The minimum depth for attic insulation in UK homes is 270mm of mineral wool or equivalent
If you are asking how much attic insulation you need, the short answer is a minimum of 270mm of mineral wool or an equivalent material that achieves the same thermal performance. Current UK Building Regulations, specifically Approved Document L (2021 edition with 2023 amendments), require loft insulation to achieve a U-value of 0.16 W/m²K (GOV.UK, Approved Document L, 2021 edition). A U-value measures how quickly heat passes through a material; the lower the number, the better the insulation. For context, 270mm of mineral wool blanket meets this standard, as does roughly 200mm of rigid foam board. Older homes with less than 100mm of existing insulation will see the greatest relative improvement in heat retention after topping up (Energy Saving Trust, 2026).
You need at least 270mm of attic insulation, typically three 100mm layers of mineral wool. Measure your loft floor area in square metres to calculate the exact amount. Check Energy Saving Trust figures for savings on your home type.
- Measure your loft floor area in square metres to calculate insulation needed.
- Minimum depth is 270mm of mineral wool or equivalent for U-value 0.16 W/m²K.
- Use three layers of 100mm mineral wool rolls or two layers of 150mm rolls.
- Top up from less than 100mm for greatest heat retention improvements.
- Check Energy Saving Trust data for cost savings on a typical semi-detached house.
- The minimum depth for attic insulation in UK homes is 270mm of mineral wool or equivalent
- The exact amount of insulation you need depends on your loft's floor area and existing depth
- Quick numbers cost, savings, and payback for a typical UK semi-detached house
- The direct answer to 'how much attic insulation do I need' is a minimum 270mm depth across the entire loft floor
- Certification and installer verification MCS and TrustMark are essential for grant-funded work
- How to calculate the number of rolls or bags of insulation you need to buy
- The payback period for attic insulation is typically 2–4 years for a gas-heated home
- Additional factors that affect the amount of insulation you need roof type and access
The exact amount of insulation you need depends on your loft’s floor area and existing depth
To work out how much attic insulation you need, start by measuring the total floor area of your loft in square metres. Multiply the length by the width of the insulated space. Next, subtract any area already covered by insulation that is at least 270mm deep. The remaining area is the amount of new material required. For mineral wool rolls, coverage per pack is typically 5–10m² at 100mm thickness. Because you need 270mm total, you will need three layers of 100mm rolls (or two layers of 150mm rolls if available). Multiply the number of layers by the coverage per pack to find how many packs to buy (Energy Saving Trust, 2026). Manufacturer data sheets from brands like Knauf and Rockwool confirm these coverage figures.
Quick numbers cost, savings, and payback for a typical UK semi-detached house
Based on a typical three-bedroom semi-detached house with gas heating, here are the key numbers for fitting 270mm of attic insulation. Costs and savings come from the Energy Saving Trust and DESNZ data.
| Material | Cost per m² | Annual saving (gas-heated home) | Payback period | Lifespan |
|---|---|---|---|---|
| Mineral wool blanket | £15–£25 | £315–£395 | 2–4 years DIY | 40+ years |
| Loose-fill (cellulose) | £20–£35 | £315–£395 | 3–5 years DIY | 30–40 years |
| Rigid foam board | £25–£40 | £315–£395 | 3–6 years DIY | 50+ years |
| Professional install (mineral wool) | £40–£60 | £315–£395 | 3–6 years | 40+ years |
Annual savings are consistent across materials because all achieve the same U-value of 0.16 W/m²K. Source: Energy Saving Trust, “Loft insulation: costs and savings”, 2026; DESNZ, “Energy consumption in the UK”, 2025 data (published 2026).
The direct answer to ‘how much attic insulation do I need’ is a minimum 270mm depth across the entire loft floor
The standard recommended by the Energy Saving Trust and required by Building Regulations for new installations is a minimum 270mm depth across the entire loft floor (Energy Saving Trust, 2026). If you have less than 100mm of existing insulation, top up to 270mm. If you have between 100mm and 200mm, top up to 270mm. If you already have 200mm or more, adding more may not be cost-effective because the additional energy saving is small. The key variable is your current depth. Measure it at several points using a ruler or tape measure pushed through the insulation to the ceiling below.
Certification and installer verification MCS and TrustMark are essential for grant-funded work
If you plan to claim a government grant for attic insulation, such as the ECO4 scheme or the Great British Insulation Scheme, the installer must be MCS-certified for insulation and registered with TrustMark (GOV.UK, “Great British Insulation Scheme: installer requirements”, 2026). Check the MCS register at mcscertified.com and TrustMark at trustmark.org.uk before hiring. For DIY installation, no certification is needed, but you must follow Building Regulations if you add more than 50mm of insulation. This means the final U-value must still meet 0.16 W/m²K. Check our guide on ECO4 grant eligibility and how to find an approved installer
How to calculate the number of rolls or bags of insulation you need to buy
To calculate the exact quantity, multiply your loft’s floor area in square metres by the required depth of 0.27 metres to get the volume in cubic metres. For mineral wool rolls, divide this volume by the coverage per roll. A typical roll of 100mm thick mineral wool covers 5–10m² per layer. Because you need three layers of 100mm insulation to reach 270mm, multiply the floor area by three, then divide by the coverage per roll. For loose-fill insulation, check the bag coverage: one bag typically covers about 1m² at 270mm depth. Always add 10% for cutting waste and irregular spaces around eaves, pipes, and hatches (Energy Saving Trust, “Loft insulation: step-by-step guide”, 2026). Manufacturer packaging data confirms these coverage rates.
Example calculation: a loft floor area of 50m² needs 50 × 0.27 = 13.5 cubic metres of insulation. For 100mm mineral wool rolls covering 8m² per roll, you need 50 ÷ 8 = 6.25 rolls per layer, rounded up to 7 rolls per layer. Three layers means 21 rolls total. Add 10% waste: 23 rolls.
The payback period for attic insulation is typically 2–4 years for a gas-heated home
For a typical gas-heated semi-detached house in the UK, fitting 270mm of attic insulation saves between £315 and £395 per year on heating bills (Energy Saving Trust, “Loft insulation: costs and savings”, 2026). The installation cost ranges from £500 to £1,200 if you do it yourself, or £1,500 to £2,500 for a professional install including materials and labour. DIY payback is 2–4 years. Professional install payback is 3–6 years. These figures assume a 50m² loft area and gas heating at typical 2026 rates. If you heat with electricity or oil, annual savings are higher, and payback is faster.
Additional factors that affect the amount of insulation you need roof type and access
For a pitched roof with a boarded loft used for storage, you cannot insulate the loft floor. Instead, you need insulation between and above the rafters, which requires rigid foam board and a different calculation. The required U-value is still 0.16 W/m²K, but the depth of rigid foam needed is typically 150–200mm depending on the product (GOV.UK, Approved Document L, 2021 edition). For a flat roof, insulation is placed above the deck, and the required U-value is 0.18 W/m²K, equating to 150–200mm of rigid foam (Energy Saving Trust, “Flat roof insulation”, 2026). Access restrictions such as small hatches, low trusses, or narrow eaves may limit how much insulation you can physically fit. In tight spaces, consider loose-fill or blown-in fibre insulation, which can be poured or blown into awkward gaps (Energy Saving Trust, “Loft insulation: types”, 2026). Read our article on loft insulation types for different roof structures
Frequently Asked Questions
For a typical 3-bed semi with a 50m² loft, you need enough insulation to cover the floor area to a depth of 270mm. That means around 15 packs of 100mm mineral wool rolls if each covers 8m² per layer. The Energy Saving Trust confirms these coverage figures.
The minimum depth is 270mm of mineral wool or an equivalent material. This meets the U-value of 0.16 W/m²K required by Approved Document L (2021 edition with 2023 amendments). GOV.UK sets this standard.
You need three layers of 100mm mineral wool rolls or two layers of 150mm rolls to reach 270mm total depth. Manufacturer data from Knauf and Rockwool confirm these layering requirements.
Yes, you can top up existing insulation if it is in good condition and dry. Add layers to reach a total depth of 270mm. The Energy Saving Trust advises topping up from less than 100mm for best results.
Mineral wool attic insulation costs around £3 to £6 per square metre for 100mm thickness. For a 50m² loft needing 270mm, total material cost is roughly £450 to £900. DESNZ data supports these estimates.
