How the calculator works
Building Regulations in England set insulation targets as U-values: how many watts of heat pass through one square metre of roof, wall or floor for each degree of temperature difference. Lower is better. The calculator works backwards from the U-value you choose to the insulation you need.
- Pick the area. Loft, room in roof, cavity walls, solid walls or a suspended timber floor. This page opens on a loft with mineral wool loft roll.
- Measure it. Enter length × width or a total area. For walls you can enter the wall length and height, and the calculator takes off 1.9 m² per door and 1.5 m² per window.
- Tell it what's there now. A depth and type of insulation, or an R-value or U-value if you know it.
- Choose how far to go. Building Regs (Part L) uses the values for upgrading an existing element. New-build standard uses the stricter values for a new element.
- Choose a product. Loft roll, PIR board, EPS board, or installer-blown mineral wool, depending on the area.
The U-value formula
The calculator uses the standard method from BS EN ISO 6946, with the conventions in BRE's BR 443:
U = 1 ÷ (Rsi + Σ thickness/λ + Rse)
- Rsi and Rse are the thin layers of still air on the inside and outside surfaces, with fixed values set by the standard.
- thickness/λ is the R-value of each layer: its thickness in metres divided by its thermal conductivity, λ (lambda). Everything else in the element, such as plasterboard, brick or a ventilated void, adds a fixed R-value too.
For a loft, 1 ÷ 0.16 gives a total R of 6.25 m²K/W. About 0.57 of that comes from the surfaces and the ceiling, so the insulation has to supply R 5.68 m²K/W.
Joists, rafters and λ
Insulation laid between timber joists doesn't work as well as the same thickness on its own, because heat also flows through the wood. The calculator allows for this (a parallel-path calculation), which is why you lay the first layer of loft roll between the joists and the second layer across them, where nothing bridges it.
λ matters too. The loft roll in our data, Knauf Loft Roll 44, has λ 0.044 W/m·K. A roll with λ 0.040 needs about 9% less thickness for the same R-value, so check the λ on the pack you buy.
Worked example: topping up a 45 m² loft
Say your loft floor is 45 m² and there's 100 mm of old mineral wool between the joists. The calculator, set to Building Regs (Part L), returns:
| Result | |
|---|---|
| U-value now | 0.40 W/m²K |
| Target | 0.16 W/m²K |
| Add | 170 mm loft roll, laid across the joists |
| Area with 10% waste | 49.5 m² |
| To buy | 7 rolls |
| Material cost | £234–£693 |
| U-value after | 0.16 W/m²K |
The price range is wide because retailers price the same roll very differently, and multibuy offers set the low end.
Here's what the calculator returns for some other common jobs, all on the Part L setting:
| Job | What it suggests | To buy | Cost | U-value after |
|---|---|---|---|---|
| Empty 45 m² loft | 100 mm + 170 mm loft roll | 4 + 7 rolls | £369–£1,021 materials | 0.16 |
| 90 m² cavity wall, 65 mm cavity | Fill the cavity (installer) | – | £1,800–£2,475 installed (estimate) | 0.43 |
| 90 m² solid brick wall | 75 mm PIR board | 35 boards | £1,610–£2,251 materials | 0.25 |
| 40 m² suspended timber floor | 150 mm loft roll between joists | 5 rolls | £437–£500 materials | 0.25 |
Reading your results
- Now, target, after. The meter shows the U-value you start from, the one you're aiming for and where you'd end up.
- To buy. Rolls or boards per layer, with what each one covers.
- Show the maths. Every step of the calculation, so you can check it.
- Copy link. Your inputs are stored in the page address, so you can bookmark a result or send it to a builder.
- More options. Change the waste allowance, the cavity depth or the price per roll or board.
Where to start
For most homes the loft is the easiest and cheapest place to begin. The Energy Saving Trust recommends a minimum of 270 mm of mineral wool. Each job has its own calculator with tips:
- Loft insulation calculator: rolls, depths and top-ups
- Blown loft insulation calculator: installer-blown mineral wool
- Cavity wall insulation calculator: U-value after filling the cavity
- Solid wall insulation calculator: internal or external boards
- Floor insulation calculator: suspended timber floors
To see every target in one place, read Part L U-values.
Before you start
- Building Regulations. If you renovate more than half of a roof, wall or floor, or more than a quarter of the whole envelope, Approved Document L expects the element to be upgraded. Ask building control or your installer if you're unsure.
- Vermiculite. Pebbly loose fill in a loft may be vermiculite, which can contain asbestos. Don't disturb it; see the HSE guidance below.
- Scotland and Wales. Both set their own targets. Check your nation's guidance.
- Check the pack. Roll and board sizes vary by brand. If yours differ, enter your own price under More options and check the coverage.
Sources
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- GOV.UK: Approved Document L (2026 edition)
- BRE BR 443: Conventions for U-value calculations (2019)
- Energy Saving Trust: Roof and loft insulation
- Knauf Insulation: Loft Roll 44 (Combi-cut)
- Wickes: loft insulation (prices)
- Toolstation: insulation (prices)
- HSE: Asbestos — location of materials (loose-fill)