UK Calculator Methodology: U-Values, Bridging & Prices

Units

The UK section works in SI units throughout: thicknesses in millimetres, areas in square metres, R-values in m²K/W, thermal conductivity (λ) in W/m·K and U-values in W/m²K. Prices are in pounds sterling.

Targets

UK targets are U-values for the whole element, from Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments), which applies in England. There are two levels:

Area	Building Regs (Part L), Table 4.3 (b)	New-build standard, Table 4.2
Loft	0.16	0.15
Room in roof	0.16	0.15
Cavity walls	0.55	–
Solid walls (internal or external insulation)	0.30	0.18
Floor	0.25	0.18

Table 4.3 gives the improved U-values for upgrading an existing element. Its single roof row covers lofts, rooms in the roof and flat roofs.
Table 4.2 gives values for new elements in an existing home. A cavity can only be filled, so cavity walls have no new-build value.
The 2026 edition of Approved Document L, published on 24 March 2026 and in force from 24 March 2027, keeps the same values for existing homes.
Wales and Scotland set their own values. We use England's, and say so on every page.

The full rules, including when they apply, are on Part L U-values.

U-value method

U-values are calculated following BS EN ISO 6946, with the conventions in BRE's BR 443:

U = 1 ÷ (Rsi + Σ thickness/λ + Rse)

To size insulation, the calculator turns this around. The R-value the insulation must supply is 1 ÷ target U, minus the surface resistances and the R of the rest of the build-up. These are the values it uses:

Area	Rsi	Rse	Rest of build-up	Timber	Timber fraction
Loft	0.10	0.04	0.43	100 mm ceiling joists	12.8%
Room in roof	0.10	0.10	0.06	100 mm rafters	8.8%
Cavity walls	0.13	0.04	0.51	–	–
Solid walls	0.13	0.04	0.42	–	–
Floor (suspended timber)	0.17	0.04	1.04	150 mm floor joists	10.8%

R-values in m²K/W. The ‘rest of build-up’ column covers everything except the new insulation and the surfaces: plasterboard, roof space, masonry, floorboards, ventilated voids and, for floors, a simplified allowance for the ground.

Timber bridging

Where insulation sits between joists or rafters, heat also flows through the timber. The calculator uses a parallel-path method. Layers fill from the bottom up. Within the depth of the timber, the effective conductivity is:

λ effective = (1 − f) × λ insulation + f × λ timber

where f is the timber fraction and λ timber is 0.13 W/m·K, a typical softwood value from EN ISO 10456. Anything above the timber, such as a loft layer laid across the joists, is calculated as plain thickness ÷ λ.

Calibration

The values for the rest of each build-up are calibrated so that reference constructions match published results:

Loft: 100 mm + 170 mm of λ 0.044 loft roll with 12.8% timber gives U 0.16, as Knauf and RdSAP 10 state.
Cavity wall: a filled 65 mm cavity gives about U 0.43, as in Knauf's tables.
Floor: 150 mm of mineral wool between the floor joists gives about U 0.25, in line with the Energy Saving Trust.
Solid wall: an uninsulated 225 mm solid brick wall gives about U 1.7, the RdSAP 10 value.

Existing insulation

If you enter a depth, existing mineral wool and polystyrene (EPS) are valued at λ 0.040, the figure in the RdSAP 10 specification, and bridged by the timber in the same way as new insulation. Vermiculite is given a conservative λ of about 0.053 and triggers a warning, because the HSE lists loose-fill insulation among materials that may contain asbestos. You can also enter a known R-value or U-value instead. If what's there already meets the target, the calculator says so and stops.

Choosing rolls and boards

Each product has a list of standard thicknesses, each with its coverage per roll or board and a price range. The calculator tries every combination of up to three layers and keeps those that reach the target. From those it picks:

the fewest layers, then
the thinnest total stack, then
the fewest rolls or boards, then
the lowest price.

Rolls or boards per layer = area × (1 + waste allowance) ÷ coverage, rounded up. If nothing fits within a limited depth, it shows the best stack that does and warns you.

Installer products

Cavity fill fills the whole cavity. It can't be installed part-way, so the calculator doesn't size it to the target: it fills the cavity width you choose (50, 65, 75, 85 or 100 mm) with λ 0.040 blown mineral wool (Knauf Supafil 40) and reports the U-value you end up with.

Blown loft insulation (Knauf Supafil Loft 045, λ 0.045) is calculated as a continuous layer to whatever depth reaches the target.

Prices

Rolls and boards use retail prices from Wickes, Toolstation and Travis Perkins, checked on 14 October 2026. Where they differ, you see a low–high range; multibuy offers set the low end. B&Q was unavailable on that date. These are material costs only, without tools, labour or delivery.
Installer prices are derived from the Energy Saving Trust's per-home figures, so they're estimates. Cavity wall insulation at about £2,200 for a typical home, assuming 80–110 m² of external wall, gives £20–£27.50 per m². Loft insulation at about £750 from 0 to 270 mm, assuming a 40–55 m² loft, gives £13.60–£18.75 per m² at 270 mm, scaled by depth. No waste allowance is added to installed prices.

You can enter your own price per roll or board under More options.

Waste and wall areas

The waste allowance defaults to 10%. It's an editorial choice that covers offcuts, trimming around obstructions and uneven spacing, and you can change it. If you measure walls by length and height, the calculator deducts 1.9 m² per door and 1.5 m² per window.

Product data

Loft roll: Knauf Loft Roll 44 Combi-cut, λ 0.044, 100/150/170/200 mm
PIR board: SOPRATHERM GA4000 (formerly Celotex) and Kingspan TP10, λ 0.022, 2400 × 1200 mm, 25–120 mm
EPS board: Jablite Jabfloor 70, λ 0.038, 2400 × 1200 mm
Cavity fill: Knauf Supafil 40, λ 0.040
Blown loft: Knauf Supafil Loft 045, λ 0.045

Limitations

Ground floors are simplified. Floor heat loss really depends on the floor's size, shape, edge insulation and the ground, as set out in BS EN ISO 13370. We use a fixed allowance calibrated to a suspended timber floor, so solid floors aren't modelled properly.
No condensation risk analysis. The calculator doesn't check for condensation inside walls, roofs or floors. That matters most for internal wall insulation and rooms in the roof.
Not a SAP or RdSAP assessment. It doesn't produce an EPC rating, estimate bills or savings, or replace a calculation for building control.
Simplified build-ups. Battens, fixings, plasterboard and render systems aren't modelled individually. Product data sheets and an installer's calculation take precedence.
Workmanship. Gaps, compression and air leaks aren't modelled. Careful installation matters as much as the numbers.
England only. Wales and Scotland have their own targets.
Prices change. They're a snapshot from one day, including promotions.

How other sections differ

The site has separate sections for the US, Germany and France, and each uses a different method. The US section follows ENERGY STAR's recommended insulation R-values by climate zone, which count the insulation alone, not the framing. The German section works from U-values in the GEG and federal funding rules. The French section works from the thermal resistance thresholds used by renovation grant schemes.

Checking our numbers

The engine is covered by automated tests, and the worked examples on our pages come from the same engine through a command-line tool, so every quoted depth, roll count, price and U-value matches the calculator. Show the maths under each result lists every step. If you spot an error, email info@loftinsulationcalc.com.

How the UK calculator works