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Insulation calculator

Work out how much insulation you need. For a loft, you get the number of rolls, how to lay them in layers, and the U-value before and after. For a wall or floor, you get the number of insulation boards.

Man laying yellow mineral wool insulation between joists in a loft

Measure the depth between the joists.

270 mm of standard mineral wool is the usual recommendation. 250 mm is enough with a 0.040 W/mK product.

%

5% is plenty for an open loft. Allow 10% or more for boards, which need cutting round obstacles.

Your result

Rolls of insulation to buy

8

100 mm + 170 mm on top of your 0 mm

Layer 1: 100 mm rolls, between joists
313.89 m² per roll
Layer 2: 170 mm rolls, across joists
58.01 m² per roll
Depth after
270 mm
Area of loft
30 m²
U-value now
2.3 W/m²K0 mm of insulation
U-value after
0.16 W/m²K270 mm of insulation
  • Roll sizes vary by brand. We use Knauf Loft Roll 44 sizes: 100 mm = 13.89 m², 150 mm = 9.18 m², 170 mm = 8.01 m², 200 mm = 6.84 m². Check the pack size and your U-value with your supplier. 200 mm rolls in particular can be as small as 5.3 m².
  • The U-values assume mineral wool at about 0.044 W/mK, laid evenly. Squashed insulation or boarding laid straight on it makes the U-value worse; raise the boards on risers.
  • Lay the first layer between the joists (split the roll along its perforations), and the next layer across them at right angles, butted tight. Keep the eaves clear for ventilation and keep insulation away from downlights and cables unless they are rated for it.

Add this to your project shopping list, then print it or take it to the merchant.

Open shopping list

How to use the insulation calculator

  1. Choose What are you insulating?: Loft (rolls) or Wall or floor (boards).
  2. For a loft, give the size as Length and width or as an Area in m². Then set Insulation you have now (measure the depth between the joists) and Depth you want. 270 mm is the usual choice.
  3. For boards, type the Area to cover and pick a Board size, or choose My own size and enter the length and width.
  4. Leave Extra for cutting and waste at 10%, or drop it to 5% for an open loft. Boards need more because of cutting round obstacles.
  5. Read the rolls or boards to buy, then tap Add to shopping list to keep them, or Share link to send the result.

Why use this calculator

It tells you how to lay the rolls, not just how many. A loft goes up to 270 mm in layers: the first between the joists, the rest across them. The calculator works out which rolls go in each layer.

It shows the U-value before and after. You can see what your loft loses now and what the new depth saves, and whether you reach the 0.16 target for an existing loft.

It handles top-ups. If you already have 100 mm or 150 mm, it works out only the extra you need, not a whole new loft's worth.

It shows its working, including the roll sizes it assumes, so you can check the sums against the pack in your hand.

How we worked it out

Your numbers, step by step. This updates as you change the calculator.

  1. Loft area = 6 m × 5 m = 30 m².
  2. You have 0 mm and want 270 mm, so you need 270 mm more.
  3. Laying plan: 100 mm between the joists, then 170 mm across the joists.
  4. Each layer covers the whole loft. With 10% extra: 30 × 1.1 = 33 m² per layer.
  5. Layer 1 (100 mm between the joists): 33 m² ÷ 13.89 m² per roll = 2.38, rounded up to 3 rolls.
  6. Layer 2 (170 mm across the joists): 33 m² ÷ 8.01 m² per roll = 4.12, rounded up to 5 rolls.
  7. U-value now (0 mm) = 2.3 W/m²K. After (270 mm) = 0.16 W/m²K. The target for an existing loft is 0.16 W/m²K, so you meet it.

The formula

Loft rolls.

  1. Work out the loft area: length × width, or the area you enter.
  2. Work out the depth to add: depth you want − insulation you have now.
  3. Plan the layers. If you have under 100 mm, the first layer is 100 mm laid between the joists. The rest go across the joists, in rolls of 100, 150, 170 or 200 mm, butted tight. 100 mm between plus 170 mm across makes 270 mm.
  4. For each layer: rolls = loft area × (1 + extra) ÷ area of one roll, rounded up.
  5. The U-value before and after comes from the loft insulation depth in the table below.

Boards for a wall or floor. Boards = (area × (1 + extra)) ÷ the area of one board, rounded up. Standard sheets are 2,400 × 1,200 mm (2.88 m²) or 1,800 × 900 mm (1.62 m²), or you can enter your own size.

U-value by depth of loft insulation

Insulation at the joists None 100 mm 150 mm 200 mm 250 mm 270 mm 300 mm
U-value (W/m²K) 2.3 0.40 0.30 0.21 0.17 0.16 0.14

These are the standard RdSAP figures. They include the timber joists and assume mineral wool at about 0.044 W/mK, laid evenly. Between the table points we work out the value in a straight line.

What we assume

  • Roll sizes: we use the cover of Knauf Loft Roll 44 rolls: 100 mm = 13.89 m², 150 mm = 9.18 m², 170 mm = 8.01 m² and 200 mm = 6.84 m². Other brands differ, so check your pack. Rolls are 1,140 mm wide and can be split along their perforations to fit joists at 400 or 600 mm centres.
  • 270 mm and 250 mm: 270 mm of mineral wool at 0.044 W/mK reaches about 0.16 W/m²K. The Part L guidance uses 250 mm of mineral fibre as a typical build-up to reach 0.16, which works for a better product (about 0.040 W/mK). That is why both depths are on offer.
  • Two layers: 100 mm between the joists and 170 mm across them is the usual retail way to reach 270 mm. It’s a convention, not a rule.
  • Target: 0.16 W/m²K for an existing loft, from Part L (Approved Document L, Volume 1, England).
  • Extra: 10% by default for waste and overlaps.

Tips

  • Measure the depth you have between the joists in a few places, not just one. The depth can vary across the loft.
  • Don't squash the insulation. Squashed insulation does less. Boarding laid straight on it makes the U-value worse, so raise the boards on risers if you want a storage floor.
  • Keep the eaves clear so air can move through the roof space. Don't stuff insulation into the gap at the edges.
  • Keep insulation away from downlights and cables unless they are rated to be covered. If you aren't sure, ask an electrician.
  • Roll sizes vary by brand, so check the pack. 200 mm rolls in particular can be as small as 5.3 m². Do the sums again with the cover on your pack.
  • Check a whole wall or floor with the U-value calculator, and see how much heat a room loses with the BTU calculator.

FAQs

How much loft insulation do I need in mm?
270 mm of mineral wool is the usual recommendation, which gets a typical loft to about 0.16 W/m²K. 250 mm is enough if your product is 0.040 W/mK or better. Choose the depth you want in the calculator.
How many rolls of loft insulation do I need for my loft?
A 30 m² loft with no insulation, going up to 270 mm with 10% extra, takes 8 rolls: 3 rolls of 100 mm between the joists and 5 rolls of 170 mm across them. With 5% extra it is 7. Your loft will differ, so enter your own size.
How thick should loft insulation be in the UK?
270 mm of mineral wool, laid in two layers, is the usual advice. A common way is 100 mm between the joists and 170 mm across them. Part L (England) aims for a roof U-value of 0.16 W/m²K when you improve an existing loft.
What U-value do I need for loft insulation under Building Regulations?
The target is 0.16 W/m²K when you upgrade an existing loft in England. A new roof has a limit of 0.15. For comparison, 100 mm gives about 0.40, 150 mm about 0.30 and 270 mm about 0.16.
How many insulation boards do I need for a floor or wall?
Divide the area, plus waste, by the area of one board. For 20 m² with 10% extra (22 m²) and 2,400 × 1,200 mm boards (2.88 m² each), you need 8 boards. Pick your own board size if yours is different.
How much does it cost to insulate a loft in the UK?
The Energy Saving Trust puts professionally installed loft insulation at around £750 for a three-bedroom semi-detached home, and MyJobQuote gives a range of about £700 to £1,200. Prices vary with the size of the house. DIY mineral wool quilt is about £10 per m² for the material. This calculator doesn't price the job.
How much cavity wall insulation do I need?
Cavity wall insulation is blown in by a specialist installer, so you don't buy it in rolls or boards. Filling a cavity takes the U-value of an older wall from about 1.5 to about 0.7 W/m²K. Ask for a survey first.

Last reviewed: 6 November 2026 · How we calculate · Report an error