Insulation works by trapping still air so heat cannot cross the wall, and it belongs on the outside of the thermal mass, facing the weather, continuous around the whole building. Using the carrier’s R-values, an 18-inch straw bale wall loses about one eleventh of the heat of a 12-inch dry earth wall of the same area at the same temperature difference (the earth wall loses about 11 times as much).
Part of Passive Solar Building Design
Preventing heat from leaving a building through walls, roof, floor and gaps, and how insulation and thermal mass divide the work.
Insulation and Mass Do Opposite Jobs
The carrier states it plainly: mass stores heat, insulation prevents heat transfer, and a building needs both in the right places. Mass goes inside the insulation envelope, exposed to the interior; insulation goes outside the mass, facing the weather. Heat leaves a building through whatever is easiest, so a single gap or bare patch of wall matters more than extra thickness elsewhere. The basics of why still air insulates, including that gaps matter more than thickness, are in Thermal Insulation Basics.
R-Value, in Both Units
The carrier quotes R-values per inch of thickness in US units (hour square foot degrees Fahrenheit per Btu). One US R-unit is 0.1761 square metre kelvin per watt. Heat flow through a wall is Q = A x dT / R, with R in square metre kelvin per watt (SI), A the area and dT the temperature difference.
What You Need
- An insulation material — see Natural Insulation Materials and Insulation Materials Ranked.
- A continuous envelope design — walls, roof, floor, door and window edges all joined.
- A dry detail — insulation soaked by rain or condensation stops working and rots.
Method: Decide Where the Insulation Goes
Step 1: List every surface that touches the outdoors or the cold ground Roof, walls, floor, door, shutters.
Step 2: Insulate from the outside of the mass Put the mass inside; wrap the building in insulation outside it. See Where to Insulate vs Where to Mass.
Step 3: Close the gaps See Air Sealing with Primitive Materials.
Step 4: Cover the windows at night Glass is the weakest part of the wall. Insulating shutters or heavy curtains on winter nights cut the loss.
Worked Example
Compare two walls of 20 m2 at a 20 K temperature difference (indoors 20 degrees warmer than outdoors), with the carrier’s per-inch R-values:
- Straw bale wall, 18 inches at about R-1.5 per inch: R = 27 US units = 27 x 0.1761 = 4.75 m2 K/W. Heat flow = 20 x 20 / 4.75 = 84 W.
- Dry earth wall, 12 inches at about R-0.2 per inch: R = 2.4 US units = 2.4 x 0.1761 = 0.42 m2 K/W. Heat flow = 20 x 20 / 0.42 = 946 W.
- Ratio: 946 / 84 = 11.3, so the straw wall loses about one eleventh as much heat as the earth wall.
The earth wall loses about 11 times as much heat. That is why the carrier says to use dry earth as the strategy only in earth-sheltered designs, where the ground itself is mass and the surrounding earth is at a steady temperature. (The 18 inches is an assumed bale thickness; the carrier says straw bales give R-30+ in a plastered wall.)
Common Mistakes
- Insulation between the sun and the mass — it stops the mass charging.
- Wet insulation — it loses most of its value and can rot; keep it dry and shed water from it.
- Thick insulation with gaps — a gap at the eaves or the floor edge can cost more than extra thickness saves.
Safety
Natural insulation can burn, rot and harbour animals
Dry straw, sawdust, leaves and cattail fluff catch fire easily: keep them well clear of flues, stoves and lamps, and plaster or lime-treat them where possible. Damp insulation grows mould whose spores harm lungs, so wear a cloth mask when handling it, and keep rodents out.
What’s Next
- Learn next: Natural Insulation Materials — what to fill the walls with
- Combine with: Thermal Mass Principles — the store inside the wrap