Guide
How much loft insulation do I need?
The common recommendation for a loft insulated at ceiling level is 270 mm of mineral wool in total, usually laid as 100 mm between the joists and 170 mm across them.
Written by the SK Trade team. Last reviewed 1 November 2026.
Two layers
Lay the first layer between the joists to fill the depth of the joists, then lay the second layer at right angles across the joists. This covers the joists, which otherwise act as cold bridges.
How many rolls?
Measure the loft floor area. For each layer divide the area by the coverage printed on the roll. Coverage varies by brand and thickness: as a rough guide a 100 mm roll covers around 8 m² and a 170 mm roll around 5 m².
Storage and walkways
If you need to store things in the loft, raise the boarding above the insulation on loft legs rather than squashing the insulation, which reduces its performance.
Ventilation
Keep eaves vents clear so air can move through the roof space and stop condensation.
Safety
Wear gloves, long sleeves, eye protection and a dust mask, step only on joists or boards, and use a light so you can see what you are doing.
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Related questions
How many rolls do I need for my loft?
Divide the loft floor area by the coverage printed on the roll, for each layer. Coverage varies by brand and thickness; a 100 mm roll typically covers around 8 m² and a 170 mm roll around 5 m².
How thick should PIR insulation be?
It depends on where it goes and the U-value required. Pitched roofs often need 100 mm or more between and under rafters; floors and walls vary. Use the maker's U-value tables or ask your designer.
Sources
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