How Much Loft Insulation Do I Need?
Quick Answer
You need approximately 6 rolls of mineral wool insulation
Based on: 8m x 6m loft = 48m², 270mm target thickness, standard rolls (1200mm x 8m = 9.6m² per roll at 100mm thick), laid in two layers
How We Calculated This
UK Building Regulations (Approved Document L) recommend a minimum of 270mm of mineral wool loft insulation for new and retrofit installations. 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 (e.g. Knauf Loft Roll 44, Superglass) are:
- 100mm thick: 1200mm wide x 8.0m long = 9.6m² per roll
- 170mm thick: 1200mm wide x 5.93m long = ~7.12m² per roll
- 200mm thick: 1200mm wide x 4.92m long = ~5.9m² per roll
First Layer: 100mm Between Joists
For 48m² of loft floor: 48 ÷ 9.6 = 5 rolls of 100mm. However, the rolls are 1200mm wide and joists are typically 400mm apart (centres). You will need to cut the rolls lengthways to fit between joists, which means less waste than a full-width lay. In practice, 5 rolls should be sufficient for the first layer.
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 ÷ 7.12 = 6.7, round up to 7 rolls of 170mm.
Total
- 100mm rolls: 5 rolls
- 170mm rolls: 7 rolls
- Total: 12 rolls (if starting from zero insulation)
If you already have 100mm of insulation between the joists and just need to top up with a cross-layer, you only need the 7 rolls of 170mm(or 6 rolls of 200mm as an alternative to overshoot the 270mm target slightly).
Note: The “6 rolls” quick answer assumes topping up an already part-insulated loft, which is the most common scenario in UK homes.
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 — upgrading to 270mm saves ~25% on heating bills
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.