Damp Around Window Frames: Causes & Diagnosis

The symptoms

Black mould or damp on the reveals and around the frame
Peeling paint and stained plaster near the opening
Worse in winter (suggesting condensation) or worse after rain (suggesting water ingress)
Soft or stained timber on the sill or frame

The building physics: two different mechanisms

The reveal — the wall surface returning to the window — is often a thermal bridge. The wall is thinner there, or the frame interrupts the insulation, so the internal surface runs cold. When that cold strip falls below the dew point of the room air, moisture condenses on it. That is condensation, driven by cold surfaces and indoor humidity, and it is worse in winter.

Separately, water can get in from outside: failed perimeter sealant or mastic, cracked render, a defective or missing sill, blocked weep holes, or poor flashing above the window. That is water ingress, and it tracks with rainfall rather than cold weather. The two look similar but need completely different remedies — which is why diagnosis matters before any work is done.

The likely causes

Cold, uninsulated reveals (a thermal bridge) condensing indoor moisture
Failed perimeter sealant or mastic letting water in
Cracked render, defective sills or blocked drainage above the window
Air leakage chilling the reveal further

How to tell which one you have

A useful first test is timing: condensation worsens in cold weather and high indoor humidity (and is often paired with window condensation), while a leak worsens during and after rain. But the two can coexist — a cold reveal that also has a minor external defect — so we confirm with thermal imaging (showing the cold bridge) and moisture readings (distinguishing surface condensation from water tracking through the wall).

Common mistakes homeowners make

Re-sealing the outside when the problem is internal condensation on a cold reveal
Repainting with 'anti-mould' paint without warming the surface
Assuming any window damp must be a leak

How RetrofitIQ investigates damp around windows

Thermal imaging of the reveals and frame to map cold surfaces and bridges
Surface-temperature and humidity readings to calculate the dew-point margin
Moisture readings to distinguish condensation from water ingress
An external inspection of sealant, render, sills and flashings
A diagnosis stating the actual cause, then a targeted remedy (insulated reveal, sealing, or both)

Common questions from clients.

How do I know if it's condensation or a leak?

Condensation tracks with cold weather and high indoor humidity; a leak tracks with rainfall. Thermal imaging and moisture readings confirm which mechanism is at work — and whether both are present.

Can a cold reveal really cause that much mould?

Yes — reveals are classic thermal bridges and are often the coldest surface in the room, so they condense and grow mould readily.

Will insulating the reveal lose much space?

Reveals are detailed with a thinner, higher-performance insulation to manage the thermal bridge without losing meaningful opening size. We specify this carefully.

My damp only appears after heavy rain — what does that mean?

That points to water ingress rather than condensation — a failed seal, cracked render, sill or flashing defect. We inspect the external detailing and confirm with moisture readings before specifying a repair.

Could it be both condensation and a leak?

Often, yes. A cold reveal can condense in winter while a minor external defect lets water in during rain. A proper investigation identifies all contributing mechanisms so the remedy addresses each.

Damp around window frames — condensation, leak or cold reveal?