Shed - condensation on perspex roof

9 Oct 2014
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United Kingdom
The last couple of months I've been having great fun building a shed/greenhouse/summerhouse in my graden. Concrete base, timber structure, windows, 2mm perspex roof at an angle of around 10 degrees.

Trouble is, the inside of the roof is constantly coated in condensation, to the point where droplets are dripping onto the floor. Any idea how I can prevent this?

Most of the advice I've seen regarding these sorts of problems is to ensure there's adequate ventilation. Well my shed hasn't had any doors on it for the past 6 weeks, so air circulation shouldn't be a problem.

The conrete has had plenty of time to dry out, and the rest of the inside of the shed is dry - no moisture on the windows, and the wood is dry (and has been treated). I've applied sealant all around the inside sides of the roof, just in case rain was seeping in.

I'm trying to work out why the condesation is forming, and what I can do to stop it. Presumably it's because the air inside the shed is slightly warmer (although I can't see how, when there are no doors on the shed) and the outside of the roof is cooler - so the water condenses out of the air.

A friend suggested placing a sheet of 1/2 inch plywood over the roof (ie sitting on top of the perspex) during the winter. It would be a bit of pain removing it in the spring and putting it back on in the autumn, but if it solves the problem, I might have to.

Before I spend 40 quid on plywood, though, does anyone have any suggestions? If I added another sheet of perpsex, say 10mm below the existing sheet, I'd end up with sort of double glazing. Would the air layer between the two sheets help prevent condensation?
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are you saying there is no doors or windows ??
what have you been doing in the space over the last few days ??
It has doors and windows on now, but the condensation has been a problem ever since I put the roof on - which was a few weeks ago
I think the polycarbonate tri wall may have been a better choice for your roof, and I'd consider fitting this as a secondary glazing system.
For sure it will be warmer than the existing, and it should cure your problem.
John :)
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The perspex is colder than the surrounding internal air, I think due to night sky radiation (similar effect to how frost forms on car windows).

The shed structure will cause a tiny positive internal temperature, enough that the internal air can then allow condensation on the underside of the poly.

Burnerman is correct that insulated polycarbonate will solve the issue, or overlaying the polycarbonate with some insulating material.
The floor is concrete, which is made of water mixed with cement and aggregate. Is it in contact with the damp ground, or is there a dpm?
Cheers for the advice, I've bought a sheet of polycarbonate so we'll see how it goes. Do I need to seal the ends of the sheet to stop air/moisture getting between the two layers?
Sealing them will make them more thermally efficient, but may allow small amounts of condensation to form internally from trapped moisture.
the place that sold the sheets probably sells end-trim and porous breathing tape that will keep dirt and insects out.

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