Loft area
Depth (270mm is the widely cited current UK target; check current guidance as this can change)
Roll size
| Top-up | Amount |
|---|
How this calculator works
The top-up depth is simply the target depth minus what's already there; if the existing insulation already meets or beats the target, no top-up is needed and the roll count is zero. The area, plus wastage for trimming around joists and obstructions, divided by the area of one roll (its length × width) gives the roll count. Loft rolls are sold rated for a specific finished depth (adding a specific-depth roll over an existing layer to reach the target, rather than a generic thickness), so buy a product rated for the top-up depth you need, not the full target depth, if you're layering over existing insulation.
Why 270mm, and why it might not be your number
270mm of mineral wool (or the equivalent for other materials, since different insulation types have different thermal performance per millimetre) is the figure most commonly cited by UK energy suppliers and installers as the current recommended loft depth, broadly aligned with Building Regulations guidance for new work. It isn't a single fixed legal minimum for existing homes and can be revised, so check current guidance, particularly if you're using a material other than mineral wool, since materials like rigid foam board or blown cellulose reach the same performance at different depths.
What this calculator does not check
This tool covers roll quantity for a flat, unobstructed loft floor only. It doesn't check ventilation gaps at the eaves (blocking these with insulation can cause damp and timber rot, so a ventilation baffle is often needed at the eaves edge), whether loft hatch, pipes and the cold water tank need their own separate insulation, or loft board and walkway removal if the loft is already boarded over the existing insulation.