If you’re insulating a garden room, shed, or timber-frame outbuilding, PIR board is almost certainly going to come up. It came up on every insulated build I’ve done over the last 20 years, and I’ve used it across garden rooms, a fully insulated workshop, a soundproofed music studio, and multiple shed conversions here in Cornwall.
This guide covers what PIR actually is, why it works well in timber-frame garden buildings specifically, what thickness you should be using, and a few things I’ve learned the hard way about where it sits in an insulation stack versus mineral wool alternatives.
If you’re just starting to plan an insulated garden room or shed, you might also want to look at my full garden room insulation guide and step-by-step guide to insulating a garden room properly alongside this one.
What Is PIR Insulation?
PIR stands for polyisocyanurate. It’s a rigid foam board bonded to a thin aluminium foil facing on both sides. That foil does two useful things: it acts as a vapour barrier, and it helps reflect radiant heat back into the building.
The foam itself is a closed-cell thermoset plastic, which means it doesn’t absorb moisture and it doesn’t compress or rot over time. That’s a key reason it works well in the sort of environments garden buildings live in, where you’re dealing with humidity, temperature swings, and limited ventilation compared to a main house.
PIR is sold under various brand names in the UK. Celotex, Kingspan, and Recticel are the three you’ll encounter most often. The boards are chemically near-identical across brands; the main differences are in foil thickness, compressive strength, and the specific product lines suited to roofs, walls, or floors.
Why PIR Works Well in Timber Frame Garden Buildings
Garden rooms and sheds are almost always timber frame, which means your insulation is going in between studs, joists, and rafters. PIR suits this application well for a few reasons.
Thermal efficiency per millimetre. Garden buildings typically have shallower stud depths than a house, often 45mm to 95mm. PIR’s lambda value sits around 0.022 W/mK, meaning you can achieve meaningful U-values in thinner sections than you could with mineral wool. In a 70mm stud wall, 70mm of PIR will perform considerably better than 70mm of glass wool.
Rigidity. Unlike mineral wool, PIR boards are rigid and self-supporting. They cut cleanly with a handsaw or sharp knife and friction-fit between studs without slumping or sagging over time. In a shed or garden room you might not be lining the inside immediately, and PIR stays put in the meantime.
Vapour control. The foil facing on PIR boards provides a degree of vapour resistance by default. In a well-built garden room this needs to be supplemented with a proper vapour control layer, but the foil buys you something in the interim and helps with continuity at board joints when taped correctly.
What Thickness Should You Use?
This is the question I get asked most often, and the honest answer is that it depends on how you’re using the building and what your stud depth allows.
Here’s how I’d approach it by building type:
Garden Room Used Year-Round
For a garden room you’re heating regularly, I’d aim for a wall U-value of 0.28 W/m²K or better (if you’re not sure what U-values mean in practice, my garden room insulation and U-values guide explains the impact on heating bills). In a standard 95mm timber stud wall, that means filling the full cavity with 90mm PIR (leaving a small gap for friction fit) or using a combination of 70mm PIR between studs plus a continuous 25mm layer across the face of the studs to break the thermal bridge.
For the roof, insulation matters more than the walls in most timber-framed garden rooms because heat rises. I’d use 100mm PIR minimum between rafters, and ideally a secondary layer of 25-50mm across the rafter face.
For the floor, 75-100mm PIR between floor joists is standard. PIR boards rated for floor use (Celotex TB4000 or Kingspan TF70 equivalent) have higher compressive strength to handle foot traffic loads. If you’re planning underfloor heating, the insulation spec matters even more — I cover how the two work together in my garden room underfloor heating and insulation guide.
Garden Room Used Occasionally or as a Home Office
If you’re in and out rather than heating all day, a slightly lighter spec still makes sense, but I wouldn’t drop below 70mm in the walls. The difference in material cost between 50mm and 70mm PIR is small; the difference in comfort through a Cornish winter is not.
Shed Conversion or Workshop
For a workshop or hobby shed that you want to take the edge off rather than keep fully warm, 50mm PIR between studs does a reasonable job. I used 25mm PIR in my original log cabin garden office conversion, as documented in my BillyOh garden office insulation guide, and it performed well for a space that wasn’t heavily heated. For a converted shed with 45mm studwork, 40mm PIR plus a 12.5mm plasterboard finish is a common approach.
PIR vs Mineral Wool in Garden Buildings
I get asked fairly often whether PIR or Rockwool (or similar mineral wool products) is better for garden room insulation. My honest answer is that the best builds often use both.
PIR wins on: thermal efficiency per mm, moisture resistance, rigidity and ease of fitting in narrow cavities, and vapour control.
Mineral wool wins on: acoustic performance, fire resistance (mineral wool is non-combustible), cost per square metre, and filling irregular gaps.
In my soundproofed music studio garden room, I used a combination of PIR in the roof and floor for thermal efficiency, and Rockwool RW45 in the walls specifically for its acoustic density. That combination gave me good thermal performance and meaningful sound isolation. If you’re building a garden room for music, podcasting, or any application where sound matters, mineral wool in the walls is worth the consideration.
For a straight thermal-only build, PIR throughout is simpler to specify and generally delivers better U-values in the cavity depths available in garden buildings.
Cutting and Installing PIR Boards
PIR cuts easily with a panel saw, a fine-tooth handsaw, or a sharp Stanley knife scored and snapped. I’d use a handsaw for anything over 75mm thick, and a knife for thinner boards. Always score both faces before snapping thinner boards to get a clean edge.
Wear a dust mask when cutting. The foam particles are an irritant and you’ll generate a fair amount of debris.
For friction-fitting between studs, cut boards around 5mm wider than the cavity so they compress slightly on install. This removes most air gaps without needing adhesive.
Tape the joints. This is the step most self-builders skip and shouldn’t. Foil tape on all board joints kills air paths and significantly improves real-world performance. Unfoiled joints let air move around the insulation and undermine your U-value calculations. I use standard foil tape from any builders merchant; Coroplast and Scapa both work well.
If you’re going for a proper continuous vapour control layer, install a separate VCL membrane over the PIR and tape all laps. The PIR foil alone isn’t sufficient as a standalone vapour barrier in a well-sealed building. I cover the full vapour control setup in detail in my guide to insulating a garden room properly.
Brands and What to Buy
The three main PIR board manufacturers in the UK are Celotex, Kingspan, and Recticel. Their product ranges are broadly equivalent:
Celotex GA4000 is the general-purpose board suitable for walls, roofs, and floors in timber-frame builds. It’s what I reach for most often for garden room builds and is widely available at Travis Perkins, Jewson, and most independent builders merchants.
Kingspan Kooltherm is a phenolic board rather than pure PIR, with a slightly better lambda value (around 0.018-0.020 W/mK vs 0.022 for PIR). It’s more expensive but allows thinner sections for the same U-value. Worth considering if stud depth is tight.
Recticel Eurowall is competitively priced and performs similarly to Celotex. I’ve used it on a couple of builds without any issues.
For small quantities, you’ll often find offcuts and part packs on eBay or Facebook Marketplace significantly cheaper than merchant prices. For a garden room, the volumes aren’t huge and it’s worth checking before buying new.
Real-World Performance: My Garden Room in Winter
The soundproofed garden room I built for music production here in Newquay uses 100mm Celotex between rafters in the roof, 90mm PIR in the walls (with a secondary 25mm layer across the stud face), and 75mm PIR in the floor. The building is roughly 4.8m x 3m.
In winter, a single small oil-filled radiator brings the room to working temperature within about 20 minutes from cold. I’d estimate heating costs for a full day’s use in the room run to somewhere around £1.50-£2.00 on a cold day, which for an unconnected outbuilding in Cornwall in January I consider acceptable.
The room doesn’t drop below around 8°C overnight even in the coldest weeks, which means condensation on surfaces isn’t an issue and the room recovers quickly when you start heating it.
That’s not a controlled experiment, but it gives you a realistic reference point if you’re specifying insulation for a similar-sized building in a similar climate.
Summary: Key Takeaways
- PIR is the right default for insulating timber-frame garden buildings because of its thermal efficiency per millimetre and moisture resistance
- For a year-round garden room, aim for 90mm in walls, 100mm in roof, 75-100mm in floor
- Tape all joints with foil tape; the boards alone without taped joints underperform against their specs
- Consider mineral wool alongside PIR if acoustic performance matters to you
- Celotex GA4000 is a reliable, widely available choice; check builders merchants and online for pricing before buying
Ben is a self-employed builder and DIY content creator based in Newquay, Cornwall, with over 20 years of hands-on construction experience. He has built more than ten garden buildings personally, including insulated garden rooms, workshops, and a soundproofed music studio. The wood-create.com site grew out of his own self-build projects and now attracts DIY builders across the UK.
