How to Stop Cold Drafts from Your Letterbox UK

Cold drafts from your letterbox can chill your home in winter and raise heating bills as your system works harder to keep warm.

How to Stop Cold Drafts from Your Letterbox UK

Cold drafts from letterboxes enter your home through gaps in the letter slot and around the flap. To stop them, install a letterbox draught excluder or seal gaps with suitable materials inside the door.

Stopping drafts in your house starts at the door. Letterboxes are a common source of cold air because they are a direct opening to the outside. Sealing this gap reduces heat loss and improves comfort.

Begin by identifying the source of drafts around your letterbox. Check for gaps around the flap, worn seals, or a loose frame. Feel for cold air on a windy day by placing your hand near the slot inside the door.

1. Measure the Letterbox
   Measure the width and height of your letterbox slot. The standard UK slot is about 250mm wide and 38mm high, but sizes vary. Accurate measurement ensures the draught excluder fits properly.
2. Choose a Draught Excluder
   Select a type of draught excluder that suits your door and letterbox. Options include brush strips, magnetic flaps, internal cowls, or full cover plates. Each blocks drafts differently and suits different needs.
3. Prepare the Door Surface
   Clean the inside face of the door around the letterbox. Remove dust and grease so adhesives or screws will hold securely. If installing a cover plate, you may need to remove the existing letterbox flap.
4. Install the Draught Excluder
   Follow the manufacturer’s instructions. Brush strips attach inside the slot with screws or adhesive. Magnetic flaps fit over the existing flap and seal tightly. Full cover plates replace the flap entirely for maximum sealing.
5. Seal Around the Letterbox
   Use weatherproof sealant or foam tape around the edges of the letterbox frame inside the door to fill any remaining gaps. This prevents air leaking through cracks beyond the flap itself.
6. Check the Door and Letterbox Fit
   Close the door and test for drafts again. Adjust or add sealing as needed. If drafts persist around the door frame, consider replacing worn door weather seals or adding an under-door seal to stop drafts coming through the bottom.

Common Causes of Cold Drafts Through Letterboxes

Letterbox design contributes to cold drafts because it creates a direct opening through the door. The flap and slot allow outside air to enter when not sealed tightly.

Pressure differences between the inside and outside of the house drive air movement through the letterbox. Wind creates lower pressure outside, pulling cold air in through gaps.

Drafts often come from gaps around the letterbox flap, a loose or warped door, or worn seals around the frame. These allow cold air to flow freely inside.

To find where cold drafts come from in your house, close all doors and windows and feel for air movement around door edges, keyholes, and letterboxes. A smoke pencil or incense stick held near gaps will show air flow direction.

1. Locate Draft Sources
   With the door closed, run your hand or a lit incense stick along edges of the letterbox, door frame, and keyhole to detect cold air.
2. Check Door Alignment
   Inspect door hinges and frame alignment. A door that doesn’t close tightly can cause drafts around the letterbox and other gaps.
3. Inspect Seals and Weatherstripping
   Look for cracked or missing weather seals on the door edges and bottom. Replace or add an under-door seal to block drafts coming in at the bottom of the door.

If drafts come from multiple points around the door, consider a full door weather seal replacement or hinge adjustment to improve the overall fit and reduce air leakage.

Types of Letterbox Draught Excluders

Letterbox draught excluders come in several types, each suited to different door materials and draft levels. The most common are brush strips, magnetic flaps, internal cowls, and full cover plates.

Brush strips attach inside the door around the letter slot. They block 70–80% of drafts by sealing gaps where air enters. However, they wear down over time as the bristles flatten and gaps appear. Brush strips work best on wooden or composite doors, not UPVC.

Magnetic letterbox draught excluders have a flap that seals completely when closed. They can be fitted inside or outside the door and provide near 100% draft blockage. Magnetic flaps suit UPVC doors well because they attach without drilling and resist weather better than brushes.

Internal cowls are curved covers fitted inside the door that redirect airflow and reduce drafts. They are effective but can make mail delivery stiffer. Full cover plates are solid plates with hinged flaps covering the entire letterbox opening, blocking all drafts. These plates slightly hinder mail delivery but offer the best seal.

Letterbox weather stripping is another term for brush strips or foam seals installed around the letter slot edges to reduce drafts. To stop drafts from a mailslot in a door, choose a magnetic flap or full cover plate for the strongest seal.

For UPVC doors, magnetic letterbox draught excluders are the best choice. They attach securely without damaging the door, withstand weather, and seal tightly against drafts.

Installation Guide for Letterbox Draught Excluders

Installing a letterbox draught excluder depends on the type chosen but generally requires only basic tools. You will need a tape measure, screwdriver or drill, cleaning cloth, and sometimes adhesive or screws.

1. Measure Your Letterbox
   Measure the width and height of your letterbox slot. A standard UK letterbox is about 250mm wide and 38mm high. Accurate measurements ensure a snug fit for the excluder.
2. Prepare the Door Surface
   Clean the inside face of the door around the letterbox with a damp cloth. Remove dirt and dust so adhesives or screws hold firmly.
3. Fit Brush Strips or Weather Stripping
   Cut the brush strip or foam seal to size. Peel off backing if adhesive or screw in place around the letterbox edges. Press firmly to secure.
4. Install Magnetic Flap
   Align the magnetic flap over the letterbox opening inside or outside the door. Attach with screws or adhesive pads included. Ensure the flap closes fully and seals against the door.
5. Attach Internal Cowl or Cover Plate
   Position the cowl or cover plate inside the door. Screw it in place, making sure it covers the letter slot completely. Check the flap moves freely for mail delivery.

Maintenance is simple: check the excluder annually for wear or damage. Replace brush strips when bristles flatten or gaps form. Clean magnetic flaps and hinges to keep them sealing tightly. Reapply adhesive if parts loosen over time.

If you find drafts around the whole door, not just the letterbox, consider replacing worn door weather seals or adjusting door hinges to improve the overall seal.

Following this guide will help you fit the right draught excluder securely and maintain it for long-term effectiveness, improving your home’s warmth and comfort.

Cost-Benefit Analysis of Draught-Proofing Your Letterbox

Sealing drafts from your letterbox can save around £30 to £60 a year on heating bills for a typical UK home. The Energy Saving Trust estimates that draughts, including those from letterboxes, waste about £60 annually in heat loss. Letterboxes are a direct gap in your door, so blocking that gap reduces cold air entering and warm air escaping.

Initial costs for draught-proofing a letterbox vary by method. A brush strip or magnetic draught excluder costs between £10 and £30 and typically lasts several years. A full cover plate with a hinged flap may cost £20 to £40 but blocks 100% of drafts. Installation is usually simple and requires only basic tools.

Long-term benefits outweigh the upfront expense. Savings on heating bills add up over time, reducing your energy use and carbon footprint. Draught-proofing also improves comfort by eliminating cold spots near the door. Most letterbox covers and draught excluders do not interfere with post delivery, but full covers with stiff flaps may require slightly more effort for the postman.

When to Replace Your Letterbox Instead of Repairing

Replace your letterbox if it is damaged, corroded, or no longer fits the door properly. Cracks or gaps around the frame that cannot be sealed indicate replacement is necessary. Also consider replacement if the flap is warped or broken, causing it to flap in the wind or fail to close tightly.

Fix a flapping letterbox flap by checking the hinges and spring mechanism. Tighten loose screws or replace worn springs. If the flap is warped, a replacement flap or a new letterbox may be the best solution to stop drafts and noise.

Upgrading to a modern, draught-proof letterbox model improves energy efficiency and security. New designs include magnetic flaps, internal cowls, and anti-theft locks. External letterboxes with locks keep mail safe and reduce the risk of theft, especially useful in high-traffic areas.

Replacement is better than repair when the existing letterbox cannot be sealed effectively or the door around it requires work. Installing a new letterbox designed for draught-proofing and security provides a long-term solution, reducing maintenance needs.

For a letterbox on a UPVC or composite door, use manufacturer-approved replacement parts to maintain door integrity and warranty. If unsure, consult a professional for fitting or door adjustments to prevent drafts around the frame.

If you want to avoid replacement, consider adding a telescopic letterbox cover or internal cowl that fits inside the door. These can reduce drafts without removing the existing letterbox, offering a compromise for rented homes or temporary fixes.

For more on fixing door drafts beyond the letterbox, see How to Fix a uPVC Door Leaking Cold Air.

Frequently Asked Questions