Loft insulation calculator
Find out exactly how many rolls or boards you need to insulate your loft floor. We compare mineral wool and PIR side by side, and check your result against current standards in your region.
This calculator covers loft floor insulation — the joists above your ceiling. For sloping rafter insulation in a converted loft, a separate calculator is coming soon.
Loft floor dimensions
If your loft isn't a simple rectangle, measure each section and add the areas together.
Existing insulation
Mineral wool
7rolls
- Add
- 250 mm
- R-value
- R 5.7
- U-value
- 0.17 W/m²K
Cheap and DIY-friendly. Watch for itch and dust.
PIR boards
12boards
- Add
- 150 mm
- R-value
- R 6.8
- U-value
- 0.14 W/m²K
Half the thickness for the same performance, but several times the price.
Regulation check
When fitted properly this meets Approved Document L (UK)
Target: 270 mm
Critical — read before you start
- Leave the eaves vent gap clearDon't push insulation right to the edge of the loft. Block the eaves and you trap moisture, which leads to mould and rotted rafters. Use rafter trays to keep an air gap above the insulation at the eaves.
- Fit fire hoods over downlightersRecessed ceiling spotlights get hot. Burying them under insulation without a fire-rated hood is a real fire risk. Fit hoods or check your downlights are IC-rated for direct cover.
Things to watch for
- Wear PPE for mineral woolGlass and mineral wool itch and irritate your lungs. Long sleeves, gloves, a mask, and goggles. This is the single biggest reason DIYers regret the job.
- Insulate the loft hatch tooAn uninsulated hatch is a hole in your insulation. Fit a piece of board to the back of the hatch and add a draught seal around the frame.
- Don't compress the woolCompressed mineral wool loses R-value in proportion. If your joists aren't deep enough for the full thickness, lay a second layer over the joists rather than squashing the first.
- Don't insulate under cold water tanksOlder UK homes often have a cold water tank in the loft. Insulate around and over the tank, but leave the floor underneath uninsulated so rising heat from the rooms below stops it freezing.