How Much Heat Do You Lose Through a Letterbox?

A single letterbox may look insignificant, but it can be a notable source of heat loss through the front door: cold air infiltration, convective currents and thermal bridging combine to make letterboxes important to consider when improving a home's energy efficiency. This post explains how to estimate the heat loss, outlines practical trade advice and offers DIY guidance on choosing and fitting a suitable letterbox draught excluder.

How heat is lost through a letterbox

There are three main mechanisms by which heat is lost through a letterbox: conduction through the metal or plastic flap, convective heat transfer as warm indoor air escapes and cold air enters, and direct infiltration when gaps around the flap allow air movement. For most standard letterboxes the dominant loss is infiltration; conduction through the small metal flap is comparatively minor.

Estimating heat loss: a simple calculation

For a realistic estimate use the convective/infiltration approach: Q = rho × cp × Vdot × ΔT, where Q is heat loss in watts, rho is air density (≈ 1.2 kg/m3), cp is specific heat of air (≈ 1005 J/kgK), Vdot is the volumetric airflow through the opening in m3/s, and ΔT is the temperature difference between inside and outside.

Example scenarios, with ΔT = 15°C (inside 20°C; outside 5°C):

• Low draft: Vdot = 0.001 m3/s (1 l/s). Q ≈ 18 W.
• Moderate draft: Vdot = 0.003 m3/s (3 l/s). Q ≈ 54 W.
• Strong draft or windy conditions: Vdot = 0.006 m3/s (6 l/s). Q ≈ 108 W.

Putting that in annual energy terms: a continuous 20 W loss equals about 175 kWh per year (20 W × 24 h × 365 / 1000). Depending on local energy prices this can represent a modest but recurring cost. The key point is that a letterbox with poor sealing can leak tens of watts continuously; during windy weather the figure increases markedly.

What a letterbox draught excluder does

A letterbox draught excluder reduces Vdot by sealing gaps around the flap, adding resistance to airflow and sometimes providing a secondary internal flap or brush. Options include external or internal letter plates with soft seals, brush strips, sprung flaps and magnetic seals. For fire doors and some flat entrance doors there are specialised intumescent letterboxes that combine draught sealing with fire and smoke protection.

Practical trade advice

• Specify the right product for the door type: timber front doors, composite doors and steel fire doors all demand different letterplate solutions. For certified fire doors use only approved fire-rated letterplates with intumescent and smoke seals that do not compromise certification.
• Measure before ordering: measure the external opening width and the depth of the door leaf to ensure a letterplate or draught excluder will fit flush. Note internal trim clearance so the inner flap does not obstruct skirting or thresholds.
• Consider weather resistance: choose stainless steel or powder-coated finishes for coastal or exposed locations; ensure seals are UV resistant and perform in temperature extremes.
• Keep maintenance simple: brushes and seals wear with use. For communal entrances or high-use locations specify replaceable brush cartridges or modular seals to reduce long-term costs.

DIY guidance: how to fit a letterbox draught excluder

Tools and materials: screwdriver or cordless drill, measuring tape, pencil, appropriate screws, silicone sealant, letterbox draught excluder kit or brush/seal product. For timber or composite doors many draught excluders are retrofit and require no new cut-outs; for new letterplates you may need to remove the existing plate and screw in the new one.

• Step 1: Inspect the existing fitting. Note gaps, warped flaps and whether there is an internal draft plate. Remove the old plate if corroded or loose.
• Step 2: Measure the aperture and door thickness. Match the draught excluder product to those dimensions. If using a brush strip choose brush length to overlap the aperture by a few millimetres for a consistent seal.
• Step 3: Fit the external plate first to establish alignment. Use countersunk screws and ensure the plate sits flush; use a bead of neutral-cure silicone to prevent water ingress around the edges where recommended by the manufacturer.
• Step 4: Install the internal draught excluder or inner flap if supplied. Test the action by opening and closing the flap and passing a hand or tissue nearby to check for airflow.
• Step 5: For timber doors, pre-drill screw holes to avoid splitting. For steel doors use correct self-tapping screws or machine bolts as required. Avoid over-tightening which can warp the plate and create unintentional gaps.

Common product choices

• Brush letterplates: good for frequent use; brushes allow mail through while breaking direct airflow.
• Rubber or silicone seals on inner plates: effective at low airspeeds and simple to replace.
• Spring-loaded double flaps: provide a weather barrier and are effective against rain driven by wind.
• Magnetic seals: offer secure closure and neat appearance for modern doors.
• Fire-rated letterplates: mandatory for certified fire doors; include intumescent liners and smoke seals and must be installed to the manufacturer’s instructions.

Checks and compliance

For trade professionals be mindful of Building Regulations and fire safety requirements when altering an existing entrance door; any modification on a registered fire door can void certification and compromise safety. For flats or leasehold properties check with the managing agent or freeholder before modifying communal entrance doors.

Recommended Products

Based on this article, you may find these products helpful:

• In-Line Window Mechanism
• Offset Window Mechanism
• E-Type Gasket - Black 5m Roll
• E-Type Gasket - Black (Per Metre)
• B003 Bubble Gasket

Conclusion and next steps

A poorly sealed letterbox can leak a continuous tens-of-watts of heat depending on wind and the size of the gap; that equates to measurable energy loss over a year. Installing an appropriate letterbox draught excluder reduces infiltration, improves comfort and can cut heating costs. Next steps: assess the current letterbox for drafts using a smoke pen or tissue, measure the aperture and door thickness, then select a seal type appropriate to the door and usage. For fire doors or communal entrances consult product certification and building managers before fitting. For product options and specification details see our dedicated resource at /guides/letterboxes or contact our trade team for technical advice and supply.