Adthrawn

With the plywood layer being sealed, should the plasterboard be foil backed or plain? Just ordering materials, and realised not clarified that...

Would an aluminium foil tape (everbuild etc) not be suitable, or do the PHS tapes perform differently?

I was concerned about condensate - but might be the way forward. Are those panels okay to screw into for fitting battens too? And guess no insulation, just air void?

Thank you for the detailed reply! 1A) Windows - the two side windows will be removed in the next phase. The neighbouring wooden garage is in a very poor state, so I'll pull that down and rebuild it. Longer lead time as will need planning, as want to change the roof direction for solar panels, and absorb space taken by other wooden "things" that have been built around it. The plan is to join the two buildings by a wide link, that will be both a passage and storage etc. This will take in the two Windows (single glazed and not very useful). 1B) Doors - replacing them with a 3-door bifold. Obviously wider opening needed, and that will be closer to the left-hand wall. The front wall appears to have been made from three "modules", so I'll have to rebuild some of the framework, but less of a job thanks to how the front has been built. I still need to source a threshold for the bifold, as want to account for the increase in floor thickness (circa 70mm with the 50mm PIR and 2x 9mm OSB (for amtico or less if a floating laminate flooring). 2) French gutter at front - totally agree with that, and around the sides and rear it's soil and been overgrown, so will scrape back soil but soil membrane down and put gravel down. Planning to leave the gravel until after building work, but if I'm just doing the inside now, then could be a "quick" job to get done first (already spent a couple of days clearing the overgrown ivy and weeds etc). The front path however will have to wait for the electrics... 3) Electrics - quite my forte, was planning to surface mount, but if I have a void, may as well hide the cabling and create a nice crisp white box... Linked to the above, the current power supply is an armoured cable buried in concrete. Not to my liking, and makes changing the path near impossible. I've already laid ducting along a fresh route, so when I pull the new cable in, I'll be free to cut the concrete in front. 4A) Ply sandwich - fair point, it's been standing for 15 years or so. Internal ventilation is definitely the big issue, as when I cleared out the old workbenches, shelving etc, the majority of the musty smell went. Still not completely gone, but a good bleaching of surfaces and properly ventilating the place should get rid of that. 4B) Air tight tape - just any foil tape, or something specific? Also, if taping the plywood, does this in effect become a vapour barrier? Is the insulated stud wall on the cold side or the warm side? Are you for/against the addition of ventilation in the top of the cladding panels? 5) The stud wall does seem the best option, and something I can chip away at progressively... Because it's a metal roof (insulated panels), is there any special treatment of the top of the frame, like extending a tape or barrier between the frame/plasterboard and the plywood? I'll use skirting and trim at the roof to finish the plasterboard off. Or is the frame/plasterboard happy to just butt against the roof as-is? 6) I plan to lay a vapour barrier poly sheet under the PIR - I'm not sure if a barrier was laid in the concrete floor originally. I'll seal that to the plywood walls as well - would that be a sufficient DPC for the stud wall? Thanks Jon

Okay, so adding vents to the top of the cladding should alleviate any moisture on the outside layers. I'll be putting a heat exchanger of some kind for ventilation - we need the fresh air through, and want to retain the heat.

I'm not sure how it's been fitted, whether it wraps under the frame or not. I'm hoping it's "loose" at the base, but it'll still be held against the bottom of the frame by the cladding. The poly is very much not a breathable moisture sheet. There's no ventilation in the panels, so the poly seems to have been fitted as a water barrier without consideration of condensate/ventilation. It's fairly lightly filled with a fibre glass insulation of some kind. So, to add ventilation, it should be an easy job of cutting a 60-80mm hole in the top of each void and putting a vent. Do you mean the poly sheet under the cladding or when the plywood skin is taped up and that'll be an airtight barrier?

I’ve cancelled them… will still get some PIR for the floor (50mm). Still deciding on finished floor - would have liked an amtico etc, but need floating flooring. That said, if I put double lapped OSB down, is that secured in anyway and suitable for adhered floors, or is that still left as a floating floor? anyway, I’ll build an internal stud frame now. Still need to figure out the external poly sheeting under the cladding. With a new stud frame, I’m guessing okay to tie it to the plywood; there’s no issues with damp etc? I’ll probably put a poly sheet under plasterboard, unless I can foil tape the uprights and silicon seal as they go on etc.

Okay, so would require removal of the old poly sheeting (it’s under the cladding at the very outer)? or would high level vents that cut through the poly be sufficient? I wouldn’t have to remove all of the cladding - I could even maybe get away with removing internal plywood as well to remove the poly if needed.

Knot exposing the polysheeting.

I’ll post some pics in a minute… The plywood is painted internally - if that’s taped at joints, will that not create a trapped void between the existing poly sheeting under the cladding and the ply?

It’s definitely something I’ll be fitting, but not quite sure what form it’ll be yet. To what degree would that change the wall buildups?

Yeah, I can cancel them tomorrow. I do plan to use some for the floor, so will keep that. The tricky bit is the current wall build up. Starting with cladding/poly/void/plywood, I’m not sure now what to do. Not sure if I can loose 160mm internally by building out, but could be the best option now. Either way, a big concern is outfitting internally to find the external shell has a damp issue etc. or all of the internal materials get damaged.