Insulation keeps ice cold by trapping still air, which conducts heat poorly, so fill the space around the ice with a dry, loose, fluffy material and keep it dry and free of gaps. Use 15-20 cm of dry sawdust around all sides of the ice and between layers, or 20-30 cm of straw, or pile at least 60 cm of earth against above-ground walls. Wet insulation fails because water conducts heat more than 20 times as well as still air.
Part of Ice Houses and Cold Storage
Cold-storage insulation is the material packed around stored ice or food to slow the flow of heat from the warm outside to the cold inside.
The Ice House Works Because The Insulation Works
Insulation between the ice and its surroundings, and between the blocks, is what makes the whole system function. Heat always flows from warm to cold, and the insulation’s job is to make that flow as slow as possible, so a winter’s ice lasts through the summer. A thin, wet or gappy layer fails quickly.
Insulation works because still air is a poor conductor. The materials that do best are those that trap the most air in the smallest pockets and keep it from moving: sawdust, straw, dry leaves, wool. The corpus’s general treatment of this is in Thermal Insulation Basics.
Your Choices
| Material | Thickness around ice | Notes |
|---|---|---|
| Sawdust | 15-20 cm | The classic choice; absorbs some meltwater; see Sawdust |
| Straw | 20-30 cm | Nearly as good; compresses and is harder to clean; see Straw and Hay |
| Dried leaves | gap fill | Usable in wall gaps; rot if damp |
| Earth | at least 60 cm against above-ground walls | Uses ground temperature; see Earth Berming |
What You Need
- The insulation — whichever of the above you can get dry and in quantity.
- A dry, roofed place to store it — damp material is spoiled material.
- A way to keep it dry in use — a drained floor and a sound roof. See Meltwater Drainage and Roof Design.
Method: Four Rules For Any Insulation
Rule 1: Keep it dry. Water in the pores replaces air. Liquid water conducts about 0.6 W/m.K against about 0.026 W/m.K for still air, a ratio of 0.6 / 0.026 = 23. Even partly wet insulation loses much of its value.
Rule 2: Keep it loose, not packed hard. Compressed material has less trapped air. Fluff it in and do not stamp it flat, except where you need to fill gaps.
Rule 3: Fill every gap. A gap lets air move, and moving air carries heat. Pack insulation into every space between blocks and between ice and wall. See Air Gap Prevention.
Rule 4: Cover every face. Ice loses heat through its weakest face. Insulate the top, sides and bottom, not just the sides.
Common Mistakes
- Using damp sawdust or straw — it conducts and rots.
- Skimping on thickness — the parent article’s minimum is 15-20 cm of sawdust; less than that and the ice is gone early.
- Leaving the top bare — the roof space is the warmest part.
- Letting insulation sag away from the wall — it leaves a gap at the top.
- Reusing wet material year after year — replace or dry it.
Safety
Insulation burns and, in a sealed pit, uses up the oxygen
Dry sawdust, straw and hay are fuel. Keep flames and hot ashes away from them, and do not smoke near an ice house. Damp organic insulation in a closed pit keeps decomposing, using up oxygen and releasing carbon dioxide that pools at the bottom. Ventilate before anyone enters, tether the person who descends, and never test the air with a flame. See Ice House Construction.
What’s Next
- Learn next: Sawdust — the classic ice house insulation
- Learn next: Straw and Hay — the farm alternative
- Combine with: Sawdust Layering — how much heat the layer lets through