A cold line of air beside the front door may come through the letterbox even when the door seals are sound. Before buying another strip for the frame, watch what the mail flap does after an envelope passes through it. A flap that stays slightly open is a small fault with a very noticeable effect.
The Energy Saving Trust’s guidance on small energy-saving jobs includes letterbox brushes or flaps and recommends measuring before purchase. Fit matters: an attractive cover that catches the existing flap may leave a larger opening than the one you started with.
Check the moving parts first
With the door secure, inspect the opening for trapped paper, damaged brushes, loose components, or a flap rubbing against its surround. Follow the product’s maintenance instructions. Do not force a spring mechanism or assume that adding lubricant is appropriate for every finish and hinge.
Measure the opening and the space available inside the door. Note nearby handles, glazing, mouldings, and anything that could prevent an internal cover from moving. A photograph with the measurements written separately helps when comparing fittings.
For a rented property or a door with a fire-resistance requirement, obtain the appropriate permission or professional advice before drilling or replacing components. A seemingly minor fitting is still part of the door assembly.
A simple acceptance check
After a suitable repair or installation, try a few representative pieces of mail while another person observes from inside. The flap should move freely and return to its closed position without help. Check a thin envelope as well as a thicker item that the letterbox is designed to accept; do not force oversized packages through.
Write down the fitting model and installation date. If the draught returns, that note makes it easier to find compatible brushes or understand what has changed.
Do not block deliberate ventilation elsewhere to make the house feel sealed. The aim here is a functioning letterbox with reduced unwanted leakage. A repair that prevents deliveries or leaves a flap wedged open has missed the practical purpose, however tidy it looks from the hallway.