Foam insulation comes in two main forms: spray foam (applied wet, expands in place) and rigid foam boards (cut to fit). Costs range from £20 to £50 per square metre depending on type and application. ECO4 can fully fund foam insulation for eligible households, and the Warm Homes: Local Grant may also cover it. But foam insulation carries risks that other insulation types don't, particularly around mortgage valuations and ventilation, so it pays to understand exactly what you're getting into before committing.
What Is Foam Insulation and Is It Right for Your Home?
Foam insulation is any insulation product made from polyurethane, polyisocyanurate (PIR), or polystyrene foam. It traps air (or gas) in tiny cells, which slows heat transfer through your walls, roof, or floors. That's the technical bit.
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The practical bit is more interesting. Foam insulation has become popular because it's effective per centimetre of thickness, meaning you lose less internal space compared to mineral wool or natural fibre alternatives. A 50mm rigid foam board can match the thermal performance of 100mm of mineral wool. For homes where every centimetre counts, that matters.
But here's what most guides won't tell you: foam insulation isn't always the right answer.
Spray foam applied to the underside of roof tiles has caused serious problems for thousands of UK homeowners. Some mortgage lenders now refuse to lend on properties with spray foam in the roof space. We see this question constantly, and the answer isn't simple, so we've given it a full section below. If you're considering spray foam specifically, read our detailed spray foam guide before doing anything else.
Rigid foam boards, on the other hand, are widely accepted and have been used in UK construction for decades. They're a standard part of new-build walls, floor insulation, and flat roof upgrades. Very different risk profile.
So the first question isn't really "is foam insulation right for my home?" It's "which type of foam, and where?"
Types of Foam Insulation: Spray Foam vs Rigid Foam Boards
These two products share a name but behave very differently in practice.
Spray foam insulation
Spray foam is applied as a liquid that expands and hardens in place. There are two types:
Open-cell spray foam. Softer, spongier, cheaper. It absorbs moisture, which means it needs a vapour barrier in most applications. Thermal performance is decent but not exceptional. Roughly 0.035 to 0.040 W/mK.
Closed-cell spray foam. Denser, harder, more expensive. It acts as its own vapour barrier and has better thermal performance, around 0.021 to 0.025 W/mK. This is the stuff that causes arguments with mortgage lenders when applied to roof rafters.
The appeal of spray foam is that it fills gaps and irregular shapes that boards can't reach. Old stone walls, awkward loft spaces, areas around pipes and wiring. In theory, you get a continuous insulation layer with no cold bridges.
In practice, the quality depends entirely on the installer. Badly applied spray foam can trap moisture against timbers, block ventilation paths in roof spaces, and bond to roof tiles so firmly that the tiles can't be replaced without destroying the insulation. We've seen survey reports where spray foam has caused more problems than it solved, and the homeowner was left with a removal bill of £3,000 to £5,000.
Rigid foam boards
Rigid boards come in standard sheets, typically 1,200mm × 2,400mm, in thicknesses from 25mm to 150mm. The main types are:
PIR (polyisocyanurate). The most common type in UK retrofits. Brands like Celotex and Kingspan dominate. Thermal conductivity around 0.022 W/mK, which is excellent. Good fire resistance.
EPS (expanded polystyrene). Cheaper, slightly less effective per centimetre (around 0.032 W/mK), but perfectly adequate for many applications. You'll find this in external wall insulation systems.
Rigid boards are used in wall insulation, floor insulation, flat roof upgrades, and as part of external wall insulation systems. They're predictable, well-understood, and don't cause the mortgage headaches that spray foam does.
Honestly, for most homeowners, rigid foam boards are the safer choice. Spray foam has its place, but only when installed by a genuinely qualified contractor who understands ventilation requirements.
How Much Does Foam Insulation Cost in 2026?
Right, let's talk money.
Type
Cost per m² (supply + install)
Typical whole-house cost
Best for
Open-cell spray foam
£20–£30
£1,500–£3,000 (loft)
Lofts, timber-frame walls
Closed-cell spray foam
£35–£50
£2,500–£5,000 (loft)
Lofts where space is limited
PIR rigid board (walls)
£25–£45
£4,000–£8,000
Internal wall insulation
PIR rigid board (floor)
£20–£35
£1,200–£3,000
Suspended timber floors
EPS (external wall system)
£80–£120 (full system)
£8,000–£15,000
Solid-wall properties
Those EPS external wall figures include the full system: boards, mesh, render, and labour. The foam itself is the cheap part. If you're looking at external wall insulation, the costs are driven more by scaffolding and finishing than by the insulation material.
For context, mineral wool loft insulation costs £300 to £600 for a typical semi-detached house. So spray foam in a loft space costs roughly four to eight times more. Whether that premium is justified depends on your specific situation. If you have a loft conversion or want to use the roof space, spray foam between rafters makes sense. If you've got an empty loft you're just insulating at joist level, mineral wool is cheaper and perfectly effective. We've covered that option in detail in our loft insulation guide.
Can You Get a Grant to Cover Foam Insulation Costs?
Two grant schemes currently cover foam insulation in some form. A third recently closed.
ECO4
ECO4 is the big one. It can fully fund insulation for eligible households, meaning you pay nothing. The scheme is funded by energy suppliers and runs until December 2026, according to Ofgem's current programme timeline.
ECO4 covers cavity wall insulation, loft insulation, solid wall insulation, and in some cases, underfloor insulation. The insulation material used depends on what your installer recommends for your property, and foam products, both rigid boards and spray foam, are included where appropriate.
Eligibility is income-based. You'll typically need to be receiving a qualifying benefit like Universal Credit, Pension Credit, or Child Tax Credit, and your home needs an EPC rating of D or below. If you're not sure where your home sits, our EPC ratings guide explains what each band means and what it qualifies you for.
One thing to flag: ECO4 installers choose the insulation type based on a whole-house assessment. You can't ring up and say "I want closed-cell spray foam in my loft, please." They'll recommend whatever meets the scheme's technical standards for your property. Sometimes that's foam. Sometimes it's mineral wool. You don't always get to pick.
Warm Homes: Local Grant
This is a newer scheme, open now and running until the end of 2028. It's administered by local authorities rather than energy suppliers, so availability and the specific measures offered vary by area. Some councils are funding insulation projects including foam-based systems, particularly for solid-wall properties where external wall insulation with EPS boards is the standard approach.
The grant amount varies by local authority. There's no single national figure. Your council's scheme might cover the full cost or contribute a portion. It's worth checking even if you don't think you qualify for ECO4, because the income thresholds can be different.
Great British Insulation Scheme (closed)
GBIS closed in March 2026. It previously offered up to £3,000 towards insulation measures including cavity wall and loft insulation. If you've seen it mentioned on other sites as currently available, that information is out of date. You can read about what replaced it on our GBIS page.
Will Foam Insulation Affect Your Mortgage or Home Sale?
This is the section that matters most if you're thinking about spray foam. And honestly, this one depends on your situation and we can't give you a straight answer without knowing the specifics.
Here's what we know for certain: several major UK mortgage lenders, including Halifax, Nationwide, and NatWest, have at various points flagged spray foam insulation applied to roof rafters as a potential issue during property valuations. The concern is that spray foam bonded to the underside of roof tiles can mask defects, prevent proper inspection of the roof structure, and make tile replacement difficult.
The Royal Institution of Chartered Surveyors (RICS) published guidance in 2023 noting that spray foam insulation on roof structures should be assessed on a case-by-case basis, but that poorly installed spray foam could affect property value.
So what does this mean practically?
If you're planning to sell your home or remortgage within the next five to ten years, spray foam in the roof space is a risk. Not a guarantee of problems, but a risk. Some surveyors will flag it. Some lenders will want an independent assessment. Some buyers will walk away rather than deal with the uncertainty.
Rigid foam boards don't carry this risk. They're a standard construction material. No surveyor is going to raise an eyebrow at PIR boards in your walls or under your floor.
And here's a tangent that's worth knowing: some spray foam companies have gone bust in recent years, leaving homeowners with no warranty and no recourse if problems develop. The spray foam sector had a bit of a wild west period from 2020 to 2023 when demand surged and quality control didn't always keep up. The industry has tightened up since then, but the legacy installations are still out there causing headaches. Anyway.
Our position is clear: if you're insulating a loft and plan to stay in the property long-term, spray foam can work well when installed correctly by a certified contractor. If there's any chance you'll sell or remortgage, think carefully and get independent advice first. For walls and floors, rigid foam boards are the standard and sensible choice.
How to Find a Trusted Foam Insulation Installer in the UK
This is always worth doing properly.
For spray foam specifically, look for installers certified by the British Board of Agrément (BBA) or who hold a current NVQ in spray foam application. The National Insulation Association (NIA) maintains a list of member companies, and membership requires adherence to quality standards.
For rigid board insulation as part of a wider project like external wall insulation, your installer should be TrustMark registered. If you're going through a grant scheme like ECO4, the scheme itself requires TrustMark registration and PAS 2030/2035 compliance, so that box gets ticked automatically.
Three practical tips:
Get at least three quotes. Prices vary enormously. We've seen quotes for the same spray foam loft job range from £1,800 to £4,500 in the same city.
Ask about ventilation. Any installer who doesn't mention ventilation when discussing roof-space spray foam is a red flag. Proper ventilation paths must be maintained, and a good installer will explain exactly how they'll achieve this.
Check the guarantee. You want a minimum 25-year product guarantee backed by an insurance-backed warranty. If the installer goes under, the warranty should still be honoured by a third-party insurer. Ask for the policy details in writing before work starts.
If you're eligible for a grant, the installer will typically be assigned through the scheme. But you can still ask questions, request references, and verify their credentials. Don't assume that because it's grant-funded, you don't need to do any due diligence.
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Grant amounts and eligibility criteria are based on publicly available government data and may change. Always verify current terms directly with the scheme provider.
Frequently asked questions
Is spray foam insulation a good idea in 2026?
It depends entirely on where you're putting it and whether you might sell your home. For roof rafters, it's effective but carries mortgage and resale risks that rigid alternatives don't. For timber-frame walls or hard-to-reach spaces, it can be genuinely useful. For standard loft insulation at joist level, mineral wool is cheaper and less problematic. The key is getting a certified installer who understands ventilation requirements.
Can I get foam insulation for free through a grant?
Yes, if you qualify for ECO4. The scheme fully funds insulation for eligible households receiving qualifying benefits, and your home needs an EPC rating of D or below. You don't choose the insulation type yourself though. The installer recommends the best material for your property based on a whole-house assessment.
Will spray foam insulation devalue my house?
It might. Several major lenders have flagged spray foam on roof structures as a concern during valuations. RICS guidance says it should be assessed case by case, but some surveyors will flag it regardless. Rigid foam boards in walls and floors don't carry this risk. If you're planning to sell within the next decade, get independent advice before committing to spray foam in your roof space.
How long does foam insulation last?
Both spray foam and rigid foam boards are rated to last 50 years or more when properly installed.
What's the difference between open-cell and closed-cell spray foam?
Open-cell is softer, cheaper, and absorbs moisture, so it needs a separate vapour barrier. Closed-cell is denser, more expensive, and acts as its own moisture barrier with better thermal performance. Closed-cell is the type most commonly applied to roof rafters, which is also the type that causes the most debate with mortgage lenders. For most loft applications, closed-cell is the better performer but at roughly double the cost per square metre.