Quantities
Loft insulation rolls for two layers
- where
- rolls for layer 1
- loft floor area to insulate (m²)
- waste and cutting allowance (%)
- layer 1 coverage per roll (m²)
- rolls for layer 2
- layer 2 coverage per roll (m²)
- total insulation depth (mm)
- layer 1 thickness (mm)
- layer 2 thickness (mm)
- insulation resistance, ignoring joists (m²K/W)
- insulation thermal conductivity (W/mK)
Explain this equation
Counts loft insulation rolls for a first layer between the joists and a second layer across them, and gives the combined depth and resistance.
Each layer covers the full loft area plus a cutting allowance. Rolls of different thickness cover different areas, so each layer is divided by its own roll coverage and rounded up. The total depth is the two layers added, and its resistance is depth over λ, with mm converted to m.
Area divided by coverage per roll, then the layer resistance formula.
Sense check: 50 m² with 100 mm then 170 mm of glass wool needs 4 and 7 rolls, giving 270 mm and about 6.1 m²K/W.
Watch out: keep eaves ventilation clear and do not squash insulation under boarding. Joists bridge the first layer, so the real U-value is a little worse than the layer figure suggests.
Counts the rolls needed for a two-layer loft: a first layer between the ceiling joists and a second layer laid across them.
Enter your values to see the result.
Good to know
Defaults are Knauf Loft Roll 44 Combi-Cut, λ 0.044 W/mK: 100 mm rolls cover 13.89 m², 170 mm 8.01 m², 200 mm 6.84 m², 150 mm 9.18 m² (standard lengths; shorter rolls cover less, check the pack). 100 mm + 170 mm gives 270 mm, the usual recommended depth, which brings a loft to about 0.16 W/m²K. Taking the full area for the between-joist layer allows a little for waste. Keep eaves vents clear, do not cover recessed downlights or bury cables that need to dissipate heat, and lift or box out water tanks and pipes so they stay on the warm side.
Common mistake
Ordering the cross layer by the plan area of the joist tops only, or squashing insulation under boarding. Compressed mineral wool loses much of its resistance, so raise storage decking on loft legs.
Limits
Simple area count. Does not deduct hatch, chimney or tank areas, or allow for joists deeper than the first layer. The thermal resistance shown ignores timber bridging; use the bridged-layer calculator for the true U-value.
Sources
- Knauf Insulation Loft Roll 44 (Combi-Cut): λ 0.044 W/mK, 1140 mm wide; 100 mm 13.89 m², 150 mm 9.18 m², 170 mm 8.01 m², 200 mm 6.84 m² per roll (standard lengths)
- BRE BR 443 (3rd edition, 2019) Conventions for U-value calculations (Rsi 0.13 / 0.10 / 0.17 and Rse 0.04 m²K/W; air-gap levels 0 / 0.01 / 0.04; timber fractions; ground λ clay 1.5, sand/gravel 2.0, rock 3.5 W/mK; ignore corrections under 3%)
- Approved Document L Volume 1: Dwellings, 2026 edition (England): Table 3.2 new or replacement elements in existing dwellings (roof 0.15, wall 0.18, floor 0.18 W/m²K); Table 3.3 existing elements: threshold / improved U-values roof 0.35/0.16, cavity fill 0.70/0.55, internal or external wall insulation 0.70/0.30, floor 0.70/0.25; para 3.14 15-year payback and 0.7 backstop. In force 24 March 2027; the same values appear in Tables 4.2 and 4.3 of the 2021 edition, which applies until then
Results are calculations from the published methods and figures shown, and depend on the data you enter. Follow BS EN ISO 6946, BS 5250, PAS 2035, Building Regulations Part L and manufacturers’ data. Whole-house retrofit needs a qualified retrofit assessor and coordinator.