Our Heating Bill Doubled — Three Engineers Missed It, A Candle Found It In Four Minutes

Where the gaps actually are

The sash meeting rail. On a double-hung window the two sashes meet in the middle, and that joint opens a little further every year as the frame moves. This is the one people find, because it is at chest height and you can feel it with a hand.

The frame-to-wall joint. This is the one that usually matters most and is almost always missed, because it does not look like part of the window at all. The gap sits behind the casing, hidden by trim on the outside and by the frame on the inside — and it often opens straight into the wall cavity.

This was mine. The porch had been rebuilt eighteen months before the first expensive winter. The builders fitted it beautifully and never sealed the joint behind it. There was a continuous gap, roughly the width of a pencil, running up both sides of the doorway and across the top. The house had been quietly ventilating itself all winter through a hole nobody could see.

The sill. Easiest to seal and easiest to overlook. Shine a torch along it from one side at a very low angle — gaps throw shadows that are invisible when you look at them straight on.