UK Loft Insulation Calculator

Heat rises, and an under-insulated loft is one of the most significant places a typical UK home loses warmth in winter. This calculator works out how much extra loft insulation you need to reach the current recommended depth, how many rolls to buy for your chosen product, and gives a rough, honest estimate of what it might save on your energy bills.

How to use the loft insulation calculator

Enter your loft's floor area, in square metres or square feet, your current insulation depth in millimetres (enter 0 if you have none), and your target depth, defaulting to the recommended 270mm. Enter your chosen product's roll width, length and thickness, using the common defaults if you are not sure, and a price per roll if you know it. The calculator shows how many additional millimetres you need, how many layers that means with your chosen product, and the total number of rolls to buy.

How the calculation works

The calculator first works out your additional depth needed, your target depth minus your current depth. It then divides that by your chosen roll's thickness to work out how many layers you need to lay, rounding up, since a partial layer does not achieve the full benefit. Each roll covers a fixed area, based on its width and length, so the calculator works out how many rolls cover your loft floor area in one layer, then multiplies that by the number of layers needed to get your total rolls required.

For the potential saving, the calculator uses broad, illustrative ranges based on your starting depth, since topping up from little or no insulation saves considerably more than topping up an already reasonably insulated loft. These figures are a rough guide only, since actual savings depend heavily on your home's size, heating system, fuel prices, and how well insulated the rest of your home is.

Worked example

Take a loft with a floor area of 40 square metres, currently with 100mm of insulation, aiming for the recommended 270mm, using a standard 1,140mm by 6.75 metre roll at 100mm thickness.

The additional depth needed is 270mm minus 100mm = 170mm. Dividing by the 100mm roll thickness gives 2 layers needed, rounding up. Each roll covers around 7.7 square metres (1.14 metres times 6.75 metres), so covering the 40 square metre floor in one layer needs 6 rolls. Across 2 layers, that comes to 12 rolls in total.

Now try changing the current depth to 0mm in the calculator above, representing a loft with no insulation at all. You will see the additional depth needed rise to the full 270mm, needing 3 layers instead of 2, and the estimated saving range shown also increases noticeably, since starting from bare joists gives the largest potential improvement of all.

Types of loft insulation

Mineral wool, sometimes sold as glass wool or rock wool, is the most common loft insulation type in the UK, typically sold in rolls and reasonably straightforward for a DIYer to fit. It is what the 270mm recommendation is generally based on. Rigid foam boards achieve the same insulating effect at a shallower depth, which can suit lofts with limited headroom, but usually cost more per square metre of coverage than mineral wool rolls.

Loose-fill insulation, blown in by a professional installer, is another common option, particularly for lofts with awkward shapes or lots of obstructions where rolling out standard rolls is difficult. It is rarely a straightforward DIY job, since specialist equipment is used to blow the material evenly to the correct depth.

Whichever material you choose, check the manufacturer's stated depth for reaching the equivalent performance of 270mm mineral wool, since materials with better insulating properties per millimetre may need a shallower depth to achieve a similar result, which changes the layers and rolls calculation accordingly.

Common mistakes DIYers make

A common mistake is compressing existing insulation by simply laying a new roll straight on top without checking it has not become flattened or damp over the years. Compressed insulation performs considerably worse than its nominal thickness suggests, so it is worth checking the condition of existing material before assuming it is still doing its job properly.

It is also easy to forget ventilation gaps at the eaves, the point where the roof meets the outer walls. Blocking these gaps completely with insulation can trap moisture in the roof space and lead to damp or timber problems over time, so leaving the recommended clear gap at the eaves matters just as much as reaching the target depth across the main floor area.

Finally, take care around loft hatches, light fittings, and electrical cables. Recessed light fittings in particular often need a specific fire-rated cover or a safe clearance from insulation to avoid overheating, and standard cables are not designed to be buried under thick insulation without checking their rating first. For anything beyond a straightforward, unobstructed loft, get advice from a qualified installer before starting work.

Related calculators

If you are budgeting for wider home improvements, our paint coverage calculator and kitchen renovation cost calculator can help with other rooms. Since loft insulation is really a long-term saving project, our savings goal calculator can help you plan towards the upfront cost, and our guide to compound interest explains how regular saving, much like a well-insulated loft, pays off more the earlier you start.