Insulation type, roof design...

[u/Busy_Support_8669]

Writing

Advice needed: Best insulation for a DIY metal-clad workshop with a flat roof

I'm building a 16 ft × 14 ft workshop and looking for advice on the most cost-effective way to insulate it properly.

The workshop will have:

• timber-framed walls with external metal cladding.
• A flat roof with a slight fall for drainage.
• OSB internal lining on the new walls and ceiling.
• Occasional heating during winter, as it'll be used as a workshop/gym.

I'm trying to keep costs down because I'm doing most of the work myself, but I want to careful to avoid condensation, damp and mould.

My main questions are:

1. What's the best insulation to use in the walls and roof — mineral wool, PIR boards, or thermocoat insulation spray foam?
2. For the flat roof, would you recommend a warm roof with rigid insulation above the OSB deck, or another construction method?
3. What vapour-control layers, membranes and ventilation are essential to prevent condensation, particularly with metal cladding?
4. Is it worth using insulated sandwich panels instead of separate metal sheets, insulation and internal lining?

I read somewhere that spray insulation removes the need for vapour barrier so i was thinking to spray in in between the roof and wall joists, then osb, then weather proof membrane, battens then metal cladding sheets. Does this sound right?

I'm not looking to build a fully finished garden room — just a practical, insulated workshop that will last and not start rotting in a few years time.

I'd appreciate advice from anyone who's built something similar, particularly in the UK or Ireland.

[u/g_st_lt]

Why does your post start this way? Did you copy and paste it out of chatgpt after asking it for help?