Insulation calculator

Rolls of loft insulation, or slabs and boards for walls and floors, for the thickness you want, layer by layer.

Sets the roll, slab and board sizes and thicknesses.

Where
Areas to insulate

Area 1

Windows, doors, a loft hatch, a water tank platform: anything inside the areas above that won't be insulated.

A starting point, not a requirement. Building regulations set the performance new work must reach, and grant schemes may ask for more: check with building control or your installer. This counts materials; it doesn't work out U-values.

Extra for offcuts, spills, breakages and mistakes.

You'll need

Layer 1: 100 mm

4 rolls

Includes 10% for waste.

  • Layer 2: 170 mm5 rolls
How it's worked out
  1. Area 1: 6 m × 4 m = 24 m²
  2. 270 mm from the thicknesses sold: 100 mm + 170 mm = 270 mm, 2 layers
  3. Lay the first layer between the joists and the next across them, so the joists don't bridge the insulation. Keep cables on top of it, leave the eaves clear for air, and don't cover light fittings unless they're made to be covered.
  4. Layer 1, 100 mm: 24 m² + 10% for waste ÷ 8.33 m² a roll → 4 rolls
  5. Layer 2, 170 mm: 24 m² + 10% for waste ÷ 5.62 m² a roll → 5 rolls

Estimates only — verify before purchase. Check the quantities against your own measurements and the product's label.

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What it works out

This works out how many rolls of loft insulation, or slabs and rigid boards for walls and floors, you need for the area and thickness you want. It builds the thickness from layers you can actually buy.

It counts materials. It doesn't work out U-values or say whether a thickness meets building regulations: building control or your installer does that.

How it works

Add the areas to insulate and take off anything you won't cover, like a loft hatch or a window. Then choose the thickness. Insulation is sold in set thicknesses, so the calculator makes yours from those, with the least left over and then the fewest layers.

For each layer, it adds the waste allowance to the area and divides by what one roll, slab or board covers, using the sizes shops in your region sell.

In a loft, lay the first layer between the joists and the next across them, so the joists don't bridge the insulation. Keep cables on top, leave the eaves clear for air, and don't cover light fittings unless they're made for it.

A worked example

A loft 6 m by 4 m, insulated to 270 mm with mineral wool rolls.

  1. Area 1: 6 m × 4 m = 24 m²
  2. 270 mm from the thicknesses sold: 100 mm + 170 mm = 270 mm, 2 layers
  3. Lay the first layer between the joists and the next across them, so the joists don't bridge the insulation. Keep cables on top of it, leave the eaves clear for air, and don't cover light fittings unless they're made to be covered.
  4. Layer 1, 100 mm: 24 m² + 10% for waste ÷ 8.33 m² a roll → 4 rolls
  5. Layer 2, 170 mm: 24 m² + 10% for waste ÷ 5.62 m² a roll → 5 rolls

The result

  • Layer 1: 100 mm4 rolls
  • Layer 2: 170 mm5 rolls

These are the calculator's own figures for these measurements, with 10% for waste. Change any of them above and it works it out again.

Common mistakes

  • Squashing the insulation. Mineral wool works because of the air in it. Compressed under boards, it insulates far less. Raise the boarding instead.
  • Covering cables and downlights. Cables buried in insulation can overheat. Lift them on top, and keep insulation off light fittings that aren't rated for it.
  • Blocking the eaves. A loft needs air moving through it, or condensation builds up. Leave a gap at the eaves.
  • Assuming any thickness meets the rules. Building regulations set the performance for new work and some grants ask for more. Check before you buy.

Questions people ask

How many rolls of loft insulation do I need?

Measure the loft floor, choose the thickness, and the calculator splits it into layers sold in shops and divides each layer's area by what one roll covers.

How thick should loft insulation be?

The calculator starts from a common figure for your region, but building regulations and grant schemes set the performance. Ask building control or your installer what your project needs.

Why two layers?

The first layer fits between the joists, and the second goes across them. That covers the joists, which would otherwise let heat through.

What's the difference between slabs and boards?

Slabs are mineral wool batts that fit between studs or joists. Boards are rigid foam, which insulates more for the thickness but costs more and must be cut to fit tightly.

Is there anything electrical to watch for?

Yes: cables under insulation can overheat, and some light fittings mustn't be covered. Nothing electrical is worked out here; see our electrical guidance.