Loft insulation calculator: top up to 270 mm

How it works

Enter your loft area, or its length and width, then measure the joists and the insulation already in place. Choose the roll sizes you plan to use for each layer. The calculator works out the depth still needed, fills between the joists only when the current insulation is shallower than the joists, and adds a cross layer when you select it.

The result shows the fractional roll requirement from loft area ÷ published roll coverage, alongside the whole rolls to buy. If a layer needs more depth than one roll provides, the calculator counts enough full courses to meet or exceed that depth. Each course is rounded up separately, so it does not assume offcuts from one layer will cover another.

This is a material estimate. Loft shape, obstructions, eaves, cut waste and how insulation is fitted can change the amount you need. Check the product pack and installation guide before buying or fitting it.

Formula

• Find the depth still needed

  Depth to add = max(0, target depth − existing insulation depth)

  If the existing depth already meets the target, the additional depth is zero.

• Fill between the joists

  Between-joist depth = smaller of (depth still needed) and (joist depth − existing depth)

  This layer is added only when the current insulation sits below the top of the joists.

• Add the cross layer

  Cross-layer depth = remaining target depth above the joists

  This part is used when the cross layer is switched on.

• Turn depth into rolls

  Calculated rolls = loft area ÷ roll coverage × courses Rolls to buy = ceil(loft area ÷ roll coverage) × courses

  Each course rounds up separately, so the estimate does not reuse offcuts between courses.

Worked example

1. 1

   Calculate the extra depth

   270 mm target − 100 mm existing = 170 mm to add

   The example has 100 mm insulation between 100 mm joists.

2. 2

   Check the between-joist layer

   100 mm existing insulation = 100 mm joist depth → 0 mm to add between joists

   The existing layer is already level with the joist tops.

3. 3

   Calculate cross-layer rolls

   40 m² ÷ 8.01 m² per roll = 4.99 rolls → 5 whole rolls

   The 170 mm cross layer brings the total to 5 rolls for this example.

Assumptions and limits

The default target is 270 mm, based on Energy Saving Trust guidance. It is a general recommendation for loft insulation, not a guarantee that every roof or building is suitable. Ask a qualified installer about damp, roof ventilation, recessed lighting, water tanks or other unusual conditions.

Published roll coverage is used as stated by the manufacturer. The calculator assumes each course covers the full loft area, does not reuse offcuts, and does not adjust for hatches, tanks, sloping edges or irregular bays. Use the joist spacing measurement to check that the selected roll can be cut to fit the actual clear gap, the calculator does not deduct unusable offcuts.

If you turn the cross layer off, the result shows the remaining depth to the target. Product thicknesses are installed in whole courses, so the resulting depth can exceed the target slightly.

Frequently asked questions