How Many Rolls of Loft Insulation Do I Need?
Quick Answer
You need approximately 10 rolls of loft insulation
Based on: 8m x 6m loft = 48m², 270mm target thickness from bare joists: 4 rolls of 100mm base layer plus 6 rolls of 170mm top layer, before any cutting allowance. If topping up existing 100mm insulation to 270mm, you need 6 rolls of 170mm only, before any cutting allowance. The calculator below defaults to a 10% wastage allowance, which takes this example to 11 rolls.
Rolls Needed by Loft Size
Use this table for a quick estimate. Find your loft's floor area (length x width in metres) and read off the roll counts. Figures use Knauf Loft Roll 44 combi-cut coverage: 13.89m² per 100mm roll, 8.01m² per 170mm roll, before any cutting allowance. Round up if your loft falls between two rows, and check the pack coverage printed on your own product. The loft insulation calculator defaults to a 10% wastage allowance on top of these counts, for cutting around obstacles.
| Loft floor area | Bare joists to 270mm | Topping up 100mm to 270mm |
|---|---|---|
| 20m² | 5 rolls (2 x 100mm + 3 x 170mm) | 3 rolls of 170mm |
| 30m² | 7 rolls (3 x 100mm + 4 x 170mm) | 4 rolls of 170mm |
| 40m² | 8 rolls (3 x 100mm + 5 x 170mm) | 5 rolls of 170mm |
| 50m² | 11 rolls (4 x 100mm + 7 x 170mm) | 7 rolls of 170mm |
| 60m² | 13 rolls (5 x 100mm + 8 x 170mm) | 8 rolls of 170mm |
| 70m² | 15 rolls (6 x 100mm + 9 x 170mm) | 9 rolls of 170mm |
| 80m² | 16 rolls (6 x 100mm + 10 x 170mm) | 10 rolls of 170mm |
Prefer to enter your exact dimensions and get a cost estimate? Use the loft insulation calculator, which also handles L-shaped lofts.
How We Calculated This
The Energy Saving Trust recommends around 270mmof mineral wool loft insulation. Building Regulations (Approved Document L) set a U-value target of about 0.16 W/m²K for a roof at ceiling level rather than a fixed depth, and 270mm of standard mineral wool is the typical depth that meets it. This is typically achieved with two layers:
- First layer (100mm): Between the joists (joists are typically 100mm deep)
- Second layer (170mm): Laid across the joists at right angles
Roll Sizes
Standard mineral wool loft rolls (for example Knauf Earthwool Loft Roll 44) are supplied combi-cut: each roll is partially perforated so it can be split into two 570mm or three 380mm strips to fit between joists, or laid full width across them. A thicker roll holds less length, so coverage per roll falls as thickness increases:
- 100mm thick: 12.18m x 1.14m = 13.89m² per roll
- 170mm thick: 7.03m x 1.14m = 8.01m² per roll
These are the same coverage figures used by our loft insulation calculator. Always check the pack coverage (m²) printed on your own product, since it varies by brand and thickness.
First Layer: 100mm Between Joists
For 48m² of loft floor: 48 ÷ 13.89 = 3.46, rounded up to 4 rolls of 100mm. The rolls are combi-cut and 1140mm wide, so they split into strips that fit between joists typically 400mm or 600mm apart, with less waste than a full-width lay.
Second Layer: 170mm Across Joists
Laid at right angles to the first layer to cover the joist tops and eliminate cold bridges. For 48m²: 48 ÷ 8.01 = 5.99, rounded up to 6 rolls of 170mm.
Total
- 100mm rolls: 4 rolls
- 170mm rolls: 6 rolls
- Total: 10 rolls (if starting from zero insulation, before any cutting allowance)
If you already have 100mm of insulation between the joists and just need to top up with a cross-layer, you only need 6 rolls of 170mm, before any cutting allowance.
Note: this assumes topping up an already part-insulated loft, which is the most common scenario in UK homes. These totals exclude wastage: the loft insulation calculator defaults to a 10% wastage allowance, which takes the 10-roll bare-joists total above to 11.
What Thickness Do I Need?
- 270mm total: Current Building Regulations target (U-value ~0.16 W/m²K)
- 300mm: Slightly exceeds Building Regs — a good choice for minimal extra cost
- 100mm: Older properties often have this. Topping up to 270mm still brings a worthwhile saving, just far less than the roughly 25% of heat lost through a fully uninsulated roof
Important Considerations
- Eaves ventilation: Do not block eaves vents with insulation. Use eaves ventilation trays to maintain airflow.
- Loft hatch: Insulate the loft hatch with a piece of insulation fixed to the top. Fit draught strips around the frame.
- Pipes and tanks: Insulate pipes and water tanks separately. Do not lay loft insulation under a cold water tank — it needs warmth from below to prevent freezing.
- Downlighters: Use loft caps (fire hoods) over recessed downlighters before covering with insulation.
- Boarding: If you want a boarded loft, use raised loft legs (stilts) to maintain the full 270mm insulation depth below the boards.
Tips
Wear a dust mask, gloves, and long sleeves when handling mineral wool. Work on a board across the joists — never stand between them, as the ceiling below will not take your weight. Loft insulation is one of the most cost-effective energy-saving measures — it typically pays for itself within 2–3 years.
Frequently Asked Questions
Multiply your loft's length by its width to get the floor area in m², then divide by the roll coverage printed on the pack, rounding up to a whole roll for each layer. For example, a small 4m x 3m loft (12m²) needs 12 ÷ 13.89 = 0.86, rounded up to 1 roll of 100mm for the first layer, plus 12 ÷ 8.01 = 1.50, rounded up to 2 rolls of 170mm for the second layer, so 3 rolls in total from bare joists before any cutting allowance. If you already have 100mm in place and are only topping up to 270mm, you only need the 2 rolls of 170mm. Use the loft insulation calculator for your own dimensions, including L-shaped lofts.
No. The Energy Saving Trust recommends 270mm of mineral wool, so 100mm on its own falls well short of current guidance, even though it is common in older UK homes. According to the Energy Saving Trust (figures based on fuel prices as of July 2026, England, Scotland and Wales), insulating a gas-heated home from a completely uninsulated loft to 270mm saves around £140 a year for a mid-terrace house, £190 for a semi-detached house and £360 for a detached house, while topping up from around 120mm to 270mm saves far less, around £13 to £35 a year (£17 for a typical semi-detached home), because most of the easy heat loss is already captured by the existing layer. With only 100mm in place you sit between these two cases, so adding a 170mm cross layer to reach 270mm is still worthwhile even though the saving will not be as large as insulating from scratch.
For standard mineral wool with a thermal conductivity (lambda) of about 0.044 W/mK, such as Knauf Loft Roll 44, 270mm is the depth the Energy Saving Trust recommends to meet the Building Regulations U-value target for a loft (roughly 0.16 W/m²K). If you use a higher-performance mineral wool with a lower lambda of about 0.035 W/mK, you need noticeably less depth for the same U-value, because a better-performing material does more work per millimetre. 300mm is a common step-up for slightly better performance at low extra cost. Use the insulation thickness calculatorto check the exact depth for your product's lambda value and target U-value.
Rockwool is a stone wool (mineral wool) brand and one of the standard choices for loft rolls alongside Knauf, so the same method applies: divide your loft area by the coverage printed on the pack and round up. A common 100mm Rockwool roll measures 2.75m x 1.2m, which is 3.3m² per roll, and is typically sold in twin packs covering 6.6m² (check whether the m² figure on the shelf label is for one roll or the whole pack before you buy). For a 48m² loft, that is 48 ÷ 3.3 = 14.5, rounded up to 15 individual 100mm rolls, or 48 ÷ 6.6 = 7.3, rounded up to 8 twin packs, for that one layer. Total rolls needed for a full 270mm build-up depends on your target depth and the second-layer product you choose, so enter your own pack coverage in the calculator's advanced options for an exact figure.
There is no single UK-wide figure because it depends on how many rolls you need, the brand and thickness you choose, and your supplier's prices, which vary by retailer and change over time. Work out your roll count first using the table above or the loft insulation calculator, then multiply by your supplier's price per roll: the calculator has an optional price-per-roll field that adds this up for you automatically, including any wastage allowance. For a full worked cost breakdown covering materials, eaves ventilation trays, hatch insulation and pipe lagging for a typical UK loft, see our guide on how to install loft insulation.