Free passive obsy dehumidifier - workable?

[michaelmorris]

Metal and other poor insulators (such as glass) radiate heat out into space every night. As a result, they can reach surface temperatures well below the ambient air temperature, hence tha fact that condensation forms on them.

As I see it, there are several solutions.

1 - Shade the material to stop heat radiating off into space. This is how a dew shield works. (I have a 'dew shield' shelf above the books in my observatory. As a result they never get damp.)

2 - Insulate the material so that its surface temperature falls much more slowly. This is what I have done on the inside of the metal roof of my observatory. I stuck a layer of polystyrene wallpaper 'veneer' just a couple mm thick (used for lining walls) to the underside of the roof. Result = no condensation. The outside of the metal roof happily radiates heat out into space, but the inside surface (the polystyrene) does not get cold, hence no condensation. The thinness of the polystyrene means that heat loss by conduction through the roof is still pretty high, still allowing the inside of the observatory and the scope to cool down pretty quickly in the evening.

3 - Very gently heat the surface of the material to raise its surface temperature above dew point (how dew heaters work).

4 - Remove moisture from the air (dehumidifier). This is an energy intensive solution - even passive dehumdiying requires a large energy input to dry out the dessicant. IMHO this should only be implemented if the other three measures are ineffective or impracticable.

[Balder]

Differential is nothing to do with cooling quicker.

Your roof (or windscreen) has a warm side (internal, assuming a closed box) and a cold side (external) thus is cooler than the "contents" eg wood/seats.

Wood is a very good thermal insulator.

Now to prove this, put a sheet of steel or aluminium (far greater heat conductor than steel) in the middle of your shed and leave it for a few hours/days. How much condensation forms on it? Not a lot's the answer - because it's at the ambient temperature.

Or have I misunderstood your idea? I think you're hanging some steel vertically from the timbers. Can you post a sketch please?