Insulating Shed and Other Woes

17 Feb 2013
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United Kingdom
Hi folks,

First post here, and I'm looking for opinions on how best to deal with an issue or two.

We've had our shed for about 2 years now, it's a pent roof model, and was built up by a company (which I wouldn't do again, TBH - See next part).

I've noticed inside that my workbench and shelving (made up of fibreboard) has a nice layer of mould on top after the winter, and I've noticed a little water staining on the roof and on one of the walls. Now, I'm going to sort the roof out so it doesn't leak, by painting over the felt joins with something appropriate, and going to paint internally with some Timbercare type paint. However, I also want to insulate my shed so this is unlikely to happen in future.

My question is, how best to tackle this as it is already built? The exterior cladding is T&G, and I don't think I'll be able to put the vapour barrier on as it should go (between the framing and cladding), so would it be acceptable to put it onto the inside of the cladding, but over the framing? If not, is it best to just put the barrier directly to the cladding as best I can?

I am going to have electrics in the shed (eventually!), so I read that rules polystyrene insulation out. I'm only looking for a thin layer of insulation; perhaps 25mm or less, and will be boarding with some form of ply, so would it be OK to use the foil type insulation, or would I get away with the polystyrene 'board' if I encase the wiring circuit within trunking or similar?

I want to make the shed well enough insulated so that it doesn't get the dampness it has at the moment, as I hope to use it a little more than I do now. I would like it to be quite stable temperature wise, and I understand this is likely to happen if the work is carried out properly.

Thanks for any and all advice. :D
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Just to add:

My floor is wood on flagged base, there are battons underneath between the flags and the floor. At the moment I just laid 12mm ply on the floor, but beginning to wonder whether I should lift it and put a layer of DPM between the ply and the shed floor?
Hello Shedwise
If your shed is only 2 years old and it's already leaking then it sounds well suspect!
When you say 'fibreboard' do you mean the type of soft insulation board? - this stuff is like blotting paper and will grow mould very quickly in cold and damp conditions. But if you mean MDF and it has mould growing on it then your shed must be leaking like a sieve.
Regarding insulation, I wouldn't get too concerned about using polystyrene - someone may correct me on this, but I would think the chances of fire risk are extremely low and, as you say, you could always use trunking if this risk worried you.
So, I would use polystyrene sheet cut to fit internally, and then face this with thin ply. I wouldn't bother about using a DPM on the walls, if you have T&G walls this really shouldn't be necessary.
Your description of the floor is not clear - do you mean you've put 12mm ply over the existing floor? You could well put a layer of polystyrene sheet between the two floor layers, but I wouldnt use a DPM as well because if the floor does get slightly damp it will not dry out so easily.
Hope this is of some help.
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Hi and thanks for your response. :)

It is the soft fibreboard on my bench and shelving, not ply. It only seems to be mouldy on the higher shelving - and I have some chipboard type shelving that isn't having the same problem, so perhaps it is just due to the cold and damp?

On the roof, where the mould was, it was a long, thin strip of black on just one of the planks (roof is t&g too I think - not ideal) and it appears to be just where the felt overlaps, though the span of the roof means it is sagging slightly, so means water may pool on the roof. This is an issue I am solving by either running a bead of the roof and gutter sealant just below the felt, or with the paint on repair from Thompsons or similar.

Sorry about not making the floor clear - There is a planked (t&g again?) floor, and I have laid 12mm ply over it, I hope to store motorbikes in it when it is complete, and the ply was to distribute the load more evenly. I will look at putting a layer of polythene between the floor and ply.

I think I will get the foil insulation for the roof and walls, and box in with a thin ply, in the hope it helps maintain something of a constant temperature, to try and avoid any effects to stuff I keep in there, surface rust etc.

Thanks for your advice :)

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