Insulation and Saving Energy

Insulation is any material or design used to slow unwanted heat transfer. It does not create warmth and it does not create cold — it simply makes the journey harder for heat.

The secret ingredient in almost every good insulator is trapped air. Still air is a very poor conductor, so a material full of small pockets of trapped air — wool, polystyrene, glass fibre, feathers, shredded paper — slows conduction badly, and because the air is trapped it cannot set up convection currents either.

That is why a fluffy jersey is warmer than a thin one made of the same wool: it holds more air, not more wool.

Insulation in a South African home:

• Ceiling insulation under a corrugated iron roof — the single biggest improvement in most houses. It keeps heat out in a Limpopo summer and in on a Highveld winter night.
• A geyser blanket wrapped around the hot water cylinder, so the water stays hot for longer between heating cycles.
• A hotbox (wonderbag) — a cushion-stuffed bag that a pot is placed into. Bring samp and beans to the boil for a few minutes, switch off the plate, seal the pot in the hotbox, and it goes on cooking for hours on stored heat. During load-shedding it is the difference between a cooked supper and a cold one.
• A lid on the pot, closed curtains at night, and filling a kettle only as far as you need.

Worked example. A pot at 95 °C is sealed in a hotbox and loses about 4 °C per hour. After 5 hours it has lost $4 \times 5 = 20$ °C, so its temperature is $95 - 20 = 75$ °C — still hot enough to serve. On an open counter the same pot would have been cold long before.