Quick answer
Condensation on the inside face of a composite door is almost always a household humidity issue, not a fault with the door - the fix is ventilation, not replacement. Condensation appearing between the panes of the glass, however, means the sealed unit has failed and needs replacing; the two problems look similar but have completely different solutions.
Overview
Condensation on the inside face of a composite door is usually a humidity problem in the hallway, not a door problem. The exception is condensation inside the glazing unit - that's a failed sealed unit needing replacement.
Symptoms
- Water droplets on inside of door.
- Mould near the door.
- Misting inside the glass.
Common causes
- High indoor humidity.
- Cold door surface + warm humid air.
- Failed sealed glazing unit.
Two completely different problems that look the same
Water droplets on a door surface can mean one of two very different things, and telling them apart takes about ten seconds. Surface condensation sits on the inside face of the slab or glass and can be wiped away, only to reappear later. Internal condensation sits between the two panes of a sealed glazing unit and cannot be wiped away from either side, because it isn't on either surface you can reach.
Getting this distinction right matters because one is a household ventilation issue costing nothing to fix and the other requires a new glazing unit. We ask every caller to describe exactly where the moisture sits before booking a visit, because it changes what we bring with us.
Why surface condensation happens at all
Composite doors have a genuinely low thermal conductivity slab, but the surface temperature still sits close to the outside temperature on a cold night. When warm, humid indoor air meets that cooler surface, moisture condenses out - exactly the same physics as condensation on a cold glass of water. Hallways with poor ventilation, drying laundry nearby, or a kitchen door close to cooking steam are the most common culprits.
This isn't a sign of a faulty door. Even a very well-insulated door will show some surface condensation if the indoor humidity is high enough and the outdoor temperature drops sharply overnight, which happens often in Cornwall's damp maritime climate.
What to check before assuming it's the door
Ventilate the hallway or porch where the door sits - open a window nearby for ten minutes and see if the condensation reduces over subsequent days. Check whether trickle vents on nearby windows are open; many get painted shut or closed deliberately for draught reasons and that alone can push humidity up enough to cause visible condensation.
If you run an extractor fan in an adjoining kitchen or bathroom, confirm it's actually venting outside rather than into a loft space, which is a surprisingly common installation fault that raises whole-house humidity without anyone noticing.
- Ventilate the hallway for a few days and monitor
- Check trickle vents aren't painted shut
- Confirm extractor fans vent outside, not into the loft
- Wipe and re-check after 24 hours to see if it returns
When it's actually the glazing unit
A sealed glazing unit is two or three panes of glass with a spacer bar and desiccant-filled cavity, sealed at the edge and usually argon-filled for thermal performance. Once that edge seal fails, humid air migrates into the cavity, meets the cold outer pane, and condenses - permanently, because there's no ventilation route to let it out again.
This shows up as a fog or haze that persists no matter how much you wipe the visible surfaces, sometimes with visible watermarks or mineral staining once it's been failed for a while. There's no repair for a blown unit beyond drilling and venting, which is a temporary bodge - proper replacement is the only lasting fix.
Why coastal Cornwall sees more of both
Salt-laden air and higher ambient humidity generally mean Cornish properties run a slightly damper baseline than inland equivalents, which makes surface condensation more likely to show up even with reasonable ventilation habits. It also means UV and salt exposure age glazing unit seals somewhat faster, so blown units can appear a few years earlier than you'd expect inland.
Holiday lets are a particular case: intermittent heating between lets means the property cycles through cold, unheated periods followed by sudden warm, humid occupation, which is about the worst pattern for provoking surface condensation on any cold surface in the property, doors included.
What we do when we're called out
We inspect the door in situ, confirm whether the moisture is on a wipeable surface or trapped in the unit, and check ventilation provision around the doorway. If it's a household humidity issue, we'll advise on ventilation rather than sell you a repair you don't need. If it's a blown unit, we measure and order a replacement to match the existing bead and glazing spec.
Typical repair costs in Cornwall
| Work | Typical price | Notes |
|---|---|---|
| Diagnosis callout | £75 | Confirms surface vs internal condensation |
| Blown unit replacement (plain) | £120-£220 | Standard double-glazed unit |
| Blown unit replacement (decorative) | £180-£380 | Leaded, bevelled or patterned designs |
Guide prices only — we confirm the cost after seeing the door.
When to call a professional
- Sealed glazing unit replacement.
Prevention
- Trickle vents open.
- Extractor fans in kitchen/bathroom.
