What a cold spot on a wall is – By double glazing Edinburgh professionals
A cold spot is any part of an internal surface (a wall, a ceiling, the plaster above a door) that sits cooler than the area around it because heat escapes through it faster.
On its own, a cool bit of plaster wouldn’t matter much.
The trouble starts when warm, moist air from cooking, showering, drying washing and breathing drifts across that cold surface.
Warm air holds more water than cold air can, so as it cools against the wall it reaches its dew point and condensation forms as tiny droplets on the plaster.
How a cold spot turns into mould
The film of moisture is usually too thin to see, so all you notice is a wall that ends up slightly damp, day after day.
Mould spores are in the air of nearly every home, and a surface that stays damp for a few days is all they need to grow.
Cold spots create that damp surface reliably, so the mould growth comes back to one patch of wall and nowhere else.
Left long enough, condensation and mould growth on a bedroom wall bring real health risks, especially for anyone with asthma or a weak chest.
What causes cold spots on internal walls
Four things cause cold spots in nearly every Scottish home, and they rarely turn up in equal measure.
Cold bridging at lintels, corners and junctions
A cold bridge, which the building trade also calls a thermal bridge, is a path of solid material running from the cold external wall straight to the warm inside surface. Heat travels out along it.
Concrete or steel lintels above windows and doors are the classic example, and the lintel is the reason the plaster directly above a window so often has its own tidemark of mould.
Corners are the other usual suspect, since two external walls meet there and the inside surface has more cold outside face pulling heat away from it.
Those cold corners are where a lot of bedroom mould starts.
Missing or failed insulation
Missing insulation does the same job as a cold bridge, only over a wider area.
Cavity wall insulation that was blown in patchily, or that has slumped and got wet over the years, leaves bands of wall with nothing behind them.
Each band stays cold while the rest of the wall warms up.
Loft insulation pushed back from the eaves does the same to the top edge of the upstairs walls.
Older solid wall properties make up a large share of Scotland’s housing and have no cavity at all.
Unless something has been added, the whole outer wall behaves as one large cold wall.
Windows and doors
Windows and doors are the weakest point in any wall for heat loss, and around them is where you’ll find the cold spots a window company can do something about.
Single glazing runs so cold on the inside that it’s a cold spot in its own right.
A badly fitted frame leaves gaps around the perimeter that let cold air straight into the reveal, and the plaster there never gets a chance to warm up.
The boxed-in area beneath bay windows is often uninsulated and particularly cold, and the timber under the sill is usually the first place damp shows.
Poor detailing around the frame, with nothing packed into the gap and just a bead of sealant over the top, was common practice in the eighties and nineties.
Those windows are now old enough for the sealant to have failed as well.
Damp coming through the wall
A wet wall is a cold wall, because water carries heat through masonry far faster than dry brick does.
Penetrating damp from external defects such as cracked render, a leaking gutter or a failed pointing joint soaks the outside wall and chills the inside of it.
Rising damp from a failed damp-proof course does the same along the bottom metre of ground-floor walls.
Where damp and mould appear together low down or in a distinct band, a proper damp and timber survey is the right first step, not more insulation.
How to find cold spots in your home
Checks any homeowner can do
Your hand is a decent enough instrument on a cold day with the heating on. Run your palm slowly along the outside-facing walls, into each corner, around windows, around doors and across the ceiling near the eaves.
You’re looking for a cold patch that’s noticeably cooler than the plaster a foot away. If it also feels slightly damp, you’ve found your problem.
Furniture pushed hard against an external wall traps a pocket of still, cold air behind it, so pull the wardrobe and the bed away and check what’s there.
Faint grey speckling in the top corners, a tidemark above the window, or condensation beading on the wall itself all point to a cold area.
When to bring in a thermal camera
A surveyor with a thermal camera can walk through and show you all the cold spots in a property as blue patches on a screen. The thermal image picks up the ones behind plasterboard that you’d never feel by hand.
If the pattern points to a failed cavity or damp instead of a window or frame, that’s the time to book a damp survey.
Preventing cold spots with heating and ventilation
Better heating and ventilation won’t prevent cold spots by themselves, but they stop condensation forming on them while the proper repair gets organised. In plenty of rooms that alone keeps the mould away.
Preventing condensation by drying the air
Condensation will form on any surface that drops below the dew point of the air touching it, so cutting the amount of moisture in the air drops that dew point.
Lids on pans help, and so does running the extractor while you cook and for ten minutes after, keeping the bathroom door shut with the fan on during a shower, and drying washing outside or in a room with the window open.
Where none of that is practical, a dehumidifier in the worst room pulls litres of water out of the air every week and cuts the mould risk noticeably.
Keep the wall warm
Setting the thermostat low and leaving the central heating on steadily through the day keeps the internal surface of the wall above dew point.
Blasting it for two hours in the evening and letting the house go cold overnight does not.
If a radiator sits under the window, keep the curtains clear of it so the heat reaches the reveal.
And open the trickle vents. Ventilation feels like the enemy when you’re paying to heat the house, but stale, damp air trapped inside is what allows condensation to form on the walls. A little cross ventilation each day costs far less heat than you’d expect.
Adding insulation and treating damp
Where the insulation goes
Where the cause is a failed cavity, an insulation gap or a solid wall, adding insulation is the permanent answer.
Topping up loft insulation and pushing it right to the eaves costs little and often sorts the upstairs corners on its own. A cavity fill that has slumped can be extracted and refilled.
A solid-walled house needs either internal or external insulation, and external is the better job because it wraps the cold bridges as well as the walls and helps prevent heat from escaping across the whole face of the building.
Lining the one problem wall from inside is the realistic option in a flat or a tenement, though, and it still cuts the energy bills for that room.
Targeted insulation on a small area needs a bit of care, because insulated plasterboard over one corner does reduce cold on that wall, but the edge of the board can create a new, sharper cold line where the board stops.
Dry a damp wall before you insulate it
Where the wall is damp as well as cold, dry it first.
Fix the gutter, the render or the pointing, let the wall dry over a season, and only then insulate.
Trapping a wet wall behind a new lining makes the problem worse.
Preventing cold spots around windows and doors
The fix for a cold spot at a window or a door is either the glazed unit itself or the way it was fitted.
The glazing
A modern uPVC window with an A-rated double-glazed unit (high on the window energy rating scale), argon gas between the panes and a warm-edge spacer bar has an inside pane that stays close to room temperature.
The condensation risk on the glass drops, and the reveal beside it warms up.
The standard double glazing we fit across Scotland has a whole-window U-value of 1.4 W/m²K, which is the level Scottish building regulations set for replacement glazing.
A U-value is the rate heat passes through the unit, and lower is better.
Our triple glazing takes that down to 0.8 W/m²K, and in an exposed north-facing room or a bay that extra step is often what finally clears the cold spot.
The frame and the fitting
The frame matters as much as the glass, because a good unit fitted badly still leaves a cold bridge around the perimeter.
A window fitted properly has the gap between the frame and the masonry packed with insulation and sealed inside and out, so cold air can’t get into the reveal. The box beneath a bay window sill can be insulated at the same time as the window is replaced.
Doors
Doors follow the same rule, and a composite or uPVC door with a properly insulated slab and a sealed frame stops the hallway being the coldest space in the house, along with the mould that grows above an old timber front door.
Our energy-efficient windows and uPVC doors come with a 15-year guarantee, and composite doors carry a 10-year guarantee on the door itself.
Questions about cold spots, condensation and black mould
What does a cold spot on a wall mean?
A cold spot means heat is leaving your home through that bit of wall faster than through the wall around it. Either something is bridging the heat out, such as a lintel, a corner or an uninsulated frame gap, or the insulation that should slow it down is missing, failed or wet. Until the cause is fixed, the spot stays damp and the mould keeps growing.
How do you avoid cold bridging?
Avoiding a cold bridge means putting an unbroken layer of insulation between the cold outside and the warm inside, with no solid material running straight through it. On an existing house that means insulating window and door reveals, packing the frame gap when a window is fitted, and wrapping corners and lintels from outside if the walls are being clad. You can’t remove a concrete lintel, but you can stop it touching the warm side.
Why does mould keep coming back after I clean it?
Cleaning removes the growth but leaves the condensation problem underneath it, and spores land on the damp surface again within days. Anti-mould paint buys a few months at most. Until the wall behind the mould stays above dew point, moisture will condense on it again and the mould will return to the same spot every winter.
Why is the corner of my room always cold?
Corners where two outer walls meet lose heat in two directions at once and have less warm room air moving across them, so they run cooler than a flat wall in the same room. Furniture wedged into the corner traps still air and makes it worse. If the corner also has mould and condensation on it, the cause is usually a cold bridge at the junction or a gap in the cavity insulation right at the corner.
When do I need a damp survey instead of new insulation or windows?
Book a damp survey when the cold patch is low down on a ground-floor wall, shows a clear tidemark, or sits below a suspected external defect like a leaking gutter or cracked render. Those patterns point to water getting into the wall from outside, and no amount of ventilation or insulation will dry it. One above a window, around a frame or in an upstairs corner is far more likely to be the window, the lintel or a gap in the cavity.
Warm up the coldest room in the house this winter
If the cold spot in your home sits around a window or a door, a free survey is usually enough to tell you what’s causing it and what it will take to fix.
We’ve been fitting energy-efficient uPVC windows and doors across Central and Eastern Scotland since 1988, and our 0% finance means the fix doesn’t have to wait for spring.
Book a free home survey or call us on 0131 370 0444, and let’s get that corner warm before the mould comes back for another winter.
