Loft insulation calculator
Count mineral-wool rolls for one accessible rectangular cold-loft floor, from the depth and pack coverage you choose.
How the number is worked out
- Measure the loft floor. This first version uses one rectangle.
- Measure what is there. Use the uncompressed depth of insulation that is in good condition.
- Choose the build-up. Enter the target depth and the thickness of the exact mineral-wool roll.
- Use the pack coverage. BuildBatch multiplies the floor by the number of equal new layers, adds the visible allowance, then rounds up to whole packs once.
This is a material quantity, not a U-value or installation design. It does not decide whether the chosen product, depth or detail complies for your home.
A worked example
A 4.2 m × 3.6 m loft floor is 15.12 m². In the generic example, 100 mm is already present and one 170 mm layer reaches a chosen 270 mm target. One example pack covers 8.01 m² and the visible allowance is 5%.
| Working | Figure | Result |
|---|---|---|
| Loft floor | 15.12 m² | 100 mm |
| New layer and total | 1 × 170 mm | 270 mm |
| Layer area and 5% allowance | 15.12 m² | 15.876 m² |
| Divide by 8.01 m² and round up | 1.982022 | 2 packs |
The exact roll width must also suit the joist spacing or top layer. Copy the thickness and square-metres-per-pack figure from the exact product.
Stop and check first
- Damp, leaks or poor ventilation. Fix the cause and keep the eaves ventilation open.
- Wiring and recessed lights. Insulation can trap heat. Follow the equipment instructions and get electrical advice where needed.
- Tanks and pipes. A better-insulated loft is colder, so the water services need the right protection.
- Boarding and access. Do not compress deep insulation under boards. Any walkway or storage needs a suitable raised system and safe access.
- Another roof type. A room in the roof, insulation at rafter level, a flat roof, spray foam, loose fill and rigid boards are outside this calculation.
Questions
Why does the example use 270 mm?
Energy Saving Trust and current UK scheme guidance use 270 mm as a common mineral-wool recommendation. It is an editable example, not a claim that one depth is right for every roof, material or nation.
What if there is no insulation?
Enter 0 mm. The calculator counts enough equal layers of the chosen roll thickness to meet or pass your target, and shows the resulting installed depth.
Can I use two different roll thicknesses?
Not in this first release. It counts equal new layers of one chosen roll. Work out different layers separately or use the complete system design.
Can I use it for a room in the roof?
No. Insulation at rafters and sloping surfaces needs a different moisture, ventilation and thermal design.