Quick answer
A composite door letting in cold air almost always means the perimeter seal has lost compression, the threshold gasket has worn, or the multi-point mechanism isn't pulling the slab tight against the frame. The slab itself, with its foam core and GRP skin, is rarely the problem - the leak path is nearly always around the edge.
Overview
A composite door should be one of the warmest points in your envelope - U-values as low as 1.0 W/m²K are standard. If yours is letting in cold air, the seal, threshold or compression has failed.
Symptoms
- Cold spots near the door.
- You can feel a draught with the back of your hand.
- Heating bills up.
Common causes
- Failed weather seal.
- Compression cam not adjusted.
- Threshold seal missing or worn.
- Letterplate draught brush missing.
Why a well-made composite door shouldn't feel cold
A composite door slab is built around a 44-48mm timber or high-density foam core, wrapped in a moulded GRP or PVC skin, with the cavity filled with rigid polyurethane foam. That construction gives a slab U-value that's often better than the wall it's fitted into. Modern doorsets, glazing included, are commonly specified between 1.0 and 1.4 W/m²K, which is roughly half the heat loss of a typical timber-panel door from the 1990s.
When a customer tells us a composite door is 'letting in cold air', the door slab is almost never the cause - we can count on one hand the times a genuine slab defect has been the culprit in ten years of fitting and repairing doors across Cornwall. What fails is everything around the slab: the compression seal, the threshold, the letterplate brush, and the multi-point mechanism's ability to pull the door fully home against the frame.
That distinction matters because it changes the fix. A cold slab problem would mean a new door. A seal, threshold or compression problem is a repair that typically takes under an hour and costs a fraction of a replacement.
Finding the actual source of the cold air
Start with a simple hand test on a cold or breezy day: run the back of your hand slowly around the full perimeter of the closed door, including the top corners, the hinge side, and along the threshold. Cold air usually announces itself as a distinct line rather than a general chill - that line tells you exactly where the seal or compression has failed.
A lit candle or incense stick is more sensitive than a hand and will flicker or bend at even a small air leak. Run it around the same perimeter, paying particular attention to the corner diagonally opposite the top hinge - that's the point most affected when a door has dropped slightly or the mechanism no longer pulls evenly.
If the cold is coming through the glass rather than the frame, that's a different problem entirely - either the glazing unit's argon fill and warm-edge spacer are past their best, or you're looking at an older double-glazed unit that predates current thermal standards.
- Hand test around the full perimeter on a cold day
- Candle or incense stick test at suspect points
- Check the corner opposite the top hinge specifically
- Feel the glass separately from the frame
Compression, seals and why they fail in Cornwall specifically
The compression seal is a soft rubber or TPE gasket, usually running in a groove around the frame, that the door slab presses against when the multi-point mechanism locks home. Over time - faster in an exposed coastal position - that seal loses its spring. Salt-laden air degrades rubber compounds noticeably quicker than an inland equivalent, and UV exposure on south or west-facing doors accelerates the same process.
On top of material ageing, the hinges themselves can settle a millimetre or two over several years, particularly on heavier doors with large glazed panels. That small drop is enough to change compression at the opposite top corner, which is why so many cold-air complaints trace back to that one spot.
A keeper or cam adjustment on the multi-point mechanism can sometimes restore compression without any parts needing replacement at all, which is why we always check adjustment before quoting for new seals.
When the threshold is the real problem
Thresholds take more day-to-day punishment than any other part of the doorset - foot traffic, grit, salt spray, pressure washing. The gasket that seals the underside of the door against the threshold blade wears fastest of anything on the unit, and once it's compressed flat it stops sealing even though it still looks intact from a glance.
A worn threshold reads as cold air at floor level specifically, often accompanied by a visible polished or flattened track along the gasket where the door has been rubbing. This is a genuinely quick repair - most modern thresholds use a screw-fixed aluminium extrusion with a replaceable gasket insert.
What we do on a callout
A proper weatherproofing callout checks five things in sequence: seal condition around the full perimeter, threshold gasket condition, compression via keep/cam adjustment, letterplate brush and flap condition, and drainage slots at the base of the door (blocked drainage can, oddly, push draughts back in through displaced water). Most jobs are diagnosed and fixed inside 60-90 minutes.
We'll always tell you honestly if the fix is a five-minute adjustment rather than a paid repair - a compression tweak costs nothing if we're already on site for something else, and it's the single most common resolution to a 'cold air' complaint we see.
Cornwall-specific considerations
Exposed coastal properties - anywhere within a mile or two of open water, and certainly anything facing the prevailing south-westerlies - see faster seal and threshold wear than sheltered inland homes. We generally recommend an annual seal check for coastal properties rather than the two-to-three-year interval that's fine inland.
Holiday lets present a particular pattern: doors used constantly by different guests, with no single occupant noticing the gradual change, so problems often go unreported until a changeover cleaner or manager mentions it. A yearly service visit ahead of the summer season catches most of these before a guest experiences a cold snap.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Compression/cam adjustment | Often free on a call-out, £75 standalone | No parts needed in most cases |
| Seal replacement (per door) | £85-£150 | Depends on profile and length |
| Threshold gasket replacement | £90-£160 | Full threshold extrusion swap if worn |
| Full weatherproofing service | £120-£180 | Seals, threshold, compression, drainage, letterplate |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Seal or threshold replacement.
Prevention
- Annual seal inspection.
