Windows · Moisture
Window condensation calculator
A window is not one surface, it is three, and they run at three different temperatures. The middle of the pane is the warmest, the frame is colder, and the inch of glass around the spacer bar is colder still — which is exactly why the black mould appears in a neat line around the edge of the glass while the middle of the window stays perfectly clear. This page works out all three temperatures and tells you the highest indoor humidity you can run before each of them goes wrong.
At what humidity do windows condense?
The indoor humidity at which water starts to form, with the room at 20 °C, for each outside temperature. Worked out by the calculator above: the centre of the glass from its Ug with Rsi 0.13, the edge and the frame from their temperature factor fRsi. Below the figure that surface stays dry; above it, it fogs.
| Coldest surface | −10 °C | −5 °C | 0 °C | 5 °C | 10 °C |
|---|---|---|---|---|---|
| Single glazing, Ug 5.7 | 22 % | 29 % | 38 % | 49 % | 62 % |
| Old double glazing, Ug 2.8 | 49 % | 56 % | 63 % | 71 % | 80 % |
| Low-E double glazing, Ug 1.1 | 76 % | 80 % | 84 % | 87 % | 91 % |
| Triple glazing, Ug 0.6 | 86 % | 89 % | 91 % | 93 % | 95 % |
| Glass edge, aluminium spacer | 44 % | 51 % | 58 % | 67 % | 77 % |
| Glass edge, warm-edge spacer | 61 % | 66 % | 72 % | 78 % | 85 % |
| PVC-U frame | 56 % | 62 % | 68 % | 75 % | 83 % |
| Aluminium frame, thermal break | 51 % | 57 % | 64 % | 72 % | 80 % |
| Aluminium frame, no thermal break | 30 % | 37 % | 46 % | 56 % | 68 % |
The glass is rarely where it starts. At 0 °C outside, modern low-E double glazing stays clear up to 84 % in the room, but its edge with an aluminium spacer gets wet at 58 %, and that is the line of water round the edge of the pane. A warm-edge spacer moves it to 72 %. Single glazing fogs at 38 %, which is why old windows run with water every winter morning.
Mould needs less than water: 80 % against the surface, so it starts at four-fifths of these figures. A blind or a curtain makes every row worse, because it keeps the room's warmth off the glass: set Rsi to 0.25 above and see.
How the three temperatures come out
Two consequences fall out of that and they explain almost every complaint about double glazing.
First, the edge is not the glass. The centre of a modern pane is warm because the cavity insulates. Around the spacer bar the two panes are bridged by a strip of metal roughly 15 mm wide, and that strip short-circuits the whole cavity. Replace the aluminium with a warm-edge spacer and the glazing edge gains three degrees for a couple of euros per metre — far more temperature per euro than anything else you can do to a window.
Second, mould comes before water. A surface has to fall all the way to the dew point before anything visibly wets. Mould only needs 80 % humidity at the surface, which happens three or four degrees earlier. So the honest failure sequence is: nothing looks wrong, then the rebate goes black, then — much later — the glass runs.
Typical temperature factors
| Component | fRsi | Note |
|---|---|---|
| Aluminium spacer | 0.55 – 0.62 | the coldest point of most windows |
| Warm-edge spacer, stainless | 0.68 – 0.72 | worth about 3 K at the edge |
| Warm-edge spacer, composite | 0.72 – 0.77 | |
| Timber frame | 0.70 – 0.75 | |
| PVC-U frame | 0.70 – 0.75 | |
| Aluminium, thermally broken | 0.60 – 0.70 | depends entirely on the break |
| Aluminium, no thermal break | 0.30 – 0.45 | will run with water most winters |
Worked example you can check by hand
The window this page opens with: 20 °C and 50 % RH inside, 0 °C outside, Ug 1.1, aluminium spacer, PVC frame.
The edge is 2.3 K above the dew point, so there is not a drop of water anywhere on the window — and it is a degree below the mould line, so the rebate will go black. Swap the aluminium spacer for a warm-edge one at f 0.70 and the edge rises to 14.0 °C, clear of the line. Or hold the room below 47 % RH, which nobody does.
What this page does not do
- It does not calculate fRsi. That needs a two-dimensional model of the actual frame and glazing rebate to EN ISO 10077-2. The values offered here are typical for the component type and are for orientation, not for a report.
- Steady state, one moment in time. Real condensation is a night-time, intermittent, thermal-mass affair: the glass cools fastest, fogs at four in the morning and clears by nine. This page tells you whether the conditions allow it, not how long it lasts.
- Curtains and blinds are only half modelled. Anything that stops room air washing over the glass makes the surface colder, which is why the worst condensation is always behind a closed blind — but the higher Rsi options here only cool the glass. The published fRsi of a frame or spacer already has a surface resistance baked into it, so a blind makes the edge and the frame colder too and this page will not show it. Read the edge and frame figures as the optimistic case.
- No radiator under the window, no draughts, no solar gain. All three move the surface temperature by degrees, and all three are why one window in a house behaves differently from the identical one next to it.
- Condensation between the panes is a different problem entirely — a failed seal, not a humidity question, and no calculation will help.
Questions people ask
- At what humidity do windows condense?
- With the room at 20 °C and 0 °C outside: single glazing fogs from about 38 % indoor relative humidity, old double glazing from 63 %, low-E double glazing from 84 % and triple glazing from 91 %. The edge of the glass gets wet first: with an aluminium spacer at 58 %, with a warm-edge spacer at 72 %.
- Why does condensation form around the edge of double glazing?
- Because the spacer bar at the edge conducts heat out and makes the edge the coldest part of the glass. At 0 °C outside and 20 °C inside, a low-E double-glazed unit stays dry in the centre up to 84 % room humidity, but its edge with an aluminium spacer is wet from 58 %. A warm-edge spacer raises that to 72 %.