HOW-TO GUIDES
How to insulate roof rafters with Celotex
Updated 07/26
By: Tommy Burgess
Reviewed: Alex Peters
When it comes to insulating homes with pitched roofs, Celotex has become one of the most popular choices for homeowners in the UK.
In this guide, we’ll look at how to insulate roof rafters with Celotex, including top tips and expert advice.
Why Celotex is such a popular choice for pitched roofs
The five steps to fitting it between your rafters
Why an air gap is non-negotiable, not optional
Warm roof vs cold roof construction, and why it matters
What Building Regulations require of your insulation
How much Celotex insulation typically costs
Celotex has become one of the most popular ways to insulate a pitched roof in UK homes, and it's easy to see why.
It's slim, lightweight, and delivers strong thermal performance without eating into valuable headroom in a loft or roof space.
This guide walks you through fitting it between your roof rafters, plus the building regs, costs, and common pitfalls to avoid.
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What is Celotex?
Celotex is a rigid insulation board made from a polyisocyanurate (PIR) core, faced on both sides with low-emissivity foil.
That foil facing helps reflect heat back into the room, which is why Celotex performs so well even at relatively slim thicknesses - boards range from 12mm up to 200mm, so there's usually an option that fits your rafter depth without excessive cutting.
Its main advantages for pitched roof insulation:
One of the thinnest insulation products on the market for the thermal performance it offers
Low thermal conductivity, which minimises heat loss through the roof
A single-layer system that's quick to install compared to multi-layer alternatives
Suitable for both new builds and retrofitting older properties
Adds minimal extra load to the roof structure
A sensible option where headroom in the roof space is tight
Celotex isn't the only PIR board on the market - Kingspan and Ecotherm are common alternatives with similar specifications. If your local merchant is out of stock of one, another PIR board of the same thickness will usually do the same job.
Warm roof vs cold roof: where does the insulation go?
Before you start, it's worth understanding which type of roof construction you're dealing with, since it changes where the insulation sits:
Cold roof construction - insulation goes between (and often also under) the rafters, leaving the roof void above cold and ventilated.
Warm roof construction - insulation sits above the rafters, so the whole roof structure stays within the heated envelope of the building.
Fitting Celotex only between the rafters, without any insulation above or below, rarely meets current Building Regulations on its own and can also increase the risk of cold bridging through the timber rafters themselves.
Most professional installations combine between-rafter insulation with a layer above or below to hit the required U-value.
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How to fit Celotex between rafters in 5 steps
1. Prepare the roof
Before fitting anything, clear the space so you have room to work and good lighting. Use a vacuum to remove dust and debris from between and around the rafters.
If you spot any signs of damaged rafters - rot, cracking, or water staining - deal with these first. Insulating over an existing problem will only make it harder (and more expensive) to fix later.
Our roof repair cost guide has more on what that kind of remedial work typically costs.
2. Take all the measurements
Measure the full roof area - from the apex down to where the slope meets the horizontal joists, multiplied by the roof's width - to work out the total area you're covering on each side.
Measure the depth of your rafters too, and check the thickness of Celotex board you're planning to use.
As a guide, aim for 100mm or more where rafter depth allows, since thinner boards may struggle to meet current U-value requirements on their own.
3. Create an air gap
Leave a gap between the insulation and the roofing felt or membrane behind it, to let air circulate and prevent damp or condensation building up.
A sensible gap is around 25mm-50mm, though always check your specific system's guidance, since this can vary.
Skipping the air gap is one of the most common mistakes in DIY rafter insulation. Trapped moisture between a sealed insulation layer and cold roofing felt can lead to interstitial condensation, which rots timber from the inside out over time.
4. Cut Celotex boards to size
Cut your boards to fit snugly between the rafters - Celotex is lightweight and straightforward to cut with a fine-toothed saw or sharp knife.
Take care with the cut edges, which can be sharp.
5. Install stop battens
Fix battens in place so the insulation boards butt directly against them, keeping everything securely positioned rather than relying on friction-fit alone.
If your rafters or roof structure need attention before you insulate, a roofer can assess the timber, while an insulation specialist can advise on the right combination of products to meet current building regs.
Building Regs: what you need to know
Celotex PIR boards are suitable for a wide range of pitched and flat roof insulation projects, both at joist level and above the roof deck.
Building Regulations set out required U-values (a measure of how much heat is lost through the structure) depending on your project type and location - these can differ slightly between Part L in England and Wales and Section 6 in Scotland.
Because the specific requirement depends on your roof construction, existing insulation, and whether it's new build or a retrofit, it's always worth checking with your local authority or an experienced insulation installer before finalising your specification - particularly if the work will need building control sign-off.
Celotex roof insulation prices
Costs vary by board range and thickness.
As a guide, excluding VAT:
Celotex GA4000 range: £16 - £32 per board
Celotex TB4000 range: £12 - £22 per board
Celotex XR4000 range: £34 - £54 per board
For related costs, see our guides to loft insulation costs and barge board replacement costs.
How much does loft insulation cost in 2026?
Loft insulation is one of the most cost-effective ways to improve your home's energy efficiency. Discover what loft insulation costs, what affects the price and how to compare quotes from checked loft insulation installers working in your area.
Hiring a professional to insulate roof rafters with Celotex
If you're not confident carrying out the work yourself, hiring an experienced installer can:
Save you time and stress
Ensure the work meets current Building Regulations
Help you choose the right thickness and combination of products for your roof
Spot any structural issues before they become bigger problems
As with any home improvement project, it's worth getting at least three quotes from different tradespeople to make sure you're paying a fair, competitive price.
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Celotex for conservatory roofs
Celotex is also commonly used to insulate conservatory roofs, where its slim profile and BBA (British Board of Agrément) certification make it a reliable option for improving thermal comfort without adding excessive weight to a lightweight structure.
Given the specific challenges of conservatory roofs, this is one job we'd generally recommend leaving to an experienced installer.
Before you insulate your roof rafters...
Insulating rafters checklist
FAQs
Yes - it's one of the most effective ways to improve a home's energy efficiency and comfort, provided it's combined with the right air gap and, where needed, an additional layer of insulation to meet Building Regs.
Yes. Leave roughly 25mm-50mm between the Celotex boards and the roofing felt or membrane behind them, to allow air to circulate and prevent condensation forming.
Generally, no - this often doesn't meet the U-values required by current Building Regulations on its own, rafter depth can limit how much insulation you can fit between them, and a single layer between the rafters does little to reduce cold bridging through the timber itself. Most compliant installations combine between-rafter insulation with a layer above or below.
This depends on your rafter depth and the U-value you need to hit, but 100mm or more is a common starting point where space allows. An insulation specialist can confirm the right spec for your specific roof.
Yes - Kingspan, Ecotherm, and other PIR boards of a comparable thickness and specification will generally perform similarly. The brand matters less than getting the thickness, air gap, and installation right.
For a competent DIYer, a single roof slope might take a weekend, factoring in preparation, cutting, and fitting. A professional installer will typically be faster, particularly on larger or more complex roofs.
It can, if done incorrectly. That's exactly why the air gap matters - without it, you risk trapping moisture in the roof void, which can lead to condensation and, over time, timber decay.
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