Best PIR Insulation Board for Garden Studios: 2025 Expert Review
DR Joinery
Expert Joiner & Garden Building Specialist
Transforming a simple summerhouse into a year-round garden studio is a popular project here in York, but without the correct insulation, you are simply heating the garden. The fluctuating temperatures and damp conditions we face mean that standard rock wool often isn't enough—especially if you want to maximise internal space.
PIR (Polyisocyanurate) boards are the industry standard for timber frame buildings for a reason. They offer the best thermal performance per millimetre of thickness, which is crucial when you are trying to preserve headroom and floor space in a compact garden structure. However, not all boards fit every application.
In this review, I've selected three options suitable for different stages of your build: a budget-friendly board for repairs or small sections, a high-specification 75mm board for primary walls and roofs, and a thinner 25mm option for tricky retrofits where space is at a premium.
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Specs vs. The Real World
The Spec: "R-Value/Thermal Performance"
"Manufacturers quote laboratory 'Lambda values' (like 0.022 W/mK) assuming perfect installation."
In a timber frame, gaps are the enemy. If you don't cut the board for a tight friction fit and tape the joints with aluminium foil tape, the cold air will bypass the insulation entirely.
The Spec: "Moisture Resistance"
"PIR boards are declared 'moisture resistant' due to their closed-cell structure and foil facing."
They resist water absorption, but they are not waterproofing. In a garden studio, you still need a breathable membrane on the outside and a vapour control layer (VCL) on the warm side to prevent condensation within the walls.
My Top 3 Recommendations
Three options to cover different needs and budgets
PIR Insulation Board 1200 x 450 x 50 mm; 47.2 x 17.7 x 2 in, One Panel Covers 0.54m²
NOVECRAFTO
This is a solid choice if you need a specific amount of insulation for a smaller project or repair without buying a bulk pack. The 50mm thickness is the absolute minimum I'd recommend for a garden office to be used in spring and autumn.
The Good
- Sold as single panels, reducing waste for small jobs
- Standard 50mm thickness fits common 3x2 studwork reasonably well
- Double-sided foil facing aids thermal reflection
The Bad
- Single unit price is higher per square metre than bulk buying
- 50mm may struggle to keep a room warm in deep winter without supplemental heating
Installer Tip:
When cutting this 50mm board, use a fine-toothed saw or an old bread knife. Cut it 2-3mm larger than the stud bay to ensure a tight 'friction fit'. This stops the board from sliding down over time.
Priced at £14.95 at the time of writing
Check Price on AmazonKingspan Thermawall Multi Purpose Insulation Boards 450mm x 1200mm x 75mm (12)
LOWENERGIE
If you are building a serious garden office for year-round use, this 75mm Kingpsan board is the professional choice. It offers significantly better thermal retention than 50mm boards, making electric heating much more efficient in the winter months.
The Good
- Superior thermal performance suitable for year-round habitable spaces
- Kingspan is a market-leading brand known for consistency
- Ideally sized (450mm width) for standard stud spacing
The Bad
- Requires deeper studwork (minimum 89mm/4x2 timber) to fit flush
- Higher upfront cost, though recouped in heating savings
Installer Tip:
Note the thickness! 75mm insulation won't fit inside standard 63mm (3x2) CLS timber studs without protruding. Ensure your frame is built from 89mm (4x2) timber, or be prepared to batten out the walls to accommodate the depth.
Priced at £139.99 at the time of writing
Check Price on AmazonPIR Insulation Board 25mm - 400mm x 1200mm for use in Roofs, Walls and Floors (12)
Generic
Sometimes you just don't have the space for thick insulation. This 25mm option is perfect for over-boarding existing walls, insulating floors where height is restricted, or slipping into narrow roof rafters in older summerhouses.
The Good
- Ideal for floor insulation where ceiling height is limited
- Can be doubled up (layered) to achieve specific depths like 50mm or 75mm
- Perfect for eliminating cold bridging over studwork
The Bad
- Low thermal value on its own—best used as a supplement or in layers
- 400mm width is non-standard (usually 450mm or 600mm centres)
- Will require more cutting if your stud centres are standard
Installer Tip:
This 25mm board is excellent for 'continuous insulation'. Nail it across the face of your timber studs before plasterboarding. This prevents the 'cold stripes' (thermal bridging) you often see on walls where the timber stud conducts the cold from outside.
Priced at £62.99 at the time of writing
Check Price on AmazonFrequently Asked Questions
What thickness of PIR board do I need for a garden office?
Do I need a vapour barrier with foil-backed PIR boards?
Can I put PIR insulation directly next to the external cladding?
Is PIR insulation better than rock wool for garden studios?
How do I cut PIR insulation boards cleanly?
Why is there a price difference between brands like insulation?
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