Sawdust is the classic ice house insulation: pack 15-20 cm of dry sawdust around the sides, under and over the ice and between the layers of blocks, using sawdust from untreated wood that has been dried before use. The parent article credits it with cutting melting by 80-90 percent compared with bare ice. A 2.5 m pit with 17.5 cm of sawdust round the ice needs about 4.5 cubic metres of loose sawdust, plus a few more between the layers of blocks.
Part of Ice Houses and Cold Storage
Sawdust is the fine waste from sawing wood, used as a loose-fill insulator in ice houses because it traps air and is cheap wherever there is a sawmill.
Sawdust Insulates By Holding Air, And Fails When It Holds Water
A mass of fine wood particles has air between them, and that trapped air is the insulation. The parent article credits 15-20 cm of dry sawdust with reducing melting by 80-90 percent compared with bare ice; that figure is the parent’s claim, and the heat arithmetic in Sawdust Layering shows what the layer thickness alone can and cannot deliver. It credits sawdust with two jobs: it traps air, and it absorbs meltwater so warm water does not pool. These can conflict. Wet sawdust loses its air, conducts heat badly and rots. The way to use both is to keep the sawdust off the wet floor, drain the floor well and renew sawdust that has soaked.
What You Need
- Dry sawdust from untreated wood — from a sawmill or from your own sawing. Avoid sawdust from wood treated with preservatives, which contaminates melt and is harmful to breathe and handle.
- A dry store — a covered, raised place to hold the sawdust before use.
- Quantity — see the worked example.
- A shovel and a sieve — to break lumps and remove chips.
Method: Use Sawdust Well
Step 1: Dry it. Fresh sawdust from green wood is wet and ferments. Spread it thin under cover and turn it until it is dry and loose.
Step 2: Lay the floor layer. Start with a 15-20 cm layer on the gravel floor.
Step 3: Pack the walls. As the ice goes up, pack sawdust between the ice and the wall to 15-20 cm thick.
Step 4: Layer between blocks. Cover each layer of ice with 5-10 cm of sawdust and fill the gaps between blocks. See Sawdust Layering.
Step 5: Cap the stack. Cover the top with 20-30 cm of sawdust.
Step 6: Top up each season. Sawdust settles and compacts. Add to it, and renew any that is wet or stained.
Worked Example: How Much Sawdust A Pit Needs
Inputs: the 2.5 m by 2.5 m pit from Ice House Construction with 17.5 cm of sawdust on the sides, bottom and top.
- Gross pit volume: pi x 1.25^2 x 2.5 = 12.3 m3.
- Ice core after the sawdust: pi x 1.075^2 x 2.15 = 7.8 m3.
- Sawdust around the core: 12.3 - 7.8 = 4.5 m3.
Sawdust between the layers is extra. So about 4.5 m3 of loose sawdust is needed before the layers. Sawdust compresses when packed, so order more than the figure.
Common Mistakes
- Damp sawdust — it conducts and rots.
- Treated or contaminated wood — a hazard to health and food.
- Letting it touch the drain — fines wash into it and block it.
- Packing it solid — it loses air. Pack firmly enough to fill gaps, not to crush.
- Reusing soaked sawdust — replace or dry it.
Safety
Sawdust burns, its dust harms lungs, and a damp heap heats itself
Dry sawdust ignites easily and its dust in the air can burn fast. Keep flames, forge sparks and hot ashes away from the store and the ice house, and do not smoke there. Fine wood dust irritates the airways, so cover the nose and mouth when shovelling it. A large damp heap can ferment and heat itself, so turn and dry it rather than piling it deep. In a closed pit, damp sawdust consumes oxygen; follow the ventilation and tether rules in Ice House Construction and never test the air with a flame.
What’s Next
- Learn next: Sawdust Layering — the heat arithmetic
- Combine with: Straw and Hay — the alternative where there is no sawmill