Guide

Loft insulation: thickness and how many rolls you need

Updated October 2026

Quick answer: Divide loft area by the coverage printed on the roll at the thickness you are buying, then round up. A 40m² loft needs five rolls if each covers 8m² for one layer. For a new 270mm mineral-wool installation, calculate a 100mm layer and a 170mm layer separately: their rolls usually cover different areas.

What thickness are you trying to achieve?

Around 270mm of conventional mineral wool is a common UK loft-floor benchmark. It is not a universal thickness for every insulation material: thermal conductivity and the required performance affect the design. Rigid boards and other materials cannot be substituted millimetre for millimetre without checking their specification.

For a cold loft, insulation normally follows the ceiling below, leaving the roof space outside the heated envelope. A converted loft or insulated roof slope is a different construction with different ventilation and condensation details. This guide concerns rolls laid on a suitable loft floor, not a room-in-roof conversion.

Measure the existing layer in several places. A nominal 100mm layer may be compressed or patchy, while a clear central area can hide gaps at the edges. If it is dry, clean and serviceable, a cross-laid top-up can be appropriate. Investigate damp or contamination before adding material.

How many rolls of loft insulation do I need?

Use area × allowance ÷ roll coverage. For a simple clear loft, an allowance of 5% can cover cutting; 10% may be more useful around trusses and awkward corners. Round each layer separately because leftover thin insulation does not replace the required area of a thicker layer.

These are illustrative pack coverages, not standardised roll sizes or brand claims. Width, roll length and thickness vary between packs. A multi-roll pack may print a combined coverage rather than the area of each piece. Use the label for the complete pack you are buying; do not assume every 170mm roll covers 8m².

The example table's final column describes alternatives, not four layers to install together. If you already have sound 100mm insulation, buy the 170mm top-up for that area. If starting from bare ceilings, add both the 100mm and 170mm rows. Do not buy only the thicker layer and assume you have reached 270mm.

Worked example: a 40m² loft from bare

Take a rectangular loft measuring 8m × 5m. The footprint is 40m². Use 100mm rolls covering 13.9m² each and 170mm rolls covering 8m² each. Both layers need a 5% cutting allowance, so each is planned against 40 × 1.05 = 42m².

1. Base layer: 42 ÷ 13.9 = 3.02, rounded up to four 100mm rolls.
2. Cross layer: 42 ÷ 8 = 5.25, rounded up to six 170mm rolls.
3. Total purchase: ten rolls, comprising two different thicknesses.
4. At illustrative prices of £58 per base roll and £52 per top-up roll, cost is (4 × £58) + (6 × £52) = £544.

Whole-pack rounding creates surplus, particularly in the base layer. Compare alternative pack sizes if available, but do not remove the cross layer merely because the base roll order contains spare area. Area and installed depth are separate requirements.

Using the insulation calculator accurately

The loft insulation calculator multiplies footprint by one or two layers and uses a single roll-coverage input and price. It is convenient when equal-coverage layers or a simplified allowance are suitable. It does not automatically account for different coverage at different thicknesses or add cutting waste.

For the example above, calculate each layer separately using “top-up only” and its actual coverage and price, then add the results. To include 5% in the tool, an estimating length of 8.4m with the original 5m width represents 42m². Use 13.9m² and £58 for the first calculation, then 8m² and £52 for the second. This reproduces four plus six rolls.

Typical UK material and installation costs

For October 2026 planning, 100mm mineral wool may cost roughly £4–£8 per m² supply only, while a 170–200mm top-up may cost around £6–£12 per m². Ordinary fitted work can fall around £12–£28 per footprint m² depending on depth and scope. These are typical ranges rather than quotes, including VAT where charged; access, removal and boarding can cost extra.

Price both required layers and their whole packs rather than multiplying a thin-roll price by the loft footprint once. Existing material can make a top-up much cheaper than starting from bare. Hatch insulation, tank and pipe protection, protective clothing and delivery may also need separate allowances.

Laying out the insulation without compressing it

The first layer commonly sits between ceiling joists, with the next across them to reduce gaps and cover timber bridges. Follow the selected system's ventilation details at eaves. Insulation should not block ventilation paths, and storage boards need a designed raised support arrangement rather than resting on compressed wool.

Identify electrical cables, downlights, tanks and pipework first. Some cables need assessment because surrounding insulation changes heat dissipation. Cover downlights only where their rating and specified protection permit it. Keep access to equipment and avoid walking between joists or loading an unsupported ceiling.

For the surrounding project sequence, read the best order to decorate a room, particularly before repairing ceiling finishes below. For a choice between DIY and professional installation, see DIY versus hiring a tradesperson. Restricted access and uncertain services can outweigh a small saving on installation.

Frequently asked questions