| Concrete | 2,400 | 880 | 2.11 |
| Rammed earth | 1,900 | 900 | 1.71 |
| Adobe | 1,700 | 900 | 1.53 |
| Brick | 1,800 | 840 | 1.51 |
| Softwood | 500 | 1,700 | 0.85 |
In round numbers: water about 4.2, concrete and dense stone 2.1-2.2, and brick, adobe and rammed earth 1.5-1.7 MJ per cubic metre per degree. Water against granite: 4.19 / 2.16 = 1.9, which supports the carrier’s “roughly 2x the heat of stone per unit volume”.
Why the Carrier Rates Wood “Poor”
Per volume, wood’s storage is not negligible, and denser hardwoods are higher than the softwood in the table. The reason the carrier gives, that wood insulates more than it masses, is about conductivity. Wood conducts heat poorly, roughly 0.1-0.2 W/m K against 0.6-0.9 for earth, so heat does not travel into it fast enough to use more than a thin surface skin on a daily cycle. See Heat Absorption & Release Cycles for how deep that skin is.
What You Need
- Stone, adobe, brick, rubble or salvaged concrete — see Adobe Mix, Pressed Earth Bricks and Adobe Bricks.
- Or sealed water containers — barrels, drums, tanks. The carrier suggests painting them dark, ideally black.
Method: Choose by Situation
Step 1: Look at what is abundant Stone, rubble and soil cost labour, not money. Water costs containers.
Step 2: Match to the place Floors and walls in the sun path take stone, brick, adobe or concrete. A narrow sunny space with no room for thick walls suits water containers placed where sun strikes them.
Step 3: Make the surface dark where the sun strikes The carrier says a dark slate floor absorbs about 95% of incoming solar energy and a light one reflects most of it.
Worked Example
Heat stored in 1 m3 for a 5 K swing (5 degrees Celsius):
- Water: 4.19 MJ/K x 5 = 20.9 MJ = 20.9 / 3.6 = 5.8 kWh.
- Concrete: 2.11 x 5 = 10.6 MJ = 2.9 kWh.
One 200 litre drum is 0.2 m3 of water, storing 0.2 x 4.19 = 0.84 MJ per degree. Concrete storing the same needs 0.84 / 2.11 = 0.40 m3, about twice the volume. Per kilogram the gap is wider: 4,186 / 880 = 4.8 times as much heat for water as for concrete, which is why the carrier calls water the best mass-to-weight ratio.
Common Mistakes
- Counting wood as mass — the thin skin of a wood floor does little.
- Relying on a light-coloured mass wall in the sun — it reflects the heat before it can store it.
- Open water tubs — they breed mosquitoes and lose heat by evaporation.
Safety
Water containers are heavy, can leak and can freeze
A 200 litre drum holds about 200 kg of water: check that the floor can carry it and keep it clear of exits. Water in a drum that freezes can split it, so keep drums in the heated space. Seal drums so that mosquitoes cannot breed and children cannot fall in, and never reuse a drum that held fuel or chemicals.
What’s Next
- Learn next: Mass Placement & Sizing — how much, and where
- Combine with: Thermal Insulation Basics — the other half of the envelope