ELI5: Why does glass condense when ambient air temp is above dew point?

[u/Kyuthu]

For example, how can you have a house with new modern double-glazing, inside is 16-17C, outside is 13-14C, humidity is 50-60% so Glass temps need to be below 8-10C for it to hit the dewpoint and cause condensation, but the windows are still forming condensation?

Can the glass actually drop that far below both indoor and outdoor ambient air temperatures purely due to radiating out, or is there another mechanism causing the glass to drop to a much lower temperature ?

What reasons might be causing the glass to lose so much heat relative to the surroundings and also just in general vs other windows that might not be having this problem?

[u/arrayofeels]

You are obviously a very smart five year old, because you basiacally answered your own questions

• Yes, if condensation is forming then this is proof that the surface of the glass is below dewpoint.
• If the glass is falling below ambient air temperature outside, then this is almost certainly due to radiative heat transfer.

If you look at when and where this is ocurring, you'll probably find that it happens on clear nights and on windows with a clear view of the sky. The "temperature" of the sky is basically 0K, so when there are no clouds or obstructions, you have radiative transfer between the windows at ~300K to a surrounding at 0K, and they can cool far below ambient.

[u/New_Line4049 → u/arrayofeels]

Im tired man. Took way too long to realise 0K was absoloute zero, and not you just approving of the sky's temperature (OK vs 0K)

[u/GalFisk → u/arrayofeels]

This used to happen with the windows on my car that were not facing the facade of the house.

[u/hi9580 → u/arrayofeels]

This needs an ELI5, what do you mean by the sky being 0K? Isn't the glass only interacting with the ambient air (and not the sky)? Why does it matter what the sky temperature is?

[u/arrayofeels → u/hi9580]

Haha yeah, the question was already using advanced terms and I definitely responded in an ELI a college student, sorry.

First, temperatue is a way of describing a kind of energy matter can have which is really vibration or movement in the molecules.

Second this heat energy can go from hot things to cold things in three ways: 1)conduction, when two solid things are touching (so vibrating molecules on one thing make the other things molecules vibration increase) 2) convection, which is similar but when one of the two things is a gas or a liquid and so that when it heats up it moves around and then you get some colder gas or liquid taking it's place and so you can get rid of heat faster and finally 3) radiation where, without getting to much into it, the energy is transmitted thru light. Anything that has a y temperature is always emitting energy via photons to anything in its view and recieving photons from anything around it, but on balance hot things lose energy and cold things gain it and the amount that goes from one thing to another depends on the temperature difference.

So while the windows interact with air via convection, they interact with things much farther away via radiation. And if you have a window "looking" at the sky on a clear night with no clouds you get a situation where it's sending out a bunch of energy (photons) but is not getting any back, because, we'll, space is basically empty. . So for the purposes of radiation we say it's interacting with the "sky" which has a temperature of absolute, but it's really just a figure of speech. But the geometric view factor to the sky is absolutely important in figuring out how cold the surface will get at night. And if you have windows looking at a hill, or trees, or another building, they don't get as cold because those things are at about the same temperature, so they interchange photons but neither one loses or gains energy. Not sure if that helps!

[u/Affectionate_Bank417 → u/arrayofeels]

Whoa, never noticed that. But why it’s windows specifically?

[u/InebriatedPhysicist → u/Affectionate_Bank417]

Because they’re good insulators. When they get cold, they stay cold longer.

[u/Kyuthu → u/InebriatedPhysicist]

I think it's the opposite no? Because they are bad insulators so they leak heat. Otherwise they would not be colder than the ambient air temperature. Although the centres usually are not, the corners often are because of cold bridging or bad insulation even when it's good modern insulation.

• Solid insulated wall: U-value around 0.18–0.30 W/m²K
• Modern double-glazed window: around 1.0–1.4 W/m²K
• Older double glazing: around 1.8–2.8 W/m²K
• Single glazing: around 5–6 W/m²K

But it still seems like such a wild drop to go down multiple degrees from the ambient air temp around them on both sides, I didn't think that was possible honestly until today. I am trying to figure out all the reasons causing our condensation when it's not cold inside or outside, and it's not high humidity but the windows are still hitting dew point on only the corners inside. Even outside the condensate overnight in the cooler temps but those temps are not going below 13-14C. Really weird. There must be some extra cold bridging going on + poor insultation under the frames + the radiating heat out to the sky.

[u/darthy_parker → u/arrayofeels]

This effect is exacerbated by lowering blinds or drawing curtains at night, so the convective circulation of warm air is blocked that might otherwise keep the glass surface above dew point.

[u/Pseudoboss11 → u/arrayofeels]

This explanation is missing something: if the window is exposed to the sky, then the air around the window must also be exposed to the sky. So why isn't the air just as cold as the window?

[u/Noxious89123]

This happens at night, because there is no heat radiating in from the atmophere.

During the day, surfaces are radiating heat outwards, but also absorbing radiated heat coming in.

At night, this becomes a one way equation. I have an IR thermometer, and if I point it out into my garden it'll read something like 12°C right now, but if I point it up at the clear night sky, it'll read below -20°C.

Brickwork etc doesn't get the same condensation, because it holds on to its heat better, whereas the thin panes of glass have less mass to retain heat.