Insulation Materials Compared for UK Homes

Why the Material You Choose Matters More Than You Think

Insulation is one of the few home improvements that pays you back every single winter, but treating it as a generic commodity is where projects go wrong. A product that performs beautifully in a ventilated loft can cause real damage in a solid brick wall. The three main families — mineral wool, rigid boards and natural fibres — differ in thermal performance, how they behave around moisture, and how much skill they demand on site. Get the match right and you will have a warmer, quieter, drier home. Get it wrong and you will be dealing with condensation, cold bridges or a bill that never quite drops.

Before you buy anything, work out the U-value you are targeting, the depth you can realistically sacrifice, and how your existing walls and roof manage water vapour. Those three answers narrow the field fast.

Mineral Wool: The Dependable Workhorse

Glass wool and rock wool remain the default choice for most UK lofts, and for good reason. Thermal conductivity sits around 0.035 to 0.044 W/mK, so a loft topped up to 270mm of mineral wool lands near 0.15 W/m²K — comfortably in line with current standards. It is cheap, widely stocked, non-combustible, and excellent at absorbing sound, which is why it doubles up well in party walls and floors.

• Best for: cold lofts between and over joists, suspended timber floors between joists, timber frame cavities, acoustic separation.
• Handling: straightforward to cut and friction-fit, though gloves, long sleeves and a mask are non-negotiable. The fibres irritate skin and airways.
• Watch out: it must not be compressed. Squeezing a 100mm roll into a 90mm gap loses more performance than the numbers suggest. Cut pieces 10–20mm wider than the gap so they grip the sides.

Also keep the eaves clear. Blocking the ventilation path at the wall plate is one of the most common causes of roof condensation we see.

Rigid Boards: Performance Where Space Is Tight

Polyisocyanurate (PIR) boards achieve roughly 0.022 to 0.023 W/mK, about twice the insulation per millimetre of mineral wool. Where a rafter depth of 100mm only allows 100mm of insulation, a PIR board gives you a U-value that mineral wool could not reach without dropping your ceiling height. Phenolic boards push slightly further still, while EPS and XPS are the cheaper, lower-performing cousins, useful below ground and under floors.

• Best for: rafter-level and flat roof insulation, dormer cheeks, internal wall lining, underfloor applications, anywhere headroom or floor space is precious.
• Installation demands: high. Boards must be cut accurately and fitted tight. Gaps of even 5mm leak heat and create a condensing surface, so every joint gets foil tape and every awkward edge gets expanding foam.
• Vapour behaviour: foil facings act as a vapour control layer when the joints are taped, which is a real advantage — but only if the taping is done properly.

Budget for wastage. Rigid boards are unforgiving of a badly measured cut, and you will use offcuts far less efficiently than you would with wool.

Natural Fibres: Breathable Options for Older Homes

Sheep wool, wood fibre, hemp, cork and recycled cellulose sit in a similar thermal range to mineral wool, generally 0.038 to 0.045 W/mK, but behave quite differently around moisture. They are hygroscopic: they absorb and release water vapour, buffering peaks rather than trapping them. In a solid-walled Victorian terrace where you cannot retrofit a cavity, that quality is genuinely valuable.

• Best for: solid wall internal insulation with lime plaster, timber frame retrofits, lofts in older properties, anyone wanting a lower-embodied-impact specification.
• Handling: far more pleasant to install than mineral wool — no itching, no respiratory irritation — but often more expensive, sometimes two to three times the cost of glass wool.
• Watch out: natural products still need correct detailing. Breathability does not compensate for a badly fitted vapour barrier in a modern airtight construction, and some wool products need checking on moth treatment.

Moisture Control: The Detail That Decides Everything

Thermal performance gets the headlines, but moisture decides whether your insulation lasts. Interstitial condensation forms inside the build-up when warm, damp air reaches a surface below dew point, and the damage is invisible until timber rots or plaster blows.

The principle is simple: keep the inside drier than the outside, and let any moisture that does get in escape. In modern timber frame and warm roof construction, that means a continuous vapour control layer on the warm side, taped and sealed at every penetration. In older solid walls, it means vapour-open materials and no impermeable barriers — a PIR lining on a solid brick wall with no ventilation can push the dew point straight into the brickwork.

Whichever route you take, pair the insulation with ventilation. Extractor fans in kitchens and bathrooms, trickle vents on windows, or a mechanical ventilation system with heat recovery are not optional extras; they are part of the package.

Installation Demands and Cost Realities

Roughly, mineral wool is the cheapest per square metre, rigid boards sit in the middle once you account for foam and tape, and natural fibres cost most. Labour differs just as much. Wool is forgiving and DIY-friendly. Rigid boards demand accuracy and patience. Natural fibres require tidy framing and careful handling to avoid slumping.

Remember that many insulation projects fall under building regulations, and building control will want to see a target U-value met and ventilation addressed. If you are insulating a loft or floor yourself, measure the joist depth first — it dictates your options more than anything else. If you are lining a solid wall internally, or converting a loft, it is worth having the detail checked before you commit. The right material, fitted well, will keep the house warmer for decades. The wrong one will simply give you a colder, damper surprise.