Thanks in advance for any answers.

I have just had a shed installed which is built onto the side of our house. It has a metal roof, with OSB board under it (there are wooden noggins to support it).

I have tried to insulate the roof with Polystyrene blocks, which we intend to cover with plasterboard.

The sides of the shed have been insulated in the same way - although there is no metal on the sides it's just wood. The main difference being, that I have used a vis screen on the cold side of the polystyrene. There have also been vents cut in the sides of the panels towards the bottom.

The floor is currently just a concrete slab with flags in between. There is nothing over the top of it.

The sides of the shed are showing no signs of mold, however the roof is - behind the polystyrene blocks. I am guessing that this is due to moisture being trapped and a lack of ventilation. Does anyone have any ideas for a solution that we could use to avoid this? Or am I better off just taking the plasterboard and polystyrene off of the roof and forgetting trying to insulate it?

Thanks again,


  • "Vis screen"? Do you mean "Visqueen", the 1950's brand name for poly sheeting?
    – isherwood
    Mar 25, 2016 at 13:56
  • 1
    What you call noggins I've always called fire stops. I learned someting today. Thank you! Mar 25, 2016 at 14:00

2 Answers 2


Condensation does cause problems. You absolutely must allow air to flow under a steel roof. I think the most common mistake that I have seen with additions on the side of the house is the lack of a vent where the roof meets the wall. And in this case, maybe a 1" space where the polysytrene/plastic meets (or actually shouldn't meet) the wall.

Also, I would avoid using plaster board (aka drywall) in areas without climate control. It simply will not last.

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Absolutely, remove the insulation & don't install any drywall. Floor venting is a start, though not needed, but mid-level or high-level venting is best. Bleach the interior down, scrub it with a broom, hose it out & leave the door(s) open for a couple of dry days to get rid of any mold.

A shed must be free to change with the outdoor conditions as quickly as possible. Organic items should not be stored in the shed. So no, Cardboard or Interior Upholstered & Fabric. Outdoor only Cushions, Table Umbrellas & Wood Furniture are mostly fine, but should be cleaned & sunbathed annually.

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