Condensation and mould around windows: causes and solutions

The January phone call is almost always the same: “My windows are covered in water every morning and a black stain is forming in the corner, around the window. Is it the frames’ fault?” Sometimes yes, sometimes no. The honest answer is that condensation and mould arise from the meeting of three factors — the window, the installation, and the habits of the people living there — and understanding which of the three weighs the most is the first step to not spending money badly. In this article I explain how the phenomenon forms, how to tell a window problem from a ventilation problem, and in which cases replacement genuinely solves it.

What physically happens on cold glass

Condensation isn’t water “coming in” from outside. It’s water vapour present in the air inside your home that turns liquid when it touches a surface cold enough. The key figure is the dew point: the temperature below which air, at a given humidity, can no longer hold on to its vapour and releases it.

A purely indicative example, to be taken as an order of magnitude and not as a guaranteed value: indoor air at 20°C with relative humidity around 60% has a dew point of about 12°C. That means any surface in the room colder than ~12°C — single glazing, a metal frame without a thermal break, the corner of the wall behind a wardrobe — becomes a “trap” on which vapour settles. If humidity rises to 70%, the dew point climbs towards 14-15°C and condensation appears even on less-cold surfaces. These are example figures: they change with the actual temperature and humidity in your home.

Two practical consequences follow from this:

• The colder the surface, the more easily it gets wet. Single glazing or a cold frame are the first to condense.
• The more humid the air, the more easily everything gets wet. A home producing a lot of vapour with little air exchange will condense even with decent windows.

Black mould around the window is the next stage: where condensation lingers for a long time on plaster and silicone, and surface humidity stays high for days, spores find the ideal environment to take hold. Visible liquid water isn’t even necessary: it’s enough for the surface to stay above a certain humidity threshold for a long time.

The real causes, in order of frequency

When I carry out a site visit, the causes I find are almost always a combination of these. I’ve put them in a table with the “typical symptom” that gives them away, so you can do a first self-diagnosis.

Cause	Where you see it	Typical symptom
Single glazing or old/degassed double glazing	Water on the glass, especially at the bottom	Heavy condensation on the glass in the morning, dry or near-dry frame
Cold frame (aluminium without thermal break, old metal)	Water and mould on the profile, not just the glass	Frame ice-cold to the touch, drops along the glazing bead
Thermal bridge from installation (sub-frame, reveal, sill)	Mould in the corner of the wall around the window	Relatively dry glass but black wall at the edges
Poor airtightness of the gaskets	Draughts and irregular condensation	Curtains that move, high energy bills, but also cold draughts
Insufficient ventilation	Condensation spread across several windows and walls	Home that “smells stuffy”, high relative humidity everywhere
Excess vapour produced	Bathroom, kitchen, bedroom	Mirrors and glass steamed up after showers, cooking, sleeping

It’s worth dwelling on a distinction that makes the difference when deciding what to buy: where the water forms.

• If the water is on the glass, the main suspect is the glass itself (single glazing, or an ageing double-glazed unit with the gas fill by now dispersed) or high indoor humidity.
• If the water and mould are on the frame, the problem is the cold profile: a metal frame without a thermal break, or an old window with worn-out hardware and gaskets.
• If the mould is on the wall around the window, in the corner of the reveal or under the sill, then the glass and frame may well be fine: the problem is the thermal bridge from the installation, i.e. the point where the window meets the masonry. This is the most insidious cause because it often survives even after the windows have been changed, if the installation was done badly.

How to tell whether it’s the window or your ventilation habits

This is the question I get asked most, and it’s the right one: before spending money, it’s worth understanding where the problem really lies. Here’s a practical method you can carry out yourself over a few winter days.

1. Buy a cheap hygrometer (a few euros) and put it in the room that condenses the most. Measure indoor relative humidity in the morning, before opening anything.
2. Keep a diary for a week. Note the humidity, where the condensation forms (glass, frame, wall), and what you did the evening before (shower, drying laundry indoors, windows always closed).
3. Do the air-exchange test. For two or three days, open all the windows for 5-10 minutes twice a day (morning and evening). If condensation drops noticeably, a significant part of the problem is ventilation, not just the window.

How to read the results, bearing in mind these are indicative thresholds and not absolute values:

• Very high indoor humidity (as an example, consistently above 60-65% in winter) with condensation on several windows and walls: the dominant factor is vapour produced indoors and poor air exchange. Changing the windows helps but may not be enough on its own.
• Reasonable indoor humidity (indicatively around 40-55%) but condensation concentrated on one specific, old window with single glazing or a cold frame: there, the window really is the bottleneck, and replacement makes plenty of sense.
• Mould only in the corner of the wall around a window, with the rest of the home dry: strong suspicion of a structural or installation thermal bridge, to be assessed with a technician.

A paradox that catches many people off guard: after installing new, airtight windows, condensation can sometimes seem to get worse. This isn’t a defect. It’s that previously humid air was escaping through the draughts of the old window, and now that the home is sealed the vapour stays inside until it’s deliberately let out. Airtight windows require conscious ventilation, whether manual or mechanical. I come back to this in the “When it’s NOT worth it” section.

Why a modern window solves it (when the problem is the window)

When the diagnosis points to the window, replacement works on two fronts at once: it raises the temperature of interior surfaces and improves airtightness. Let’s see how.

Thermal-break or PVC frame. A multi-chamber PVC profile or thermal-break aluminium keeps the inner face of the frame much warmer than an old metal profile. If the dew point is ~12°C and the inner surface of the frame sits at 16-17°C instead of 8-9°C, it simply stops condensing. PVC windows are inherently low-conductivity, and here that’s a physical advantage, not a marketing one.

Low-emissivity double glazing with gas fill. The jump from single glazing (or old air-filled double glazing) to a modern insulated glass unit with a low-emissivity pane and argon fill is enormous specifically for the inner surface temperature of the glass. The low-emissivity coating reflects heat back inwards and keeps the inner pane warmer, raising it above the dew point in most conditions. We have a dedicated in-depth guide on how low-emissivity and selective glass works, because it’s the component that, more than any other, decides whether the glass mists up or not.

Airtightness. New gaskets and hardware that closes evenly eliminate localised draughts, which besides letting in cold create irregular condensation points and cool down the reveals.

The table below compares, qualitatively, typical configurations against condensation risk. The Uw values are indicative orders of magnitude, not guarantees: they depend on the specific product and its certification.

Configuration	Indicative Uw (order of magnitude)	Indoor surface temperature in winter	Condensation risk on glass
Single glazing, old metal frame	over 4-5 W/m²K	Very cold	Very high
Air-filled double glazing, ageing frame	~2.5-3.0 W/m²K	Cold	High
PVC + low-emissivity double glazing + argon	~1.0-1.3 W/m²K	Mild	Low
PVC/thermal-break aluminium + triple glazing	~0.8-1.0 W/m²K	Mild	Very low

To work out the right Uw values for your climate zone, and understand how far to push it, you’ll find everything in the guide on thermal insulation, Uw and climate zones.

Installation: the part many people forget

You can buy the most expensive window in the catalogue and still end up with mould in the corner. It happens when the joint between window and wall isn’t insulated and sealed correctly. That joint is a thermal bridge: if cold outside air and humid inside air meet within the thickness of the reveal, hidden condensation forms there and feeds mould on the plaster.

Installation carried out to the UNI 11673 standard works precisely on this: continuity of insulation around the frame, sealing with the three levels of tightness (airtight inside, insulating middle layer, vapour-permeable but water-impermeable outside), and correct tapes and foams. It’s the difference between a “window propped in a hole” and a “window integrated into the wall”. I’ve explained in detail what installation to UNI 11673 actually means, because it’s the item that separates a job that solves the mould from one that merely shifts it a few centimetres.

The product’s reference standard is EN 14351-1, which, together with the CE marking required under CPR 305/2011, defines the window’s declared characteristics. But note: CE marking concerns the product, not the installation. A certified window installed badly won’t protect you from condensation within the wall. This is why, when the suspicion is a structural thermal bridge, it’s worth starting from an overall assessment of the wall and not just from replacing the window. You can see the whole range on the windows page.

Morning external condensation: a “defect” that’s actually a strength

There’s a phenomenon that generates a lot of worried phone calls and deserves an explanation, because it’s counterintuitive. On new, highly insulating windows — typically triple glazing or excellent low-emissivity double glazing — you can find dew on the outer face of the glass in the morning, outside, especially in spring and autumn on clear nights with cold temperatures.

It looks like a defect. It’s exactly the opposite. That external condensation means the outer pane is so well insulated from the indoor heat that its outward-facing side cools down almost as much as the night sky, dropping below the dew point of the outside air. In practice it’s physical proof that the window insulates extremely well: the heat from inside the home can no longer reach the outer pane to “dry it out”. It disappears on its own as soon as the sun warms the glass or the air moves. It causes no damage and doesn’t get into the house. It’s a sign of quality, not a fault.

When it’s NOT worth it

Changing the windows isn’t always the solution, and saying so is part of my job. Here are the cases where spending money on the window risks not solving the problem, or not being enough.

• The problem is almost entirely humidity and ventilation. If indoor humidity is consistently very high because laundry is dried indoors, a lot of cooking is done without an extractor hood, the home is never aired out, and condensation is spread across several windows and walls, new airtight windows can even make the sensation worse. Habits need correcting first, or mechanical ventilation (MVHR) installed, or at least vents/micro-ventilation. Then, if still needed, the windows.
• The mould is clearly a wall thermal bridge, not a window one. Corners, columns, uninsulated beams, cold north-facing walls: there, the window has little to do with it. The solution lies in external wall insulation or targeted insulation of that zone, and changing the window won’t move the needle.
• Rising damp or infiltration. If humidity is rising from below or entering through cracks and roofs, that’s liquid water, not condensation. It needs to be tackled at the source; no window will stop it.
• Windows that are already modern and in good order. If you have recent, well-installed low-emissivity double glazing and condensation appears only in extreme, occasional conditions, there’s probably nothing to replace: you just need to manage air exchange better.
• Limited budget and only one move possible. If you can only make a single intervention, the hygrometer and a week-long diary will tell you whether it’s better to start with the window or with ventilation. Spending “at random” is the surest way not to solve it.

In all these cases, an honest site visit is worth more than a quick quote: better to find out beforehand that the problem lies elsewhere.

FAQ

Is condensation on the glass the fault of new windows? Not necessarily. Often new windows, being airtight, bring to light an excess of indoor humidity that previously escaped through draughts. If condensation appears after replacement, the first step is to improve air exchange, not blame the window. If instead it’s the old window with single glazing that’s condensing, then that really is the problem.

If I change the windows, will the mould in the corner of the wall disappear? Only if the mould depended on the old window or on poor installation. If it’s a wall thermal bridge (corner, column, uninsulated north wall), changing the window isn’t enough: that zone needs insulating. This is why it’s important to understand exactly where it forms before buying.

How high should indoor humidity be to avoid condensation? As an indicative order of magnitude, in winter a relative humidity around 40-55% is a good comfort target and greatly reduces the risk of condensation. Above 60-65% the risk grows. These are guideline thresholds: they depend on temperature and on the cold surfaces present.

Does triple glazing eliminate condensation? It greatly reduces the risk on the glass because it keeps the inner surface warmer, but it doesn’t eliminate the problem if the cause is excess indoor humidity or an installation thermal bridge. Glass is one of the levers, not the only one. It needs to be paired with a frame that isn’t cold and with correct installation.

Why do I find water outside the glass in the morning, on the outer face? That’s morning external condensation: a sign that the window insulates very well. The outer pane cools below the dew point of the night air because the heat from inside the home no longer reaches it. It disappears on its own and isn’t a defect.

Is a single new window enough to solve the problem in a room? If the room has only one problematic window and reasonable humidity, it can be enough. If, however, excess vapour is being produced or there are several cold spots, a single window will improve things but won’t close the problem. Self-diagnosis with a hygrometer helps you understand this before spending money.

In summary

Condensation and mould around windows arise from the meeting of cold surfaces and humid air. The window matters — single glazing, a cold frame and poor airtightness are concrete causes solvable with PVC or a thermal break, low-emissivity insulated glass and UNI 11673 installation — but it isn’t always the sole culprit: sometimes the problem is ventilation, sometimes a wall thermal bridge. The smart move is to understand where the water is really forming before spending money. If you’d like an honest assessment of your case, with CE-certified Salamander products and fully Italian-run management, request a quote: we start from the cause, not from the sale.