What the table shows
The table above lists the U-values the calculator uses, by area. Lower is better: a U-value is the heat, in watts, that passes through one square metre for each degree of temperature difference.
- Building Regs (Part L) uses the improved values in Table 4.3 of Approved Document L 2021, for upgrading an existing element.
- New-build standard uses Table 4.2, for a new element in an existing home, such as a new roof or wall.
Here are both tables, with the Table 4.3 thresholds:
| Element | Threshold, Table 4.3 (a) | Upgrade to, Table 4.3 (b) | New element, Table 4.2 |
|---|---|---|---|
| Roof: loft at ceiling level, room in roof or flat roof | 0.35 | 0.16 | 0.15 |
| Wall: cavity insulation | 0.70 | 0.55 | – |
| Wall: internal or external insulation | 0.70 | 0.30 | 0.18 |
| Floor | 0.70 | 0.25 | 0.18 |
All values are in W/m²K. Table 4.3 has a single roof row covering a loft insulated at the ceiling, a room in the roof insulated at the rafters, and a flat roof. The cavity wall row applies only to walls suitable for cavity fill; you can't make a cavity wider, so there's no new-build value for cavity fill. Table 4.2's wall value of 0.18 is for a new wall.
When the rules apply
You don't have to insulate just because the table exists. The values kick in when you do certain work:
- Renovating more than 50% of an individual element, such as one wall or the roof, or
- A major renovation affecting more than 25% of the surface area of the whole building envelope.
Renovation includes adding a new layer of cladding or render, dry-lining, stripping an element back to its structure and rebuilding it, replacing a flat roof membrane, and providing cavity wall insulation. When it's triggered, the guidance expects you to bring the whole element up to the column (b) value.
There are limits built in:
- If reaching the column (b) value isn't technically or functionally feasible, or wouldn't pay for itself within 15 years, upgrade to the lowest U-value that would.
- As a general rule, an upgraded element shouldn't end up worse than 0.70 W/m²K.
- Notes to Table 4.3 allow a lesser standard in some cases: for example, where insulating a solid wall to 0.30 would reduce a room's floor area by more than 5% (note 6), or where it would significantly affect adjoining floor levels (note 8).
If you're not sure whether your project is covered, or what standard is acceptable, ask your local building control body before you start.
Worked example: Part L or new-build standard?
Here's what the two levels mean in practice, from the calculator:
| Job | Building Regs (Part L) | New-build standard |
|---|---|---|
| Empty 45 m² loft, loft roll | 270 mm (100 + 170 mm), 11 rolls, £369–£1,021, U 0.16 | 300 mm (200 + 100 mm), 12 rolls, £399–£722, U 0.14 |
| 90 m² solid brick wall, PIR board | 75 mm, 35 boards, £1,610–£2,251, U 0.25 | 120 mm, 35 boards, £2,100, U 0.17 |
| 40 m² suspended timber floor, loft roll | 150 mm, 5 rolls, £437–£500, U 0.25 | 250 mm (150 + 100 mm), 9 rolls, £573–£828, U 0.16 |
| 90 m² cavity wall, 65 mm cavity | Filled, U 0.43, £1,800–£2,475 installed | – |
Costs are for materials, except cavity fill, which is an installed estimate. Two things stand out. First, products come in set thicknesses, so you often beat the target: 75 mm of PIR gets the solid wall to 0.25, not just 0.30. Second, roll prices vary so much by thickness that the deeper option isn't always dearer at the top of the range, so check prices before you decide. In the floor case, 250 mm only works if your joists are deep enough to hold it.
How the calculator uses these values
The calculator converts the U-value into the R-value the insulation must supply, using the BS EN ISO 6946 formula:
U = 1 ÷ (Rsi + Σ thickness/λ + Rse)
It takes off the surface resistances and the rest of the build-up, allows for timber joists bridging the first layer of insulation, and finds the fewest layers, then the thinnest stack, of real products that get there. It's calibrated to published results: 100 + 170 mm of loft roll gives U 0.16, as Knauf and RdSAP 10 state. The U-value calculator explains the formula step by step.
The 2026 edition, Wales and Scotland
A 2026 edition of Approved Document L was published on 24 March 2026 and takes effect on 24 March 2027. Its tables for existing homes carry the same values as the 2021 Tables 4.2 and 4.3, so the figures here stay valid.
These are England's values. Wales and Scotland set their own, and some are tighter for new elements. Check your nation's guidance before you buy.
Next steps
- Size the job with the loft insulation calculator, cavity wall insulation calculator, solid wall insulation calculator or floor insulation calculator
- See how many millimetres each product needs in loft insulation thickness
Sources
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- GOV.UK: Approved Document L (2026 edition) — takes effect 24 March 2027
- BRE BR 443: Conventions for U-value calculations (2019)
- RdSAP10 Specification (BRE, February 2024)
- Knauf Insulation: UK & Ireland Insulation Solutions Guide (June 2026)