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Project guide

How much loft insulation do I need for 270mm?

The Energy Saving Trust recommends 270mm of mineral wool in a loft. Here is how many rolls that takes, from scratch or as a top-up.

Updated Based on manufacturer guidance, sources below

The short answer

From empty, lay 100mm between the joists and 170mm across them: 270mm in total. Rolls = loft area / coverage per roll. For an 8 x 6m loft (48 m²) using Knauf Loft Roll 44 combi-cut rolls, that is 5 rolls of 100mm and 8 rolls of 170mm.

The formula

  • Loft area (m²) = length x width (the floor area of the rooms below is close enough)
  • Layer 1 rolls = area / coverage per 100mm roll
  • Layer 2 or top-up rolls = area x 1.05 / coverage per roll
  • Top-up thickness = the thinnest roll that takes you to at least 270mm

Worked example

An 8 x 6m loft with no insulation:

  • Area: 48 m²
  • 100mm between the joists: 48 / 11.09 m² per roll = 4.3, so 5 rolls
  • 170mm across the joists: 48 x 1.05 / 6.50 m² = 7.8, so 8 rolls
  • If you already have 100mm, you only need the 8 rolls of 170mm.

Work it out for your job

Change the measurements and the quantities update straight away.

Loft floor area
48 m²
Build-up
100mm + 170mm = 270mm

Your list

  • 5100mm rolls between the joists (11.09 m² each)
  • 8170mm rolls across the joists (6.5 m² each)
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Materials checklist

  • 100mm mineral wool rollsThe first layer, between the joists.
  • 170mm mineral wool rollsThe second layer, across the joists at right angles, to reach 270mm.
  • Top-up rollsIf you already have some, the thinnest roll that takes you to 270mm.

You'll also need

  • Dust mask (FFP2 or better), gloves, goggles and long sleeves
  • Crawl boards to kneel on
  • Loft legs if you want to board over without squashing the insulation
  • Insulation and a draught strip for the loft hatch
  • Pipe lagging and a tank jacket
  • A long knife or insulation saw

Step by step

  1. Check what is thereMeasure the depth of any existing insulation. Most UK lofts have some.
  2. Measure the loftLength x width of the loft floor. The floor area of the rooms below is close enough.
  3. Lay the first layerFrom empty, roll 100mm between the joists.
  4. Lay the second layerRoll 170mm across the joists at right angles, which also covers the cold joist tops.
  5. Finish the detailsInsulate the hatch, lag pipes and the tank, and keep the eaves clear for ventilation.

Tips from the manufacturers

  • Roll coverage varies a lot by brand and thickness. The figures here are from the Knauf Loft Roll 44 data sheet (combi-cut rolls): 11.09 m² for 100mm, 7.35 m² for 150mm and 6.50 m² for 170mm. Check the label on the roll you buy.
  • Lay the top layer at right angles to the first, as the Energy Saving Trust and Centre for Sustainable Energy advise. It covers the joists, which would otherwise let heat escape.
  • Keep the eaves clear so air can flow and the loft does not get damp.
  • Leave the area directly under a cold water tank uninsulated so warmth from below stops it freezing, and lag the tank and pipes instead.

Mistakes to avoid

  • Squashing insulation under boards or stored boxes: compressed insulation works much less well. Use loft legs.
  • Covering recessed downlights that are not rated for it.
  • Blocking the eaves and causing condensation.
  • Working without a mask and gloves: mineral wool irritates skin, eyes and lungs.

Frequently asked questions

How thick should loft insulation be?

The Energy Saving Trust recommends 270mm of mineral wool. That is usually 100mm between the joists and 170mm across them.

How many rolls of loft insulation do I need?

Divide the loft floor area by the coverage on the roll. For a 48 m² loft from empty with Knauf combi-cut rolls, that is 5 rolls of 100mm and 8 of 170mm.

Is 270mm loft insulation a Building Regulations requirement?

You do not have to top up an existing loft, but 270mm is the Energy Saving Trust's recommended depth and is in line with the U-value Building Regulations expect when a loft is upgraded.

Can I put loft insulation on top of old insulation?

Yes. Lay the new rolls across the joists over what is there, as long as the old insulation is dry and in good condition.

Can I board over loft insulation?

Only if the boards sit above the insulation, for example on loft legs. Boarding straight onto the joists squashes 270mm down to the joist depth.

Sources

Quantities are estimates for typical jobs, using the coverage figures above. Products vary, so check the data sheet for the product you buy, and talk to a professional for structural, electrical or gas work. SmartBuild is independent and not affiliated with any of the manufacturers listed.

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