Quick answer
Draughts through a composite door come from the perimeter seal, the threshold, or a hardware gap - never from the slab itself, which is essentially airtight once manufactured. Trace the draught to its exact point before assuming the whole seal set needs replacing; most jobs turn out to be a single corner or the letterplate.
Overview
The slab of a composite door is essentially airtight - 44mm timber core, polyurethane foam and GRP skin. Any draught is coming from the perimeter seal, the threshold, or a hardware opening (letterplate, cylinder gap).
Symptoms
- Draught felt when standing near the door.
- Curtains moving in a closed room.
Common causes
- Perimeter seal.
- Threshold seal.
- Letterplate brush.
- Cylinder not flush.
The slab is airtight - the perimeter isn't
A composite door slab is a sealed sandwich: GRP or PVC skin, rigid foam fill, timber or foam core. There's no realistic path for air to pass through the slab material itself. Every genuine draught complaint we've investigated has traced back to the perimeter - the seal, the threshold, or a gap around hardware.
That's a useful thing to know before spending money, because it narrows the search dramatically. You're not looking for a mystery leak through solid material; you're looking for a specific, findable gap around the edge of the door or at one of a handful of known weak points.
The four places draughts actually come from
In order of how often we find them: the compression seal (worn, torn, or under-compressed at one point), the threshold gasket (flattened by years of foot traffic), the letterplate (no brush, torn flap, or oversized aperture), and the cylinder or handle spindle (a gap where the hardware doesn't sit flush against the slab).
Each has a distinct feel. Seal draughts are usually a thin, consistent line felt with the back of a hand moved slowly around the frame. Threshold draughts are felt low down, often with a visible cold patch on the floor just inside the door. Letterplate draughts are obvious and localised - hold your hand near the flap on a windy day.
- Compression seal - thin line around the frame
- Threshold gasket - cold felt at floor level
- Letterplate - localised, worse in wind
- Cylinder/spindle gap - small but persistent
Testing methods that actually work
The candle or incense stick test remains the most reliable DIY diagnostic - lit smoke will visibly bend or flicker at even a small leak that your hand might miss, especially in a warm hallway where air currents are already moving. Run it slowly around the entire perimeter, including corners, before making any assumptions about where the problem is.
A second useful check: close the door on a strip of paper at several points around the frame and try to pull the paper out. If it slides free easily, compression isn't reaching that point. This works especially well for confirming a single weak corner rather than a seal that's failed all the way round.
Why draughts often appear seasonally
Rubber and TPE seal compounds stiffen slightly in cold weather, which can either improve or worsen a marginal seal depending on how it was sitting beforehand. More significantly, wind-driven pressure differences mean a seal that's adequate in still air will leak visibly in a gale - which is exactly why Cornish coastal properties report draughts far more often in winter storms than in summer.
If your draught only appears in strong wind and is absent on calm days, that's not a sign the seal is fine - it's a sign the seal is marginal, and it will get worse as it ages further rather than better.
Are draught excluders worth fitting?
A quality letterplate with an internal brush and a properly fitted flap is worth having and is a one-off, permanent upgrade. Stick-on foam or brush strips applied to an existing seal are almost always a temporary botch - they compress unevenly, peel in salt air, and rarely last a full season outdoors.
If a seal has genuinely failed, the correct fix is a push-fit replacement in the matching profile, not an add-on strip. We stock the common profiles used by the major composite door brands and can usually match on the day.
What a seal-and-compression service actually involves
We check the full perimeter for consistent compression, adjust the keeps or cams on the multi-point mechanism where the door has settled slightly, inspect and if needed replace the threshold gasket, and check the letterplate and cylinder for flush fit. Most draught complaints resolve with adjustment alone, with seal replacement needed less often than people expect.
For coastal or heavily exposed properties we'll also flag whether a heavier, storm-rated seal profile would be worth fitting - these hold compression better under sustained wind loading than the standard profile supplied on most doors.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Draught diagnosis callout | £75 | Waived if repair proceeds same visit |
| Compression adjustment | Included in callout or £75 standalone | No parts in most cases |
| Letterplate upgrade (brushed, flapped) | £45-£95 fitted | Permanent fix, not a stick-on strip |
| Seal replacement | £85-£150 | Full perimeter or single length |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Seal replacement.
Prevention
- Silicone-treat seals annually.
