Condensation

[Mike Barnes]

Today I noticed condensation on the bathroom windows in that clichéd "Christmas card" style - a strip of condensation round the edge of each pane, and the middle of the pane clear.

Why does that happen?

[Phil L]

It's when warm moist air meets a cool surface.

[TheScullster]

"Mike Barnes" wrote

It is difficult to avoid condensation completely particularly in a moisture laden bathroom. To get close you need: Sufficient heat in the bathroom to keep the moisture in the air rather than simply condensing on the nearest cold surface. A very effective, well placed extractor fan and duct arrangement to remove the warm moist air and replace with warm dry air (a hole through the wall with a cheapo B&Q axial fan is unlikely to cut it in a bathroom of any size).

Phil

[LSR]

I think the OP is asking why the condensation forms in a particular pattern, not why it forms at all.

[Mike Barnes]

Sorry, I should have made myself clearer.

Why only in a strip round the edge of the pane? Why not in the middle as well?

[Roger]

The message from "Phil L" contains these words:

Yes, but Mikes question was not why the condensation but why just round the edges of a window.

I don't know the answer for sure but I think the most likely explanation is that the area adjacent to the window frame is relatively sheltered and retains a marginally higher level of humidity as a result.

Another possibility is that the frame itself is colder than the pane but I think that is extremely unlikely.

[Bob Mannix]

The centre of the pane may warm up quicker than the frame of the window, having less mass. As warming occurs, condensation will leave the centre of the pane and remain around the edge. One would expect the opposite phenomenon on cooling. IMHO

[Mike Barnes]

Seems fair enough. So it would most likely occur in a room where the temperature has just risen sharply, such as a bathroom after a shower. Makes perfect sense now. Thanks.

[The Natural Philosopher]

Cold bridges clearly marked?

[The Natural Philosopher]

You think wrong then.

[Andrew Gabriel]

The edge of the pane has cold bridges to the outside, in the form of the frame and the glass pane separator in the sealed unit. Also, you have a crevice formed where the glass meets the frame, and natural drafts will tend to skate over a volume of dead air in the crevice, allowing the crevice air to cool and more likely drop below the dew point, where condensation will start.

[Andrew Gabriel]

Depends when the window was manufacturered.

[Mike Barnes]

You guessed correctly that it's sealed double glazing. However the same pattern is seen in pictures that pre-date sealed double glazing, and I always assumed that a single sheet of glass was a better conductor than a traditional wooden frame. Could be wrong, of course.

It just struck me that this pattern of condensation is something that's often seen and accepted without questioning why.

[The Natural Philosopher]

Not usually. The frit seal on the edges of DG units are a classic case in point as are aluminium PVC covered window frames.

Only single glazing in wood tends to be colder in the middle.

[<me9]

Two reasons,

1: The air can circulate better away from the corners, allowing condensation to dry more easily as the room warms/dries. The lower edge is usually the worst, as the cold air falls down the inner surface and condenses out the moisture when its temperature drops below the dew point.

2: With DG units, the spacers are usually more thermally conductive than the air (or argon) between the panes reducing the temperature around the edges. Modern units are usually better in this respect.

[The Natural Philosopher]

Lead/putty outside is one (possible) issue.

The other is less air movement.

A third is draughts round the frame edges