Thermal mass is any heavy, dense material that absorbs heat while the sun is shining and gives it back slowly after dark, which keeps a room steadier than the weather outside. With enough mass the indoor swing can be held to 5-8 degrees Celsius while the outdoor swing is 20 degrees or more, provided the mass is inside the insulation and in the sun.
Part of Passive Solar Building Design
The idea of heat storage in building materials: what it does, why it works, and the two placement rules that decide whether it helps.
Mass Is a Battery for Heat
Imagine a sunny winter day. Sun comes through the south windows and strikes a stone floor. Instead of the air heating up fast, the floor takes up much of the heat and warms slowly. After sunset the floor, now warmer than the room, gives the heat back and keeps the room from cooling quickly. The room is cooler at midday and warmer at midnight than a light-built room would be. The carrier’s figure is that with adequate mass the indoor temperature swing can be limited to 5-8 degrees Celsius even when the outdoor temperature swings 20 degrees or more. The carrier calls thermal mass “the battery of a passive solar building”.
What Makes a Good Store
Three properties matter, in this order for a beginner:
- How much heat it holds per volume — water holds the most; the carrier says it stores roughly 2x the heat of stone per unit volume. Stone, brick, adobe and concrete are good. See Mass Materials: Stone, Earth, Water.
- Where it is — in the path of the winter sun, exposed to the room, not covered or hidden. See Mass Placement & Sizing.
- How it is wrapped — mass goes inside the insulation envelope, exposed to the interior; insulation goes outside the mass, facing the weather. See Where to Insulate vs Where to Mass.
Mass Needs Sun and Needs Insulation
Mass in a dark corner does almost nothing: it never gets the heat. Mass outside the insulation heats the outdoors instead of the room. And a house with lots of mass but little sunlight is a cold house: the mass sits at the average temperature of the place and takes heat from you. Mass and glass work as a pair; neither helps alone.
What You Need
- Dense materials — stone, adobe, brick, salvaged concrete or water-filled containers.
- Sun on them — a south-facing glazed wall.
- Insulation on the outside — see Insulation Strategies.
Common Mistakes
- Mass in shade — a heavy wall that the sun never reaches is just a heavy wall.
- Insulation between the sun and the mass — it stops the mass charging.
- Covering the mass — thick rugs, panelling and furniture over a stone floor hide it from the sun and the room.
- More mass to fix overheating without ventilation — mass slows the swing but needs night ventilation to dump the day’s heat in summer.
What’s Next
- Learn next: Mass Materials: Stone, Earth, Water — what to use
- Combine with: Solar Orientation & Geometry — pointing the sun at it