A cold room is only as good as its insulation, because every watt that leaks in through the walls has to be pumped out again. Heat flow through a wall is the material’s conductivity times area times temperature difference divided by thickness, so doubling the thickness of dry sawdust from 15 to 30 cm halves the leak, and keeping the insulation dry matters as much as how much of it there is.
Part of Refrigeration
Choosing and placing insulation around a cold box or room from natural materials, and calculating the heat that still leaks in.
Conductivity, Area And Thickness Set The Leak
The corpus gives the rule in Thermal Insulation Basics: heat flow through a flat layer is Q = k × A × ΔT ÷ L, with k the conductivity, A the area, ΔT the temperature difference and L the thickness. Insulation works by trapping still air, which conducts about 0.026 W/m·K; the better materials are fluffy, dry and slow to compress.
The corpus table in Natural Insulation Materials gives conductivities that this page uses, in W/m·K: wool 0.041, sawdust 0.058, straw 0.096, dry leaves 0.144. Lower is better. The refrigeration carrier’s table ranks cork and wool as excellent, sawdust, straw, dried grass and charcoal as good, and says wet sawdust insulates poorly. The wider comparison is in Insulation Materials Ranked.
The carrier’s recommended approach is a double wall with a 15 to 30 cm gap filled with sawdust, straw or wool, the outer wall to keep weather out and the inner wall to hold the insulation in place.
What You Need
- Dry insulation, in the quantity to fill the gap fully with no voids.
- Inner and outer walls with a gap set by the thickness you want.
- A vapour barrier on the warm side. The carrier suggests animal skins with fat, waxed cloth, or pitch-coated fabric. Pitch and Waterproofing covers one of these options.
- A rigid floor and base, insulated like the walls.
Method: Build The Walls
Step 1: Frame the inner box first, sized to the shelves you need and no larger. Every cubic metre you cool costs energy.
Step 2: Build the outer wall at the chosen distance. Leave the gap open at the top for filling.
Step 3: Fill in layers and tamp lightly. When it is right, there are no voids, and the fill is not compressed hard. Compressing insulation drives out the air that does the work.
Step 4: Seal the top and the corners. Gaps at corners and at the door frame carry far more heat than the same area of good wall.
Step 5: Put the vapour barrier on the warm side of the insulation. Moisture moving into cold insulation condenses or freezes there and ruins it.
Worked Example: What Thickness Buys
A cold room of 1 × 1.5 × 2 m inside, the carrier’s size. Its inside surface is 2 × (1 × 1.5 + 1 × 2 + 1.5 × 2) = 2 × (1.5 + 2 + 3) = 13 m². Assume 30 °C outside and 4 °C inside, so ΔT = 26 K. These are assumptions for the arithmetic.
- Sawdust, 0.30 m: Q = 0.058 × 13 × 26 ÷ 0.30 = 65 W.
- Sawdust, 0.15 m: Q = 0.058 × 13 × 26 ÷ 0.15 = 131 W.
- Straw, 0.30 m: Q = 0.096 × 13 × 26 ÷ 0.30 = 108 W.
- Wool, 0.30 m: Q = 0.041 × 13 × 26 ÷ 0.30 = 46 W.
Halving the thickness doubles the loss, as the formula says. At the same thickness straw leaks 108 ÷ 65 = 1.7 times as much as sawdust, and wool 46 ÷ 65 = 0.71 times. These are floors: the area used is the inside surface only, the calculation counts conduction through dry insulation, and doors, corners, gaps and damp all add to it.
Every one of those watts leaks in around the clock and has to be pumped back out by the machine, which is why a thicker wall is usually cheaper than a bigger plant.
Common Mistakes
- Letting the insulation get wet. Wet sawdust insulates poorly, as the carrier says; damp straw and leaves are worse.
- Voids at corners and the door frame. A gap short-circuits the whole wall.
- Compressing the fill. It squeezes out the still air that insulates.
- Skipping the floor. The carrier notes that an uninsulated floor bleeds cold into the ground.
- Vermin. Straw invites mice; line the gap against them.
Safety
Sawdust, straw and dried grass burn, and damp fill grows mould
Keep insulation away from flues, lamps and hot machinery, particularly near the compressor or the generator of an absorption machine. Handling dusty or mouldy plant material releases spores that can damage the lungs, as the corpus warns in Natural Insulation Materials; wear a mask and work outdoors when filling.
What’s Next
- Learn next: Cold Room Design — doors, seals, floors and vapour barriers
- Combine with: Ice Houses — the same insulation principles around stored ice