How to stop condensation on corrugated plastic?

Lieutenant Scott wrote:

Transparent or opaque sheeting fastened to the underside of the rafters?
The water should drop onto it, and then run down to the edges, where you can deal with it easily by some form of gutter arrangement.
Of course, this won't deal with the cause of the condensation, just the symptom.
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On Tue, 14 Feb 2012 19:28:30 -0000, John Williamson

As long as the condensation formed on the inside of the original layer, and not the new one...
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My garage roof is made of box profile steel and in the centre there's one sheet of see-through plastic to make a skylight. The metal sheet has a lining of OSB board but of course the window bit is not lined with OSB and that produces lots of condensation just like you describe.
I don't think changing the roof slope would be practical - you'd still get as much condensation - but it might all run down to one end.
Could you line the ceiling with some sort of boarding? That would stop it.
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I have a similar problem in similar Scottish conditions, and do have to just live with it. In my case there really is nothing I can do as it's clear plastic and is the only daylight source for the lean-too section of the garage and contains my lathe, and is in near enough daily use. Certainly the angle of my roof is pretty low to the extend that in the recent winters I have to go and remove the snow from it.
But the condensation is not significant - if it was the lathe would suffer and it is quite OK under an old groundsheet, so I do wonder quite why you are getting so much moisture - I guess there may be no damp proofing on the floor. If in fact your shed roof does not require to give light then lining it would solve the problem.
Rob
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On 14/02/2012 22:39, robgraham wrote:

sheet of see-through plastic to make a skylight. The metal sheet has a lining of OSB board but of course the window bit is not lined with OSB and that produces lots of condensation just like you describe.

If you don't mind what it looks like you could try lining it with horticultural grade bubble wrap available in large sheets. That should provide sufficient insulation and decrease contact with cold outer skin.
Insulation and/or stopping saturated air reaching the cold surface and possibly with active ventilation when the relative humidity is low. Maybe an application at last for a solar powered fan ;-)
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On 15/02/2012 08:16, Martin Brown wrote:

The bubble wrap works well in my cold frame. It's gaffer taped to the pvc panels. Bone dry in there
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On 14/02/2012 19:03, Lieutenant Scott wrote:

A subject close to my own heart as well ! Scotland too ..
I have a largely sandstone outbuilding with a clear plastic corrugated roof, that suffers lots of condensation.
About 18 months ago, I (in my wisdom at that time), I lined the roof with 6mm ply, the idea being I created a "cushion" so that the water couldn't condense on the cold plastic, instead I had "warm" plywood.
It's not really helped ! The ply is rotting and going black. Well, it was anyway .. Left a half door open for months and months and the roof seems to be clearing a bit, but am not sure if it's the open door, or the large bucket of road salt that has helped !!
Anyway, have now closed the door and removed the bucket of road salt, and instead stationed 3 diy dehumidifiers around the shed, which are basically road salt in a muslin square or tea towel hanging over a bucket, the salt draws in the water, then drips it into the bucket.
So, over the next few months I'll see if the underside of the roof stays drier or not. I can still see water condensing on the cold walls.
I have a theory that one wall in particular is sat in water and the water is drawn into the shed that way. The whole outbuilding, which is pretty big, is on an incline and I've convinced myself that the wall at the bottom of the incline is the one causing trouble. The parts of the building higher up the slight incline are definitely drier.
Am currently lining one wall with shelving which as part of the prep, I've put polystyrene in and will board over with 6mm ply. Of course, this means more water will condense onto the other bare walls !
Anyway ... line the roof, don't bother ! Of course though, there's not as many drips of water into the shed as the poor old ply lining gets most of it.
I think the diy dehumidifier is worth a shot. I also intend to scatter around the shed some tins (with holes) with wood charcoal in them to act as dehumidifiers as well.
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The road salt idea is brilliant! I will do that if the sheeting isn't enough. Much better than paying for a dehumidifier and some electrocity.
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Albeit not a steep enough roof to allow the water to run down I suggest that if you cannot make it steeper to dry thoroughly, spray with silicone spray to provide a less water adhesive surface and allow the water to run more easily to the edge to a collection gutter.
Apart from that install double wall polycarbonate sheeting such as eBay item 320705397522 underneath to give better insulation and therefore less condensation.
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Silicone spray sounds like a good idea, I'll get some of that.

As the shed isn't heated, how does insulation help? Or is it in the evening when the outside cools and the inside does not that condensation occurs?
Also looks expensive!
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The condensation is on your side, if it doesnt happen your stored goods will probably mould or rot. Just direct the condensate better.
NT
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I don't store rottable things in there. Bicycle, mower, hedge trimmer, electric saw, that kinda thing. But large amounts of water falling on them might cause rust.
I'm currently planning (form advice in here) to spray the roof with silicone spray to make the water slide to the bottom, and if that fails, add some polythene sheeting below it to catch the drips.
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On 14/02/2012 19:03, Lieutenant Scott wrote:

You don't say whether it is clear or solid opaque plastic, so this may or may not not be suitable. I had the same problem with a corrugated steel roof on an outbuilding. I found that pushing 2" thick expanded polystyrene sheet (something I had lying around - thinner may work as well) up between the rafters and making sure it was well sealed around the edges cured the problem.
Colin Bignell
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wrote:

It's tinted blue, but you can see through it.

Are you sure there is no condensation above the sheet causing mould?
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On 15/02/2012 11:14, Lieutenant Scott wrote:

The problem is temperature differential, so condensation above the foam would be improbable. At least, nothing collapsed in the c 20 years it was in place before I sold the house.
Colin Bignell
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wrote:

Surely your roof would just have cooled more slowly, and the condensation would form more gradually over the whole night?
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It does it (perhaps moreso) with the door left ajar. I assumed that was simply allowing damper air back in once the air inside had dried out due to condensation occurring.
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On Tuesday, February 14, 2012 7:03:04 PM UTC, Lieutenant Scott wrote:

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I've sorted it by increasing the slope of the roof.
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Thanks colonel peter / uncle scott.. Tara
Jim K
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