Why your loft is the first place to look

Heat rises, and in an uninsulated home roughly a quarter of your heating bill disappears straight through the roof. That is money you have already spent, drifting into a cold sky. The good news is that loft insulation is one of the cheapest and least disruptive energy upgrades you can make, and most UK homes have the space for it. The trickier part is choosing what to fill that space with.

The three materials you will come across most often are mineral wool, sheep wool and rigid foam boards. Each has a different personality: one is the dependable budget workhorse, one is the natural option, and one is the high-performance problem solver for tight spaces. Here is how to tell them apart.

Mineral wool: the reliable all-rounder

Mineral wool comes in two guises, glass wool and rock wool, and it is what most people picture when they think of loft insulation. It arrives as rolls or batts, usually in a yellowish or greenish tint, and it is made from molten rock or glass spun into fine fibres.

• Cost: the cheapest option by a comfortable margin, often the reason it wins.
• Performance: typically around 0.035 to 0.044 W/mK thermal conductivity, so you need roughly 270mm depth to hit the recommended loft U-value of 0.16 W/m²K in most of the UK.
• Installation: straightforward to lay between and across joists, and easy to cut with a bread knife or insulation saw.
• Watch out for: it irritates skin, eyes and airways, so wear gloves, long sleeves and a mask. It also compresses if you squash it, and squashed insulation performs far worse.

If you have a standard loft with generous joist spacing and a modest budget, mineral wool is hard to beat. It is also non-combustible, which matters if your loft contains wiring or a boiler flue.

Sheep wool: natural, breathable, kinder to handle

Sheep wool insulation has grown steadily in popularity, and it is easy to see the appeal. It is a natural fibre, usually treated with borax to resist moths, mould and fire, and it feels far more pleasant to work with than mineral wool. No prickling, no itching, no cloud of dust.

• Cost: considerably more than mineral wool, often two to three times the price for the same coverage.
• Performance: comparable to mineral wool, with thermal conductivity around 0.038 to 0.040 W/mK.
• Moisture: wool can absorb and release moisture vapour without losing performance, which suits older, breathable properties where trapped damp is a real risk.
• Green credentials: a renewable by-product with lower embodied energy than mineral wool, though how much that matters depends on your priorities.

Wool really earns its keep in period homes, converted lofts and anywhere you are nervous about condensation. If your roof is older and less than perfectly sealed, wool's ability to manage moisture is a genuine advantage rather than a marketing line.

Rigid foam boards: maximum performance, minimum depth

Rigid foam boards, usually polyisocyanurate (PIR) or expanded polystyrene (EPS), are the high-performance option. They deliver far more insulation per millimetre than wool, which makes them the material of choice when space is tight.

• Cost: the most expensive per square metre, though you need less thickness.
• Performance: PIR boards reach around 0.022 to 0.028 W/mK, so roughly half the thickness of mineral wool does the same job.
• Best uses: loft conversions, rafter-level insulation, dormer cheeks and any spot where headroom or joist depth is limited.
• Installation: must be cut precisely and fitted tightly, with edges taped or foamed. Gaps of even a few millimetres create cold bridges that undermine the whole job.

Boards are also the go-to choice for insulating between and above rafters when you want to keep the loft usable. They are less pleasant to cut, produce dust, and need careful sealing, but nothing else packs this much performance into so little space.

Choosing between them

Start with your loft, not the material. Ask yourself three questions.

• How much depth can you spare? If you have joists sitting well below the roofline and no plans to board over them, wool is fine. If headroom is tight, go rigid.
• What is your budget? Mineral wool will almost always give the best cost per unit of warmth.
• Is moisture a concern? In older, breathable buildings, wool's hygroscopic behaviour is a real benefit.

A common approach is to mix materials: mineral wool between the joists and a rigid board layer across the top to eliminate cold bridging. This hybrid often gives the best balance of cost and performance.

Practical points before you start

Whatever you choose, the details matter more than the label. Aim for a continuous layer with no gaps, and never compress the material to make it fit. Keep insulation clear of downlights unless they are rated for contact, and maintain a small air gap around any flue. If your loft floor has cables running through it, consider routing them above the insulation rather than burying them.

Finally, check whether you qualify for support. Several UK schemes and grants still help with loft insulation costs, particularly for older properties or lower-income households, and your local authority can point you in the right direction. Get the depth right, fit it neatly, and your loft will quietly pay you back every winter.