Looking for some advice…
Have a wooden workshop building, that’s been built with 2x4s, 8-10mm internal plywood skins, insulated voids (fibreglass? Not checked), and external horizontal lapped cladding. It’s about 12-15 years old, but in excellent condition (and seems to have been built pretty well). Has an industrial roof as well, made of 100mm insulated metal panels.
I’m converting to be an all-year office/studio space, and looking for some advice/reassurance that my plans are sound.
There’s a musty smell - the majority of the smell actually reduced when I removed old workbenches and shelving. The internal walls don’t feel damp - there’s no damp or mould etc. Theres also no airflow/trickle vents - one job will be to add some kind of venting.
The plywood skin is screwed in, and there’s sealant etc so I’m loathed to pull the internal skin off; the cladding nails feel loose, so I think removing boards externally will be much easier, and I can repair/replace any external boards at the same time. I’ll be able to see the inside of the plywood and frame, so if any do need replacing, it can be addressed.
So my plan is to take the cladding off, remove the old solid plastic membrane and replace with a proper tyvek/building paper for moisture control and breathing. The old membrane can be seen in gaps, and appears to have degraded in a few places, albeit, it seems a good/thick membrane.
In the process, I’ll check the internal wood frame for damage, moisture, woodworm etc and treat it as well. I’ll likely spray woodwork preventive treatment into the frame as well.
I’ll strip the old insulation out, and replace with PIR insulation panels.
The plan will be to skin the existing plywood skin with plasterboard; screwed directly on use PB screws. I’m assuming I should use plain plasterboard and not one with a foil back etc?
I’ve taken shelves mounted to the plywood off, and the screws were all rusted were they were in the plywood. Is this typical, or indicative of damp/trapped moisture? We’ve only just moved in, and the weather has been pretty good, so I have no idea how damp the building is. It feels pretty dry up to now.
The frame appears to be sitting directly on a concrete base, no plastic etc under the frame. The concrete pad is about 6” above ground level at its lowest point (and about 12” on the furthest side). There has been Ivy ingress on one corner (I’ve removed all traces externally), but doesn’t appear to have been any water ingress.
So, I’m hoping that the <cladding> <vapour barrier> <insulation> <plywood> <plasterboard> order will work to provide good insulation and minimise damp/moisture? Plus ventilation!
what kind of membrane should I fit? I’ve seen recommendations ranging from breathable to solid plastic sheets, to no membrane and relying on the foil on the PIR plus foil tape over the wood frame.
Where should the membrane be? On the outside of the insulation, or on the inside?
Being 4” framing, I’m planning to use 90mm PIR (I’ve heard that 100mm doesn’t fits into 4” frames, so 90mm works best.
I’ve been reading datasheets for insulation, but any mention of wood frames is related to brick cavity walls with internal wooden walls, so hard to get a feel for the correct assembly sequence.
Thanks!
