Brush and flap seals wear out
Most modern letterplates have either a brush seal, soft bristles that close around whatever passes through, or a spring loaded internal flap, or both. Bristles flatten and gap over years of post, hands and the odd cat, and internal flaps lose their spring tension the same way any moving hardware does.
You can usually see wear by looking closely at the brush from inside: gaps, flattened patches, or missing sections along the strip all let air straight through. This is a wearing part, not a manufacturing fault, and it is a cheap, straightforward swap.
Plate size versus letter size
Letterplates come in a small number of standard opening sizes, and some are cut generously large to accept bulky post or magazines. A larger opening lets in more draught for the same seal quality, simply because there is more gap to cover. If your letterbox seems unusually draughty compared with a neighbour's similar door, differing plate sizes are often the reason.
| Symptom | Likely cause |
|---|---|
| Draught only right at the letterbox | Worn brush or flap seal, or an oversized plate |
| Cold air whistling on windy days specifically | Gaps around the plate's fitting, not just the flap |
| Draught all round the door, not just the letterbox | Door seal or hinge alignment issue as well |
| Post gets stuck or damaged going through | Plate opening too tight for what you receive |
Fitting an internal cowl or box
An internal letterbox cowl, a small fabric or plastic tunnel fitted on the inside of the door, catches post while creating a second barrier against draughts and is one of the simplest upgrades for an older letterplate that is otherwise sound. They are inexpensive and fit most standard plates without replacing the whole letterbox.
- Check the brush or flap for visible gaps or flattening
- Compare your plate's opening size with what you actually receive through it
- Consider an internal cowl if the plate itself is in good condition
- Replace the whole letterplate if the seal cannot be renewed on its own
When it is really the door, not the letterbox
If you feel a similar draught along the top or sides of the door frame, not just around the letterbox, the letterbox is unlikely to be the whole story. A worn door weather seal, a dropped door no longer closing tightly, or a hinge out of adjustment can all let air through the main body of the door, and no amount of letterbox work will fix those separately. Feeling around the whole door on a windy day, rather than just at the letterbox, is the quickest way to tell which you are dealing with.
Letterbox brushes: inside or outside?
Brush strips and cowls are fitted on the inside face of the door. The outer flap keeps rain off; the inner brush stops the air movement that makes the flap rattle and lets cold air in. Fitting brushes outside exposes them to weather and wears them out quickly.
When the letterbox isn't the whole problem
If the flap still moves on a still day, air is being pushed through the house from another gap, often a worn door seal or a gap around a frame. Check the door edges with the back of your hand on a windy day; a draught along the edges means the door seals need attention too.
To stop a letterbox flapping in the wind, fit an internal brush seal or draught cowl behind the flap, or replace the flap with one that has a stronger return spring. Brushes go on the inside of the door, where they close the gap without catching post.
Useful pages on this site
Cold air coming through the letterbox?
Tell us what your letterbox looks like now and whether the draught is only there or all round the door, and we can point you to the right fix.