Quick answer
A composite door not sealing properly lets in daylight, draughts, and sometimes rain, and is usually caused by the handle not being lifted before locking, an uneven reveal from misalignment, or a perished weatherseal. It's diagnosed with a simple paper-slip test around the frame and fixed with adjustment or seal replacement, not sealant or tape.
Overview
A properly-sealed door compresses its weather seal all the way round when the handle is lifted. If it isn't sealing, either the handle isn't being lifted before locking, the door is misaligned, or the seal itself has failed.
Symptoms
- Daylight visible around the door.
- Draughts.
- Whistling in wind.
- Rain penetration.
Common causes
- Handle not being lifted (multi-point locks compress the seal).
- Misalignment leaving a gap on one side.
- Seal perished, torn or missing.
- Keeps not deep enough.
How a composite door is actually supposed to seal
Composite doors rely on compression sealing: lifting the handle drives the multi-point mechanism's hooks and bolts outward, which pulls the entire door slab tighter against the compression seal running around the frame. A door that's simply pushed shut without the handle being lifted is not sealed at all, regardless of how good the seal itself is — this is the single most common reason we're called to a 'sealing problem' that turns out to be user habit rather than a fault.
This is worth understanding because it changes the whole diagnostic approach: before assuming the seal or door is faulty, the very first check is whether the handle is being lifted fully every time the door is locked, particularly by anyone in the household who might not know this is necessary — children, guests, or holiday let visitors.
The paper test, and what it tells you
A simple way to assess where a door isn't sealing is to close it — with the handle lifted and locked — on a sheet of ordinary paper positioned at various points around the frame, then try to pull the paper out. Where it slides out easily, the seal isn't compressing properly at that point; where it grips firmly, the seal is doing its job.
Doing this at eight or so points around the door — both top corners, both bottom corners, and the midpoints of each side — builds a map of exactly where the sealing failure is, which tells an installer immediately whether the problem is isolated to one area (pointing to localised seal damage or misalignment) or general all the way round (pointing to compression setting or a seal that's simply worn out everywhere).
- Paper pulls out easily at one corner only: localised misalignment or seal damage.
- Paper pulls out easily all the way round: compression cam needs resetting, or seal needs full replacement.
- Paper grips everywhere except where daylight is visible: check for a physical gap or tear rather than a compression issue.
Perished seals: what they look like and why they fail
Weatherseals typically last 8 to 15 years, but that range narrows considerably in exposed coastal conditions where UV exposure and salt air both degrade rubber compounds faster than inland, sheltered installations. A perished seal shows visible cracking, a hardened or brittle texture where it should be soft and pliable, and sections that have pulled away from their retaining groove.
Once a seal has hardened, it stops rebounding after compression, meaning even a correctly-set door with good compression pressure will still let draughts through simply because the seal material itself can no longer do its job. This is a straightforward replacement, but it needs the correct seal profile for your door manufacturer — generic seal isn't interchangeable across brands.
Why sealant or draught tape is the wrong answer
It's tempting to reach for a tube of silicone sealant or a roll of foam draught tape when a door is letting in cold air, but both are poor solutions on a composite door. Sealant applied around a moving door will tear or crack within weeks and can gum up the compression mechanism if it gets into the wrong place. Foam tape changes the compression distance the mechanism was engineered for, which can prevent the hooks from fully engaging their keeps — turning a draught problem into a security problem.
The correct fix is always to address the actual cause: adjust the compression setting on the hinges, replace the specific seal section that's failed, or correct the misalignment causing an uneven gap.
Wind-driven rain: Cornwall's specific sealing challenge
Cornwall's coastal exposure means many properties experience genuinely wind-driven rain — water being pushed horizontally into a door rather than simply falling on it — which is a much more demanding test of a seal's condition than draught alone. A seal that's just good enough to stop a draught in still conditions can still allow water ingress during an exposed-coast storm if it's even slightly compromised.
For properties on very exposed elevations, we sometimes recommend an upgraded double-seal configuration or additional drainage checks at the threshold alongside standard seal maintenance, specifically because the standard seal spec assumes less extreme weather exposure than parts of the Cornish coast regularly see.
A case from a coastal cottage near Portreath
A cottage on the exposed north coast near Portreath had a persistent draught along the top of the door that the owner had tried to fix twice with draught excluder tape, with no lasting improvement. A paper test showed the seal gripping fine everywhere except the top third of the latch side, where the seal itself had visibly hardened and cracked from years of direct salt-air exposure. Replacing that seal section, rather than adjusting the door itself, solved it completely.
Frame fixings, packing and their role in seal performance
A weatherseal can be in perfect condition and a door still fail to seal properly if the frame it closes against has moved even slightly out of true. On granite and cob openings where the frame relies on packers behind its fixings to sit square, any settling of those packers over the years changes the compression the door applies to the seal at that point, leaving a gap that no amount of seal replacement alone will close. We check frame square and packer condition before recommending a new seal, because replacing a seal against a frame that's shifted just relocates the same draught rather than curing it.
Seasonal timing for spotting and fixing seal failures
Seal failures are easiest to diagnose in the depths of winter, when wind pressure differentials across the door are at their highest and any gap makes itself known as an audible whistle or a cold draught you can feel with a hand run along the frame edge. Conversely, seal replacement work is best carried out in a dry, mild spell if possible, since fitting new seal into a wet or very cold frame can affect how well the adhesive backing (used on some seal types) beds in. We often recommend a late-autumn check specifically so any seal issue is caught and fixed before the worst of the winter storms arrive.
How a failing seal progresses if it's left unaddressed
A seal that's lost a little compression rarely stays that way — once wind and water start finding a small gap, the seal edge itself is exposed to moisture it wasn't designed to face directly, which speeds up the perishing process further. Rubber and EPDM seals harden and crack once regularly wetted at an exposed edge, so a minor draught in November can become a seal that's visibly split by the following spring, and by then water may already be tracking behind it into the frame timber.
In Cornwall's wetter coastal micro-climates this progression is often faster than inland, and we've seen seals that would have lasted eight to ten years inland fail within five or six years on very exposed elevations, simply from the volume of wind-driven rain they're dealing with each winter.
Holiday-let implications of a door that isn't sealing
For a holiday let, a door that doesn't seal properly shows up fastest as guest complaints about draughts or noise, often during exactly the weeks the property should be earning its best rates. Beyond comfort, a poorly sealed door lets damp air into the porch or hallway, which over a season can contribute to condensation and mould complaints that are far more costly to resolve than the seal itself. We'd recommend including seal condition explicitly in any changeover or seasonal maintenance checklist for a let property, since it's one of the easiest faults to miss on a quick visual check but one of the most noticeable to guests.
Guarantee position on seal replacement
Weatherseals are a wear item and are typically covered for a shorter period than the door slab itself, often two to three years, and cover usually assumes normal use rather than damage from being propped open or exposed to unusually harsh conditions. Once outside that period, seal replacement is standard maintenance work rather than a warranty claim, and as a CERTASS registered installer we always match the replacement seal profile to the original so compression and appearance stay consistent with the rest of the door.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Paper test diagnosis and compression check | £65–£90 | Establishes whether it's habit, seal, or alignment |
| Seal section replacement | £80–£150 | Common where seal is localised damage or age-related |
| Full seal replacement (all-round) | £150–£260 | Where seal has perished throughout |
| Compression cam recalibration | £90–£140 | Where the door and seal are fine but pressure is wrong |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Seal replacement.
- Keep adjustment.
Prevention
- Always lift the handle before locking - engages the compression.
- Silicone-treat seals annually.
