Insulating a shed

Thanks for your question — you’re right, there’s a lot of conflicting advice out there, especially when it comes to insulating sheds and outbuildings. The “right” approach depends on what you’re trying to achieve. Since you’re looking for a conditioned workspace with AC, used for painting and sanding, you’ll need to think about insulation, airtightness, moisture control, and ventilation together. 1. Understanding the Challenge Most sheds are lightweight timber structures, designed to “breathe” and dry quickly, not to be sealed and conditioned. Once you insulate and start controlling temperature, moisture becomes a key risk — without proper vapour control and ventilation, you can get hidden condensation and rot. Activities like painting and sanding add extra vapour and dust loads, so controlled ventilation is essential. 2. Wall Build-Up (Best Practice for a Timber Shed) A good approach is to treat it like a timber-frame building: External cladding (timber, composite, etc.) Breathable membrane (to shed wind-driven rain while allowing outward drying) Ventilated cavity / battens (10–25mm gap to allow airflow behind cladding) Structural shed wall (usually OSB or timber boarding) Insulation between studs (Celotex/PIR, mineral wool, or similar — tight fit, no gaps) Vapour control layer (VCL) on the inside face (very important in a conditioned space) Internal finish (plasterboard, or plywood if you want a more workshop-friendly surface) 3. Floor If it’s sitting on a concrete slab, yes — a DPM (damp-proof membrane) under the slab is best. If that’s not in place, you can still add a DPM sheet over the slab before insulation. Then add rigid board insulation (Celotex/PIR) on top, with 18mm plywood/OSB flooring. 4. Roof/Ceiling If pitched: insulate between rafters (leaving a 50mm ventilation gap under the roof covering), then add a VCL and plasterboard/plywood. If flat: you’ll need warm roof construction (insulation above the deck) or a ventilated cold roof with careful detailing. 5. Ventilation & Air Conditioning Your AC will manage temperature, but it won’t deal with fumes or dust. You’ll need a proper extract fan with filtration to keep the workspace healthy and avoid buildup of VOCs or fine particles. A small MEV (mechanical extract ventilation) or workshop-grade extractor will do the job. 6. Key Pitfalls to Avoid Don’t skip the vapour control layer — it’s essential in a heated/cooled shed to stop condensation inside the walls. Don’t leave unventilated voids behind insulation. If you’re creating a cavity, make sure it can breathe to the outside. Don’t over-rely on AC — without extraction, fumes from paints can linger. 7. Who Can Help A good general builder/joiner can handle the stud insulation, VCL, and boarding. For the ventilation, a domestic ventilation installer or even a competent electrician with experience fitting extractor systems could help.