Three Heating Contractors Missed the Draft — 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 wood swells and shrinks and the frame racks slightly out of square. This is the one people find, because it is at chest height and you can feel it with a hand when you stand close. It is also the one that matters least, because the gap is narrow and it runs horizontally across only the width of the window. Seal it anyway — it takes five minutes and four dollars of weatherstripping — but do not stop there.

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 or door 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, which means you are heating the inside of your walls all winter.

This was mine. The porch had been rebuilt eighteen months before the first expensive winter. The builders fitted it beautifully: new jambs, new brick mold, everything tight and square and painted. What they did not do was run a bead of caulk between the new doorframe and the old framing before they nailed the trim over it. There was a continuous gap, roughly the width of a pencil, running up both sides of the doorway and across the top. I could not see it from inside or out, but the candle found it immediately. The flame leaned hard toward the wall at baseboard height, again at waist height, again at the top of the frame, all in a perfectly vertical line four inches from the corner.

When I pulled the trim off — carefully, because I wanted to reuse it — I could see straight through to the tar paper on the outside sheathing. The gap was a quarter inch wide in some places. That is not a draft. That is a ventilation system.

The sill. Easiest to seal and easiest to overlook. The joint where the sill meets the jamb or where it meets the wall below often opens up as the house settles, and it sits low enough that you do not notice it unless you crouch down. Shine a flashlight along it from one side at a very low angle — gaps throw shadows that are invisible when you look at them straight on. I found three sill gaps this way that I had walked past for two years without seeing.