Put thermal mass inside the insulation, exposed to the room and the winter sun, and put insulation outside the mass facing the weather. Mass outside the insulation heats the outdoors instead of the room: a 20 m2 stone wall 0.3 m thick holds 36 kWh for a 10 degree warming that never reaches the house.
Part of Passive Solar Building Design
The arrangement of insulation and thermal mass in a wall, floor and roof so that heat gathered by the sun stays in the house.
The Order of the Layers Is the Design
The carrier gives the rule and the two common errors. The rule: mass goes inside the insulation envelope, exposed to the interior; insulation goes outside the mass, facing the weather. The errors: putting insulation between the sun and the mass, which prevents the mass from charging; and putting mass outside the insulation, which heats the outdoors, not the indoors.
Surface by Surface
| Surface | Mass or insulation | Reason |
|---|---|---|
| Sun-facing floor | Mass, exposed | Struck by the winter sun |
| Interior wall opposite the glass | Mass, exposed | Carrier: mass walls opposite south windows |
| Exterior walls (not sun-struck) | Insulation outside; mass behind it is a bonus | Mass inside the wrap stabilizes the room |
| Roof | Insulation | Heat loss; mass overhead rarely sees sun |
| Floor over ground | Mass floor on insulated edge or underlay | Keep the ground from draining the mass |
| North wall | Insulation, few openings | Never receives direct sun |
| Glass | Covered at night | Weakest part of the envelope |
What You Need
- A section drawing of each wall, roof and floor, from outside to inside.
- A list of materials with their role: stores or resists.
Method: Read the Layers From Outside In
Step 1: Draw the layers From the weather side to the room: finish, insulation, mass, interior surface.
Step 2: Check that insulation is outside If a heavy masonry layer lies outside the insulation, it is outside mass.
Step 3: Check thermal bridges A concrete or stone edge that passes through the insulation (a floor slab running out under the wall, a stone sill, a masonry buttress) carries heat out. Break the connection or insulate it.
Step 4: Check the sun path Nothing between the sun and the mass: no insulation, no thick coverings, no furniture.
Worked Example
Outside mass: a stone wall 20 m2 in area and 0.3 m thick, on the weather side of the insulation, volume 20 x 0.3 = 6 m3.
- Volumetric heat capacity of stone about 2.16 MJ/m3 K (granite, see Mass Materials: Stone, Earth, Water).
- Warming it by 10 degrees takes 6 x 2.16 x 10 = 130 MJ = 36 kWh.
That 36 kWh is an amount comparable to the whole of a good winter day’s solar gain through ten square metres of glass (30 kWh, an assumed value, in the example at Mass Placement & Sizing), and it goes into the weather side of the wall, which then simply follows the outdoor temperature. The insulation between it and the room means the heat cannot reach the room. Moved inside the insulation, the same wall would serve the room.
Common Mistakes
- Mass outside — it warms in the sun and returns the heat to the air outside.
- Insulation over the sunlit floor — it stops charging.
- Thermal bridges — a heavy element through the insulation leaks heat.
- Insulating a south mass wall from the room — the mass can no longer exchange heat with the room.
Safety
Heavy walls on thin foundations fail
Masonry mass inside a building adds a large permanent load. Check the foundation and floor can carry it before adding mass walls or stone floors, and see Load-Bearing Capacity.
What’s Next
- Learn next: Air Sealing with Primitive Materials — closing the gaps in the wrap
- Combine with: Insulation Strategies — choosing the insulation