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Recticel insulation is a rigid PIR foam board. It has one of the lowest thermal conductivities of any mainstream insulation material, at around 0.022 W/mK. A 100mm Recticel board can outperform 150–180mm of mineral wool for the same U-value. That’s why you’ll find it in lofts, extensions, floors and cavity walls where space is tight.
This guide explains what Recticel insulation is, where it works best, which thickness suits your project, and the mistakes that catch people out on site.
Maybe you’re stood in a builders’ merchant car park with a van full of boards. Maybe you’re working it out at the kitchen table before you commit the budget. Either way, this guide is written for you, self-builders, tradespeople fitting it for a client, and architects or specifiers who want to know what happens once the drawings are signed off.
What Is Recticel Insulation?
Recticel makes PIR insulation boards. PIR stands for polyisocyanurate, a rigid, closed-cell foam. If you’ve used Celotex or Kingspan, Recticel sits in the same family. It’s a rigid foam core, faced on both sides with foil or a mineral-fibre facing. Its job is simple: stop heat moving through the building fabric.
The Properties That Actually Matter On Site
Thermal performance. The newer, thinner Recticel ranges sit at around 0.022 W/mK. That means less board thickness does more work. This matters in a loft conversion with restricted rafter depth. It also matters in a floor build-up, where every millimetre affects the finished ceiling height below.
Closed-cell structure. The foam cells are sealed, not open. This gives PIR its low conductivity. It’s also why the boards don’t sag, slump or lose performance over time. Some fibrous insulation can lose performance once it’s compressed or damp, Recticel boards hold their shape.
Compressive strength. Recticel is a rigid board, not a soft quilt, so it can take load. This matters under a concrete floor slab, under a green roof, or anywhere it’ll be walked on or built over during construction.
Moisture resistance. The closed-cell foam doesn’t easily absorb water. With the right facing, it also helps your vapour control strategy. A soaked mineral wool quilt can lose a huge chunk of its performance until it dries out. PIR is far more forgiving of site conditions during the build.
Lightweight and easy to handle. One person can carry a board up a loft ladder alone. That means it gets fitted properly, instead of wedged in badly because it’s too heavy to move.
Long service life. PIR boards shouldn’t degrade, settle or need replacing within the working life of the building. The price per board is higher than some fibrous alternatives. But that’s a big part of the whole-life cost case for using it.
Where Is Recticel Insulation Used?
Recticel insulation is suitable across a wide range of project types:
- New builds
- Extensions
- Loft conversions
- Garage conversions
- Garden rooms and outbuildings
- Commercial buildings
- Timber frame construction
- Solid wall (internal or external) upgrades
What Thickness of Recticel Insulation Do You Need?
This is the question we’re asked more than any other. The honest answer: it depends on your target U-value. That target depends on the application, and on which Building Regulations apply to your project and location.
Typical U-Value Targets by Application
| Application | Typical target U-value* |
| Pitched roofs (loft conversions) | 0.15–0.18 W/m²K |
| Flat roofs | 0.15–0.18 W/m²K |
| Ground floors | 0.13–0.15 W/m²K |
| Walls (cavity, timber frame or solid) | 0.18–0.28 W/m²K |
*Rough, non-binding guide for England under current Part L targets. Always confirm with Building Control for your specific project.
How to Get Your Exact Thickness Right
The right thickness depends on three things: the specific product range, the rest of your build-up, and any thermal bridging at junctions. Your build-up means the other materials in your wall, floor or roof.
That’s why we always recommend a proper U-value calculation before you order. Don’t guess based on what a similar-sounding project used. Differences in your wall construction, roof pitch or floor build-up can change the numbers.
If you’re not sure, get in touch with our team with your build-up details. It’s a five-minute conversation that can save you from an expensive reorder.
Common Recticel Insulation Mistakes to Avoid
- Guessing the thickness instead of calculating it. This is the single most common and costly mistake.
- Leaving gaps around board edges. Even small gaps significantly undermine performance.
- Not staggering joints in multi-layer builds. This creates a direct thermal bridge through the insulation.
- Compressing boards to force a fit. This reduces effective thickness right where you can least afford it.
- Ignoring ventilation requirements in cold-roof and suspended floor applications. This risks condensation.
- Skipping vapour control detailing, particularly on solid wall projects. This risks interstitial condensation and long-term timber decay.
- Using the wrong grade board for load-bearing situations, such as under a concrete floor slab.
- Not checking Building Control requirements before ordering. U-value targets are enforced, especially on lofts and extensions.
Recticel Insulation FAQs
What thickness of Recticel insulation do I need?
It depends entirely on your target U-value, the application (roof, wall or floor) and the rest of your build-up. There’s no single universal answer, always get a proper calculation done for your specific project rather than copying a figure from a different job.
Is Recticel insulation as good as Celotex or Kingspan?
All three are PIR boards from the same family, with broadly similar thermal performance. No brand is universally “better.” The right choice usually comes down to the specific product range, thickness options, facing type and price for your project. A proper U-value calculation will tell you what any of them need to achieve for your build-up.
Does Recticel insulation need a vapour barrier?
Often, yes, it depends on the application and facing type. The foil facing on many Recticel boards helps with vapour control. But solid wall projects and some roof build-ups still need careful vapour control detailing. Skipping this is one of the most common mistakes on site.
Can Recticel insulation get wet during installation?
It handles typical site conditions far better than fibrous alternatives. The closed-cell foam doesn’t easily absorb water, so an odd shower during first-fix won’t ruin it the way it would ruin an exposed mineral wool quilt. That said, keep it dry and detail it properly once installed.
How long does Recticel PIR insulation last?
Recticel boards shouldn’t degrade, settle or need replacing within the working life of the building. This holds true as long as they’re installed correctly and protected from physical damage. It’s part of why the whole-life cost case stacks up, even with a higher price per board than some alternatives.
Get Help Choosing Your Recticel Boards
Every project’s build-up is different. The safest next step is a quick U-value calculation, not a guess. Send our team your build-up details, and we’ll confirm the right thickness and board before you order. Or browse our full range of Recticel insulation to see current stock and pricing.
