Why does steam tend to gather on glass?

Any time you have a shower running, the first things which gets fogged up are windows and mirrors. Other smooth non-porous surfaces (e.g. metal, porcelain) don’t get nearly as much moisture. And it’s not just about visibility - you can feel the difference in the different surfaces.

Why is this?

One part of this is you don’t see the moisture condensing on other surfaces as well as you can on glass and mirrors. The moisture condenses on any surface cooler than air and steam in the room and mirrors on medicine cabinet doors and windows may be cooler than the rest of the room. The windows are likely cooler than the rest of the room because they are exposed on the outside, and the mirrors on medicine cabinets can be cooler on the back side also. But that only makes a small difference, the steam is condensing on any cool surface and as I mentioned first, it is more apparent on glass and mirrors because it distorts the light passing through or reflecting from the back of the glass.

I had thought about that. That’s why I wrote in the OP “And it’s not just about visibility - you can feel the difference in the different surfaces.”

Your mirrors are higher than the porcelain and metal in your bathroom. Warm air rises, causing more condensation on higher surfaces.

I also wonder if there’s some confirmation bias here. Your eyes tell you that the mirrors have much more condensation because the steam is much more visible. We’d have to use SCIENCE! to determine the differences in condensation on variable surfaces and heights in a bathroom. A fingertip swipe isn’t enough.

I’m sure somebody has already done the SCIENCE! but I couldn’t find anything in a cursory interweb search.

The heat capacity of glass is considerably higher than metal, which means it can absorb more heat and condense more steam before the temperature equilibrates.

Other factors are the density of nucleation sites on the surface and relative surface energies of the surface and water, affecting whether water will “wet” the surface. If it wets, then the surface won’t look “foggy”.

I’ll note that porcelain has a similarly high heat capacity and usually has a vitreous surface, so, all else being equal, it should act like glass. If it really doesn’t, which can be hard to tell, it’s probably that those surfaces aren’t as cold as the glass, as @TriPolar suggested.

On glass and mirrors you have the combination of senses to detect the moisture, and your sense of touch alone will throw you off. But the different surfaces are also different temperatures. Ceramic tiles mounted on the wall will not radiate heat from the backside the way glass and mirrors do. If mirrors are mounted on the wall like tiles the effect should be similar on both, but again it will be more apparent on mirrors.

Mentioned above are the different heights of the materials in the bathroom, and that brings up the issue of ventilation. A fan or open window will keep most of the steam in the upper portion of the room affecting mirrors and windows more, but the steam should generally condense on faucets, the sink and toilet, light fixtures and switches in similar levels as a mirror. A fan or window is required by most building codes in a bathroom to clear steam from a room because it can cause damage to the structure, plumbing, and electrical fixtures over time. The codes often recommend a fan be located close to a shower to be most affective. And if you look up, depending on the ceiling materials, you may see a lot of condensation on the ceiling. You can don’t need to feel it tell if you see water beading up and drops falling from the ceiling. Turn off your fan or close the window and this should be very apparent.


It’s water vapour, not steam.

The glass is also a better conductor of heat. The whole plane of glass is absorbing the heat of the warm air not just the surface area of the glass. So more air can be cooled to the point that the moisture condenses on the glass surface.

Relative humidity. It the reason water droplets form on a glass of ice water.

Glass is a very poor thermal conductor

Compared to most of what bathrooms are made of it’s not. Moisture will definitely condense on exposed copper pipes and other metal fixtures in a bathroom. Diamonds, gold, and mercury are used less often in bathroom construction.

Much better that sheetrock and paper. I read some years back that simple double pane glass has a R value of 1. Put hot water in a glass then in a ceramic cup. The glass will very soon be the temperature of the water. The cup a little longer.

That’s because they have higher heat capacity (see also thermal mass), particularly when full of water.

Who said anything about diamonds or gold? The thermal conductivity of copper is almost five hundred times greater than glass.

Those are among the things with greater thermal conductivity than glass. It is a better thermal conductor than most of what a bathroom is made. Water vapor considers it an excellent thermal conductor in those circumstances.