The Letterbox

Two jobs, not one

The standard letterbox does almost nothing to stop air moving through it. The flap — that thin sprung plate on the outside — is there to keep weather out of the hall, and it does that adequately. What it does not do is stop the slow, cold transfer of air that happens when the flap sits imperfectly against its frame, or when there is no secondary barrier on the inside at all. Feel behind a letterbox on a cold day and you will usually find a small, reliable stream of cold air, even with the external flap closed.

There are two ways to address this, and ideally you use both.

The external flap. Check whether the existing flap seals evenly against the plate. Press it shut and hold a lit match or smoking taper close to the edge — the same technique used to find the gaps first elsewhere in a closed room. Any flicker tells you where the seal is failing. On most letterboxes the flap is held by a pair of small screws and can be lifted off. If the rubber or brush seal around the edge has hardened, compressed or gone missing, you can press a length of self-adhesive foam draught strip around the rebate before refitting the flap. Use the thinnest profile that still lets the flap close without forcing it: a flap that has to be pushed shut will not stay shut.

The internal brush seal. This is the more effective of the two fixes and the one most letterboxes are missing. It fits on the inside of the door, over the opening, and consists of a pair of nylon-bristle brushes mounted in a simple frame that screws directly to the door face. Post passes through the brushes easily; air does not. The frame is usually held by two or four screws and takes about fifteen minutes to fit without any particular skill. The only measurement worth checking beforehand is the size of the letterbox opening, since most frames come in one of two standard sizes. If you have an old internal brush seal already, check whether the bristles have splayed outward over time — once they splay, they no longer meet in the middle and the seal is gone.

Neither fix requires removing the door or any work beyond a screwdriver and a tape measure. Done together, they turn the deliberate hole in your front door from a reliable cold draught into something close to a real seal — and you will feel the difference in the hall the same evening.