Insulation works by trapping still air in fibres or foam, because still air conducts heat poorly, about 0.026 watts per metre per kelvin. Doubling the thickness around a pot does not halve the heat loss, since the insulation shell wraps round it, and a real haybox loses several times more heat than an ideal shell of the same material, because gaps and compressed spots carry heat out.
Part of Retained Heat Cooking
Conduction through a still, dry layer of fibre or foam is the main route heat takes out of a haybox. Its rate depends on the material, the area, the thickness and the temperature difference.
Fluffy And Dry Beats Dense And Damp
The overview says insulation effectiveness depends on trapping still air, and the best materials are fluffy, dry and compress slowly. Heat flow through a flat layer is Q = k × A × ΔT ÷ L, where k is the conductivity of the material, A the area, ΔT the temperature difference and L the thickness. Air that cannot move conducts about 0.026 W/m·K (a standard value); dry fibre materials such as hay or wool are typically in the 0.03 to 0.06 W/m·K range (typical handbook range, not from the overview). Liquid water conducts more than 20 times as well as still air (about 0.6 W/m·K), which is why wet insulation fails.
Worked Example: An Ideal Shell, And What The Overview’s Figures Imply
A shell around a roughly spherical pot has a conductance UA = 4π × k × r₁ × r₂ ÷ (r₂ − r₁), where r₁ is the pot’s equivalent radius and r₂ the outer radius (the standard result for a spherical shell; a box is only approximately this).
Take these assumptions: a pot of 5 litres full of water (5 kg), equivalent radius r₁ = (3 × 0.005 ÷ 4π)^(1/3) = 0.106 m; hay-like insulation with k = 0.05 W/m·K; room at 20 °C.
- 10 cm shell: r₂ = 0.206 m. UA = 12.57 × 0.05 × 0.106 × 0.206 ÷ 0.10 = 0.137 W/K.
- 15 cm shell: r₂ = 0.256 m. UA = 12.57 × 0.05 × 0.106 × 0.256 ÷ 0.15 = 0.114 W/K.
Going from 10 to 15 cm cuts the loss by only 17 percent (1 − 0.114 ÷ 0.137), because the outer surface grows as the shell thickens. Even an infinitely thick shell would still lose 4π × 0.05 × 0.106 ≈ 0.067 W/K, so thickness alone has a floor.
The pot’s heat capacity is 5 kg × 4,186 J/kg·K = 20,930 J/K, so an ideal 15 cm shell has a time constant of 20,930 ÷ 0.114 ≈ 184,000 s = 51 hours. Cooling from 100 °C to 60 °C would take 51 h × ln(80 ÷ 40) = 51 × 0.693 ≈ 35 hours.
The overview says a well-built haybox holds above 75 °C for 4 to 6 hours. Cooling from 100 to 75 in that time means a time constant of 4 ÷ ln(80 ÷ 55) = 4 ÷ 0.375 = 10.7 hours (and 6 ÷ 0.375 = 16.0 hours). That is 3.2 to 4.8 times lossier than the ideal shell: 51.1 ÷ 16.0 = 3.2 and 51.1 ÷ 10.7 = 4.8. Gaps, a leaking lid, compressed insulation and the pot’s rim account for the difference, so sealing them is worth more than adding thickness. These figures are a model on assumed inputs, not a measurement; check your own box with a thermometer.
Common Mistakes
- Adding thickness while leaving gaps. Gaps carry the heat out.
- Letting insulation get damp. Wet material conducts much more heat.
- Using compressed insulation. It loses the air it trapped.
Safety
Insulation can burn or melt
Hay, paper, sawdust and leaves burn, and polystyrene melts near a hot object. Keep the pot and any hot stone from touching them without a non-combustible barrier. Keep the box away from stoves and flames, and dry. Mould in damp insulation is a respiratory hazard.
What’s Next
- Learn next: Heat Loss Pathways — where the heat actually goes
- Combine with: Insulation Materials Ranked — what to fill the box with