Need guidance on Shed Insulation. Mould forming inside

6 Nov 2018
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United Kingdom
I recently got a shed built after seeing one made by the builder for a neighbour nearby. The one I saw was in good condition and no mould etc. I had the same one built. Wooden frame, concrete floor, plywood and cladding on the outside, GRP roofing and plywood inside on walls and roof. No insulation. Plywood inside was painted with external wood paint. The floor was painted with masonry paint as well.

I am now seeing mould appear on mine (not sure why neighbour was not getting it as the only difference between his and mine is the paint) and having done research now, think some sort of insulation, airflow, vapour barrier etc is needed.

Could some experts please advise on how I can get the builder to fix it?

I have done some research on these forums so some points below.

1) Celotex or Rockwool insulation? I read somewhere that Celotex is not suitable for sheds but have seen some people use it. Any links to products I can use would be appreciated
2) Do I need to leave an air gap or fit the insulation right up to the cladding
3) On the inside, do I need a vapour barrier on top of the insulation as well? Something like this
4) Are any air vents needed?
5) I guess the mouldy wood panels on the inside have to be thrown away and new ones used? I can't just use a mould cleaner to clean them (although mould on the surface is removed easily. Just spray and wipe down)

Lots of questions but I am attaching some pictures. I wanted to keep this shed weatherproof and use it for storage so thank you in advance for any suggestions or advice.


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You are getting mould on the painted plywood because its a hard smooth surface.

moisture from the air is condensing on the painted surface and attracts the mould

Im not sure insulation is the answer -if the shed is unheated the painted surface will still get as cold.

I personally would recommend applying insulation panels to the walls -thin insulation panel with a paintable surface

like this for example: ErfurtMav Professional Thermal Insulation Panels

or this

or even insulating paint like this

the pro stuff is this:
Thanks for the quick response and giving a few choices. I understand why my neighbour isn't getting it. Its because of the paint which has done more harm than good haha

So these are the paints I used:

Walls/Ceiling -
Floor -

Starting from what I can do myself. Shall I just use a mould spray and clean the mould, wait for it to dry and use the insulating paint you mentioned to go over the old paint. Would this paint work on wall and floor?

Failing that, I can ask the builder to install the insulating panels on top on the existing wood, or replace the old painted wood panels with new plywood sheets.


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The UPVC door is probably doing more harm than good, it will have a good seal, you want a bit of ventilation.

A belt sander (and extraction/mask) would get rid of the paint fairly easily.

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