Loft insulation: the complete UK homeowner guide

Around a quarter of the heat in an uninsulated home escapes through the roof (Energy Saving Trust). Because warm air rises, an under-insulated loft quietly drains heat all winter, which is why loft insulation is usually the first upgrade worth making and one of the quickest to pay back. This guide is written to help you answer one question honestly: what is the best loft insulation for your property, application, budget and the space you have to work with?

Types of loft insulation

There is no single best material. The right choice depends on where it is going (over the loft floor, or in the slope of the roof), how much space you have, whether you want to board over it, and your budget. The main options for UK lofts are:

• Mineral wool and glass wool quilt. The most common and usually cheapest option for a cold-loft floor. It is rolled out between and over the joists. Typical thermal conductivity (lambda) is around 0.040 for glass wool and 0.038 for rock wool, so you need a fair depth to hit a good U-value, but there is plenty of headroom in most lofts. See our mineral wool guide and mineral wool vs Rockwool vs PIR comparison.
• PIR board. A rigid foam board with a much lower lambda (around 0.022), so it achieves the same U-value in roughly half the thickness of mineral wool. That makes it the go-to where space is tight, for example insulating the slope of a roof in a loft conversion, though it costs more per square metre.
• Multifoil. Thin, reflective foil-and-wadding rolls that work partly by reflecting radiant heat and need air gaps either side to perform. Useful where every centimetre counts, but it must be judged on its tested build-up performance, not a single lambda value. See our honest multifoil insulation guide.
• Loose-fill. Blown or poured granules, useful for awkward, irregular or hard-to-reach loft spaces where rolls will not sit neatly.

The practical rule: for a standard cold-loft floor, mineral wool quilt is usually the sensible, low-cost choice; where you are insulating the roof slope or are short of height, a rigid board such as PIR often earns its higher price. Judge any product on its declared performance for your application, not on marketing claims.

How thick should loft insulation be?

The Energy Saving Trust recommends a depth of around 270mm of mineral wool for a loft floor. At a typical lambda of 0.040, 270mm of mineral wool gets a loft to roughly a U-value of 0.14 W/m2K, which is inside the sort of target current Building Regulations expect for a loft (around 0.16, but always verify against the current regulations for your project). A lower-lambda board such as PIR reaches the same target in less depth. If your existing insulation is thin, patchy or compressed, topping it up to a full modern depth is usually worthwhile. Our loft insulation depth guide goes into this in detail, and you can check any build-up with our free U-value calculator.

How much does loft insulation cost?

Costs depend on your loft size, the material, and access. As a sourced guide:

We do not publish an invented national average. For a fuller breakdown by house type and per square metre, see our loft insulation cost guide, and always compare a few quotes rather than accept the first.

How much could you save?

Savings are largest when you go from an uninsulated loft to a full 270mm. The Energy Saving Trust's figures for going from 0 to 270mm (England, Scotland and Wales) are roughly £360 a year for a detached house, £190 for a semi and £140 for a mid-terrace. Loft insulation lasts around 40 years, so over its life it pays back many times over. One honest caveat: if your loft is already insulated to around 120mm, topping up to 270mm saves far less (of the order of £17 a year), so the big wins are for lofts with little or no insulation. We will always tell you when a job is unlikely to be worth it.

Between the joists or between the rafters?

There are two ways to insulate a loft, and they suit different goals. A cold loft is insulated at floor level, between and over the joists, which keeps the heat in the rooms below and lets the loft itself stay cold and ventilated. This is the standard, cheapest approach for a loft you only use for storage. A warm loft is insulated in the slope of the roof, between and under the rafters, which keeps the loft space itself warm and is what you need for a loft conversion or a room in the roof. Insulating the rafters usually calls for a lower-lambda board such as PIR, or a carefully installed multifoil build-up, because there is less depth to work with. For an honest look at which approach holds its performance over the years, see our comparison of multifoil vs mineral wool.

Boarded lofts and loft conversions

If you want to board your loft for storage, remember that laying boards straight onto the joists squashes the insulation and leaves you under-insulated. The fix is to raise the boarding on loft legs or a raised platform so the full depth of insulation sits undisturbed beneath it. For a full loft conversion, insulation moves to the roof slope and the walls, and Building Regulations and ventilation both matter, so it is worth getting proper advice. We can match you with installers who understand both.

Loft insulation and condensation

A common worry is that loft insulation causes damp. It rarely does directly. Insulation makes the loft colder, which is normal, but condensation is a ventilation and moisture problem, most often caused by insulation being pushed into the eaves and blocking airflow during a top-up. The answer is to keep your insulation and restore the ventilation, not to rip it out. Our loft condensation and damp guide explains the full picture, and if you have already found mould in the loft we cover that too.

How to choose, and get it fitted well

Insulation only performs if it is installed properly: the right depth, no gaps, eaves kept clear for ventilation, and no compression under boards. Whichever material you choose, workmanship matters as much as the product. When you are ready, we can match you with vetted, accredited installers so you can compare no cost quotes side by side. See how to pick a good one in our loft insulation installers guide, or tell us about your loft in under a minute and we will do the legwork.