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I read in the NZ Herald this week that one of Air New Zealand’s energy-saving strategies was to make sure an aircraft’s insulation is dry. Apparantly, 200 kg of water can be sucked out of a plane’s insulation.  Sounds impressive.

The article appeared to be pushing the weight saving as the main cost saving. Two hundred kilograms corresponds to about two-and-a-half luggage-laden passengers. A saving that’s worth taking, if it’s available to you, but I don’t think one that will make a massive dent in the company’s fuel bill.  For example, an unladen Boeing 777-300 weights in at about 160 tonnes (isn’t Wikipedia wonderful?), so 200 kg amounts to little more than a thousandth of the unladen weight.

There’s another saving, though I’m not sure how significant it will be. Damp insulation isn’t a particularly effective insulator. Planes need good insulation – with about minus 70 celcius outside, it’s a touch chilly up at 10 km altitude. Also, when you’re in one on the ground on a sunny day, it can get pretty unpleasant – I know that from experience. A decent air-conditioning system is certainly needed.

How much water does 200 kg on a large aircraft correspond to?  Our Boeing 777 fuselage is about 70 m long and 6 m wide, giving it a rough surface area of 1400 metres squared. Assuming the fuselage is wrapped in insulation, that would give about 0.14 kg or 140 ml) of water for every metre squared of insulation, or one litre of water for every 7 square metres. Imagine taking a metre-square of roof insulation and pouring half a can of soft-drink on it before you install it. It’s not going to perform at its best…

Now, if and when I can procure some U-values for dry and damp aircraft insulation, I could give you a rough estimate of just how much heat is lost – i.e. what the heating / aircon bill is for an aircraft and how much it changes when the insulation is damp. At the moment, I don’t have them, so this post has to end a bit speculatively…