Letterboxes are one of the most underestimated sources of heat loss in a home. Because they sit directly in the front door, they create a direct opening between the indoors and the outside. On colder days, even a small gap around the flap allows cold air to move in and warm air to escape.
Older letterboxes are particularly prone to this, as they often lack proper brushes or internal covers. Even when the flap appears to close fully, air can still pass through the edges. This steady movement of cold air affects the temperature of nearby rooms, especially hallways, which is why entrance areas often feel colder than the rest of the house. That issue is explored more fully here: Why Your Hallway Always Feels Colder Than the Rest of the House.
Adding a brush seal or an internal cover significantly reduces this airflow without affecting how the letterbox is used. Once the opening is better sealed, the area around the front door tends to feel less exposed, and the home retains heat more effectively.
If heat loss near the entrance is still noticeable after sealing the letterbox, it’s worth checking whether warm air is escaping elsewhere at floor level, which can compound the problem. This is often linked to gaps under doors, explained here: Why Heat Escapes Under Doors.
When combined with sealing other draught points around the house, improving the letterbox helps the heating system work more efficiently overall. For a wider approach to improving heat retention, this guide brings everything together: How to Keep a UK Home Warm for Cheap (Complete Guide).
