Wool insulates best of the natural materials at about R-3.5 per inch, then cattail fluff at R-3.0, wood shavings R-2.5, straw and moss R-1.5, dry leaves R-1.0, and dry earth only R-0.2. To reach the R-27 of an 18-inch straw bale wall you need about 7.7 inches of wool, 10.8 of shavings, or 27 of dry leaves.
Part of Passive Solar Building Design
Locally gathered materials that trap still air in walls, roofs and floors, with their insulating value, drawbacks and thickness needed.
Per-Inch Values From the Carrier, Converted
The R-values are the carrier’s, in US units per inch of thickness. The conversion to SI uses 1 US R-unit = 0.1761 m2 K/W; thermal conductivity (k, in W/m K) is the thickness in metres (0.0254 per inch) divided by that per-inch R in SI. A lower k is better.
| Material | R per inch (US) | m2 K/W per inch | k (W/m K) | Inches for R-27 | Notes |
|---|---|---|---|---|---|
| Wool | 3.5 | 0.62 | 0.041 | 7.7 | Excellent but scarce; works even when damp |
| Cattail fluff / milkweed | 3.0 | 0.53 | 0.048 | 9.0 | Tedious to harvest in quantity |
| Wood shavings / sawdust | 2.5 | 0.44 | 0.058 | 10.8 | Fire risk; treat with lime |
| Straw | 1.5 | 0.26 | 0.096 | 18.0 | Excellent in bale form |
| Moss | 1.5 | 0.26 | 0.096 | 18.0 | Naturally fire-resistant |
| Dry leaves | 1.0 | 0.18 | 0.144 | 27.0 | Settles and compacts; better than nothing |
| Earth (dry) | 0.2 | 0.035 | 0.72 | 135 | Poor; only in earth-sheltered designs |
“Inches for R-27” is 27 divided by the per-inch value. R-27 is the figure for an 18-inch straw bale wall at R-1.5 per inch (27 x 0.1761 = 4.75 m2 K/W), chosen as a benchmark; the carrier states R-30+ for bale walls.
What You Need
- Dry material — gather on dry days and store off the ground and under cover.
- A way to hold it — a cavity between boards, straw-clay infill, bale walls, or an attic floor.
- A pest and fire plan — see below.
Method: Pick and Place
Step 1: Choose by what is plentiful Straw, leaves and shavings are cheap in volume; wool is the best but scarce. A material that needs three times the thickness may still be the right choice if it is free.
Step 2: Fit the thickness to the space A thin cavity needs a better insulator. For a 10 cm cavity (about 4 inches), wool gives 4 x 3.5 = R-14, shavings 10, straw 6.
Step 3: Pack evenly Fill without voids. Settling opens gaps at the top: leaves and shavings compact, so overfill or retop.
Step 4: Keep it dry and out of reach of fire Seal it behind plaster, board or lime-wash. See Plaster and Lime Wash.
Worked Example
A roof space needs R-27. Using straw at R-1.5 per inch needs 27 / 1.5 = 18 inches (457 mm). The same job in wool needs 27 / 3.5 = 7.7 inches (196 mm), and in dry leaves 27 / 1.0 = 27 inches (686 mm). Leaves also settle, so the real depth needed is more.
Common Mistakes
- Packing insulation tight — packing too hard squeezes out the trapped air that insulates.
- Leaving gaps at corners and edges — they bypass the insulation.
- Using wet material — wet straw rots; wet leaves mat.
- Putting dry earth in walls for insulation — it is a poor insulator.
Safety
Fire, dust and rot
Sawdust, shavings, straw and fluff burn quickly: keep them well away from flues, stoves and lamps, and treat with lime where the carrier advises. Handling dusty or mouldy plant material releases spores that can cause serious lung illness, so wear a cloth mask and work outdoors where you can. Mice and rats nest in insulation: seal the edges with plaster or fine mesh.
What’s Next
- Learn next: Where to Insulate vs Where to Mass — which surface gets which
- Combine with: Straw Bale Construction — straw in its best form