How Much Loft Insulation Do I Need?

The short answer

For wool-based insulation laid over a loft floor, 270mm is the depth most commonly used as a benchmark in the UK. It is normally built up as one layer between the joists and a thicker layer laid across them. That figure applies to mineral and similar wools; other products — foil systems, rigid boards, sheep wool — have their own specifications, so 270mm is a useful rule of thumb rather than a number to apply to everything.

Where 270mm comes from

The benchmark reflects the point at which adding more wool gives diminishing returns for a typical loft floor. Older homes were often insulated to 100mm or less when standards were lower, so a great many lofts sit well below today’s benchmark. The Energy Saving Trust is a good independent source for current guidance on roof and loft insulation, and our loft insulation guide explains how we approach it.

How to check what you already have

You can usually get a rough idea yourself: with a torch and a ruler, measure from the ceiling (between the joists) up to the top of the existing insulation in a few places. If the wool is level with or below the tops of the joists, you almost certainly have room to improve. Bear in mind old insulation often settles and compresses, so it may be thinner than it looks. A survey measures this properly and checks the condition of the material.

What we actually find on surveys

Most lofts we survey fall well short of 270mm. Two recent jobs show the pattern. In an Ingatestone bungalow the old insulation was under 100mm deep, flattened and torn in places, with no roof ventilation. In a loft in Malton the old fibreglass had thinned out and was covered in dust and debris. In both cases a top-up was not the right answer: the old material came out, ventilation went in, and a full new layer was laid.

The depth on its own does not tell the whole story. A loft can look full but hold far less warm air than it should if the wool is compressed, damp, dirty or patchy. That is why we measure in several places and look at the condition, not just the number on the tape.

Depth and your joists

The joists in many older homes are only around 100mm deep, so the first layer fills the gaps between them and the second, thicker layer is rolled across the top at right angles. Once the insulation is above the joists you can no longer walk on them safely or see where they are, which is worth thinking about before the work if you use the loft for storage.

If you need a storage area, plan it first. Boards laid straight on the joists squash the wool underneath. Raised boarding sits on legs above the full depth, so you keep both the storage and the insulation. Our guide on boarding over loft insulation covers the options.

Other materials

The 270mm figure is for mineral wool. Sheep wool and recycled plastic rolls are also laid in layers, but the depth needed depends on the product, so it is checked against the maker's data sheet. Foil and board systems work differently and have their own specifications. See our mineral wool page for how the most common option is installed.

When adding more is worth it

If you have little or no insulation, a first installation is usually very worthwhile. If you already have a moderate, sound layer, a top-up to the benchmark often still makes a sensible difference. There is a point of diminishing returns, though — going far beyond the benchmark rarely pays for itself, and the space near the eaves must be kept clear for ventilation rather than crammed full.

Don’t forget ventilation and boarding

More insulation slows heat escaping, which can make the loft colder and more prone to condensation if airflow is poor. Insulation and ventilation are best considered together. And if you store things in the loft, remember that boards laid straight on the joists will compress the wool — a raised boarding system keeps the depth you have paid for.

The honest bottom line

270mm is a helpful target for wool, but the genuinely useful answer for your home depends on what is already up there, the condition it is in, the roof’s ventilation and the material chosen. An assessment gives you a clear written recommendation rather than a guess.