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

Matching the material to the gap

Moving joints — V-strip weatherstripping, about $4. Anything that has to open again: the sash meeting rail, the edge of a casement. Press it into the channel with the open side facing the draught. Cut it a centimetre long and compress it into place rather than stretching it to fit — stretched vinyl relaxes over one winter and the gap comes straight back, which is why this repair has a reputation for not lasting.

Static joints under 6mm — paintable acrylic-latex caulk, about $6. The frame-to-wall joint, the sill, skirting gaps. Smooth it with a wet fingertip within a minute, before it skins.

Use acrylic-latex, not silicone, anywhere you intend to paint. Silicone rejects paint permanently — it beads up and will not take a coat however carefully you prepare it. That single choice is the difference between an invisible repair and a shiny line across your trim forever.

Gaps over 6mm — low-expansion foam, then trim and caulk over. Use low-expansion specifically: standard foam can bow a frame as it cures.

When to stop and call someone

If the frame is soft when you press a screwdriver into it, if the glass moves in the sash and the putty is crumbling, or if you can see daylight through a crack in the frame itself, none of this will hold. Rot does not take caulk, and sealing over it traps moisture and makes it worse.

My repair came to eleven dollars and about an hour, most of it spent easing the trim off without splitting it. February came in at a hundred and sixty. March was a hundred and ten.

The part I still find hard to accept is that three professionals with meters missed it, and a retired joiner with a candle found it in four minutes — because he was the only one who asked what had changed.