Why are my windows so drafty?

17 January 2010

Question

Why are my windows so drafty? My understanding of the second law of thermodynamics is that heat moves from a hot body to a cold one. So, can you explain to me why cold air is flowing into my very nice warm room through a gap in a badly fitted window? I’ve proved that cold air is moving in this direction by placing some soft tissue paper in the gap.

Answer

You're entirely correct about thermodynamics. If you've got two stationary objects and you join them together thermally, heat will always flow from the hotter object to the colder object. It's actually pretty much how we define temperature.

So what's happening with your window is because cold air is definitely coming into your house, this situation's a bit different. You're not touching two objects together and cold is moving into your hot house. What's actually happening is that cold air is moving in to your house. The laws of thermodynamics say nothing about where air goes, they don't disallow this. So basically, large lumps of objects can move around as they like, not ignoring the laws of thermodynamics, but they aren't constrained by them in that way.

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