Energy bills in 2026 are lower than the 2022 peak but still well above where most of us remember them, and the fastest way to spend less is to stop paying to heat the street. Insulation is the least glamorous upgrade you can make and almost always the best value — yet the moment you start pricing it up you hit a wall of materials, brand names and salespeople who all swear their product is the only sensible one.
This guide is a plain-English round of uk insulation types compared — the three families you will actually be offered (mineral and natural wool, foam, and rigid boards), what each costs in 2026, and which one belongs in your loft, your walls and under your floor. There is no single best insulation; there is the right one for the job, the moisture conditions and your budget, and getting that match wrong is how people end up with damp, a wrecked EPC or a house a lender will not touch.
The three families of insulation, quickly
Almost everything on a UK merchant's shelf falls into one of three groups. Knowing the group tells you most of what you need before you ever look at a brand.
Mineral and natural wool (rolls and batts)
This is the soft, fibrous stuff most people picture: glass wool and rock (mineral) wool, plus natural options like sheep's wool and wood fibre. It comes in rolls or semi-rigid batts, it is cheap, it is easy to cut and stuff between joists or studs, and it is naturally non-combustible in the mineral versions. Its weakness is that it needs thickness to perform, so it is brilliant in a loft where you have all the headroom you want and poor where space is tight.
Foam — PIR/PUR boards and spray foam
Rigid foam boards (PIR and PUR — the familiar foil-faced boards from brands like Celotex and Kingspan) give you the best warmth-per-millimetre of any mainstream product, which is why they win wherever thickness is limited. Spray foam is a different animal: a liquid foam sprayed in place that expands and sets, used mostly in lofts and hard-to-reach voids. The two share a chemistry but behave very differently in a house, and as you will read below, one of them has become a genuine problem at sale time.
Rigid insulation boards (PIR, phenolic, EPS, XPS, wood fibre)
"Boards" overlaps with foam but is worth treating as its own category because it includes materials that are not foam at all — expanded and extruded polystyrene (EPS and XPS) for floors and external walls, phenolic boards for the very tightest U-value targets, and rigid wood-fibre boards for breathable retrofit on older solid-wall homes. Boards are the go-to when you are insulating a flat surface — a floor, a wall, a flat roof — and want a predictable, high-performance layer.
Compared: performance, cost and moisture
Three numbers decide most insulation arguments: how well it insulates, what it costs, and how it handles water. Here is where each family lands in 2026.
As a one-line summary before the detail: wool wins on price and breathability where you have space, foam boards win on performance where you do not, and natural wool or wood fibre wins on any older home that needs to breathe. Almost every good insulation decision is really a decision about which of those three constraints matters most in the room you are treating.
Thermal performance
Insulation is rated by lambda (thermal conductivity, W/mK) — lower is better. Mineral wool sits around 0.032–0.044, natural wool and wood fibre slightly higher (they insulate a little less per millimetre but breathe better), PIR/PUR around 0.022, and phenolic board the best of the mainstream lot at roughly 0.018–0.021. In practice that means a phenolic or PIR board hits the same warmth as mineral wool in about half to two-thirds the thickness. If you have the depth, wool gets you there for less money; if every millimetre counts, foam board earns its premium.
Cost in 2026
As a rough merchant-price guide before fitting: mineral wool loft roll is the cheapest per square metre, natural sheep's wool and wood fibre cost noticeably more for the same coverage, PIR/PUR board sits in the middle-to-upper range and has been volatile since the 2021–22 shortages, and phenolic board is the priciest per board. Fitting cost often outweighs material cost — a straightforward loft top-up you can do yourself for the price of the rolls, while external wall insulation is a scaffold-and-render job running into five figures. The gap between the cheapest and dearest quote for the same wall is routinely thousands of pounds, and the cheapest is not always the false economy people assume — what matters is a TrustMark-registered installer, a proper survey and a written guarantee, not the brand name on the board. Treat any single quote as a starting point and get three.
Moisture and breathability
This is where homes get damaged. Foam boards and spray foam are largely vapour-closed — great for keeping water out, risky if they trap moisture against timber or masonry that needs to dry. Mineral wool is vapour-open but loses performance if it gets wet. Natural wool and wood fibre are the breathable choice, which is exactly why they suit pre-1919 solid-wall and stone homes that were built to let moisture pass through. Match the material to how your wall was designed to behave, not to whichever product has the best lambda on paper.
Which insulation for which job
Loft and roof
For a standard cold loft, mineral wool is almost unbeatable on value: lay it to a total depth of around 270mm (a first layer between the joists, a second across them) and you are done. Only reach for boards in a loft when you are creating a warm room in the roof and need performance in a shallow rafter depth. This is also the job most homeowners can safely do themselves in a weekend — leave a ventilation gap at the eaves, do not squash the wool flat under boarding, and never cover downlights or a recessed extractor without the right fire-rated cover. If you want to board part of the loft for storage, use raised loft legs so the insulation keeps its full depth underneath rather than being crushed to half its value.
Cavity and solid walls
Cavity walls (most homes built after roughly 1930) are typically filled by injecting mineral wool or bead through the mortar joints — a professional, guaranteed job, not DIY. Solid walls (most pre-1930 homes) need either internal insulation (boards or breathable wood fibre on the inside face, which steals a little room) or external wall insulation (boards and render on the outside). Solid-wall work is the single biggest EPC lever most older homes have, and also the most expensive and the easiest to get wrong on moisture — get a survey first.
Floors
Suspended timber ground floors are usually insulated with mineral wool held on netting between the joists, or rigid boards cut to fit. Solid concrete floors take rigid boards (EPS or PIR) laid under a new screed, normally only worth doing during a bigger renovation. A draught-proofed, insulated ground floor makes a room feel warmer at foot level long before the thermostat reading changes.
Spray foam and your mortgage — read this before you sign
My blunt view: do not let anyone spray foam onto the underside of your pitched roof, whatever the doorstep salesperson tells you. Closed-cell spray foam can trap moisture against your roof timbers and hide the very rot it causes, and open-cell holds water like a sponge. The bigger, more immediate problem in 2026 is money: a large share of UK lenders now either decline or heavily caveat a mortgage on a home with sprayed roof insulation, because their surveyor cannot inspect the timber underneath. Homeowners who took a "free" or grant-funded spray-foam loft in good faith have found they cannot sell or remortgage without paying thousands to have it stripped out and an inspection report commissioned.
If your roof genuinely needs insulating at rafter level, use rigid boards cut between and under the rafters, or breathable wood fibre — both leave the timber visible and lenders comfortable. Spray foam has legitimate uses, but a domestic pitched roof marketed to you cold on the phone is almost never one of them.
EPC, building regs and grants in 2026
Insulation is one of the biggest movers on your EPC, and the EPC matters more than it used to: it is required to sell or let, it gates most grant funding, and proposed rebanding is tightening what counts as a good score. Loft and cavity-wall insulation are the cheapest points on the certificate; solid-wall insulation moves the most but costs the most.
Government support still exists but is means- and property-tested. The Great British Insulation Scheme and ECO4 can cover loft and cavity-wall work for eligible households, and the Energy Saving Trust keeps an up-to-date rundown of what is available and the numbers behind each measure. Any grant-funded or notifiable work should be done by a TrustMark-registered installer under PAS 2035, which protects you if something goes wrong — and gives a future buyer the paperwork they will ask for.
What to do in the next 30 minutes
• Check your loft depth with a tape measure. If the insulation is under about 200mm, a top-up in mineral wool is the highest-return upgrade in the house.
• Find your latest EPC on the government register and note the specific insulation recommendations and their estimated savings.
• Work out your wall type: cavity (post-1930, two skins of brick with a gap) or solid (older, single thick wall). It decides every wall option you have.
• If anyone has ever pitched you spray foam for the roof, dig out the paperwork now — you will want it before you next remortgage or sell.
• Save your EPC, any insulation guarantees and installer certificates in one place so they are ready when you sell.
A free Home+ account lets you track insulation upgrades in one place — log what you have done, store the guarantees and installer certificates, and keep a note of the next measure worth doing. It is a quieter way to raise your EPC over time without living permanently inside a renovation. If you eventually want quotes for cavity, solid-wall or floor work, you can find vetted installers when you are ready — no pressure, no doorstep.
Frequently asked questions
What is the best insulation for a UK loft?
For a standard cold loft, mineral wool laid to around 270mm total depth is the best value by a wide margin and something many homeowners fit themselves. Only choose rigid boards if you are turning the loft into a warm room and need performance in a shallow rafter depth.
Is spray foam insulation a problem when selling a house?
It can be. Many UK lenders in 2026 decline or restrict mortgages on homes with spray foam on the roof because a surveyor cannot inspect the timber beneath it. If you already have it, expect to need a specialist inspection report — and possibly removal — before some buyers can get finance.
How thick does insulation need to be?
It depends on the material's lambda value. Mineral wool needs roughly 270mm in a loft to hit current standards; a foil-faced PIR or phenolic board reaches the same warmth in around half that thickness, which is why boards are used where space is tight.
Will insulation improve my EPC rating?
Yes, usually significantly. Loft and cavity-wall insulation are the cheapest points on the certificate, and solid-wall insulation moves the score the most. Your EPC lists the specific measures and their estimated annual savings for your property.
Is natural wool or wood fibre worth the extra cost?
On a modern cavity-wall home, rarely — mineral wool does the same job for less. On a pre-1919 solid-wall or stone house it often is, because breathable wool and wood fibre let the wall dry the way it was built to, where a vapour-closed foam board can trap moisture and cause damp. Match the material to the age and construction of the wall.
Can I get a grant for insulation in 2026?
Possibly. Schemes such as ECO4 and the Great British Insulation Scheme fund loft and cavity-wall work for eligible households, based on income, benefits and property band. Check current eligibility on gov.uk and use a TrustMark-registered installer working to PAS 2035.
