Why Getting Your Loft Insulation Right Matters

A quarter of the heat in an uninsulated home escapes straight through the roof, which is why the loft is usually the first place to tackle when you want a warmer house and smaller bills. In a typical UK semi-detached home, topping up loft insulation properly can cut heating costs noticeably and pay for itself within a few years. But the material you choose matters as much as the thickness, and the three main options — mineral wool, sheep wool, and rigid boards — suit very different lofts and budgets. Here's how they compare in practice.

Mineral Wool: The Dependable Workhorse

Mineral wool, often called glass wool or rock wool, is what most people picture when they think of loft insulation. It comes as rolls or slabs in a dense, fibrous texture, with a thermal conductivity of roughly 0.035 to 0.044 W/mK, meaning it performs well for the money.

For a cold loft with joists at ceiling level, the current guidance is to build up to around 270mm of mineral wool, usually a base layer between the joists plus a cross-layered top-up at right angles. This gives you a U-value close to 0.16 W/m²K, which is where the building regulations sit for most roof upgrades.

• Cost: the cheapest option by a wide margin — expect roughly £4 to £12 per square metre depending on thickness and whether you buy rolls or slabs.
• Fitting: forgiving and quick. It squashes into awkward corners, cuts with a bread knife or insulation saw, and doesn't need precise measuring.
• Watch out for: itchy fibres, so wear gloves, a dust mask, goggles and long sleeves. It also loses performance if compressed, so never squash it down to fit under boards.

Sheep Wool: Natural, Breathable, and Pleasant to Handle

Sheep wool insulation has grown steadily in popularity, particularly in older rural properties where breathability matters. Its thermal conductivity is around 0.038 to 0.040 W/mK, so it sits close to mineral wool in performance, but it behaves quite differently in a damp British climate.

Wool is hygroscopic, meaning it can absorb and release moisture vapour without losing its insulating value, which helps regulate humidity in a loft. It's also naturally resistant to fire, and it's treated against moths and mould during manufacture. For anyone insulating a stone cottage or a timber-framed barn, that breathability can be a real advantage.

• Cost: significantly more than mineral wool, typically £12 to £20 per square metre for a 100mm layer.
• Fitting: a joy compared with glass wool. No itch, no special PPE beyond a basic mask, and it holds its shape well between joists.
• Watch out for: it needs a dry loft. If your roof leaks or you have persistent condensation, fix that first, because wool can hold moisture and encourage problems if it stays wet.

Rigid Boards: The Space-Saver for Tight Spots

Rigid insulation boards — usually PIR or phenolic foam with a foil facing — offer the best thermal performance per millimetre, with conductivity as low as 0.022 to 0.023 W/mK. That makes them ideal where depth is limited: between rafters in a warm loft conversion, or under a boarded floor where you can't afford to lose headroom.

Because they're so effective, you can hit the same U-value with far less thickness — often 100mm of PIR does the job of 150mm or more of wool. The foil face also acts as a vapour control layer when taped at the joints, which helps keep warm, moist air out of the structure.

• Cost: the priciest per square metre, roughly £15 to £25 for a 100mm board, though you buy less of it.
• Fitting: slower and fussier. Boards must be cut accurately, gaps filled with expanding foam or offcuts, and edges sealed. Dust from cutting is fine and irritating, so cut outside where you can.
• Watch out for: they don't bend around obstructions. Pipes, cables and awkward trusses all need careful cutting around, and any gap dramatically reduces performance.

Cost, Performance and Effort at a Glance

• Cheapest and easiest: mineral wool, especially for a simple cold loft.
• Best for old, breathable buildings: sheep wool, where damp management matters as much as U-values.
• Best for limited space: rigid boards, particularly in warm lofts and rafter-level jobs.
• Best thermal value per pound: mineral wool for large areas, PIR where thickness is the constraint.

Getting the Details Right

Whichever material you choose, a few fundamentals decide whether the job actually works. Keep the eaves ventilation clear — use rafter trays or wool trays so air can still flow — and never block soffit vents. Insulate and drape the loft hatch, and lag any pipes or water tanks sitting up there, because a cold loft plus a warm tank is a recipe for frozen pipes in January.

If you're laying wool over existing joists, consider raising them with loft legs or timber battens so you keep storage space without compressing the insulation. And if you're unsure whether your loft is a cold or warm roof design, check before you buy: putting the wrong material in the wrong place can trap moisture and cause rot. Done properly, though, a well-insulated loft is one of the most satisfying jobs in a rural home — quiet, warm, and noticeably cheaper to run.