Element-by-element heat-loss calculations for walls, roofs, floors and glazing — the evidence building control asks for on extensions, conversions and new builds. Calculated to the recognised conventions, delivered as a clean report you can submit directly.
The essentials
A U-value measures how quickly heat escapes through one square metre of a building element for each degree of temperature difference — W/m²K. Lower is better: a solid Victorian brick wall sits around 2.0, while a well-insulated new cavity wall achieves 0.18.
Part L sets maximum U-values for new and upgraded elements. When your building inspector asks "can you show me this wall build-up achieves 0.18?", a certified U-value calculation is the answer — and guessing with a manufacturer's generic figure often isn't accepted, because real build-ups have timber fractions, air gaps and bridging that change the result.
We calculate to the recognised conventions (BR 443 and the BS EN ISO standards), accounting for repeating thermal bridges like studs and mortar joints — the detail building control expects to see in the report.
| Element | New element in existing dwelling | New dwelling (limiting) |
|---|---|---|
| External wall | 0.18 W/m²K | 0.26 W/m²K |
| Roof | 0.15 W/m²K | 0.16 W/m²K |
| Floor | 0.18 W/m²K | 0.18 W/m²K |
| Windows & glazed doors | 1.4 W/m²K | 1.6 W/m²K |
Indicative values from Approved Document L (England, 2021 edition). Requirements differ in Wales and Scotland and by project type — we confirm the correct targets for your project.
What we calculate
Cavity walls, solid masonry with internal or external insulation, timber frame, SIPs and steel frame systems — including the timber-fraction and mortar-joint corrections that generic figures skip. We'll also tell you the cheapest way to close the gap if your proposed build-up misses the target.
Pitched roofs insulated at ceiling or rafter level, warm and cold flat roofs, room-in-roof conversions and dormers. Loft conversions are a frequent building-control flashpoint — a proper calculation showing the rafter build-up achieves the target usually settles it in one email.
Ground-bearing slabs, suspended timber and beam-and-block floors. Ground floor U-values depend on the perimeter-to-area ratio, not just the insulation thickness — which is why the same build-up passes on one plot and fails on another, and why a calculated result matters.
For internal-insulation and flat-roof projects we can add a condensation risk (Glaser) analysis, showing whether moisture will accumulate within the build-up — increasingly requested by building control alongside the U-value itself, and cheap to bundle in the same report.
Need the whole-dwelling picture? U-values feed directly into SAP. If your project involves a new dwelling or a heavily glazed extension, our SAP calculation service uses these element calculations as inputs — one supplier, one consistent set of numbers.
Common questions
The build-up, layer by layer: materials, thicknesses, and the insulation product (or the target, and we'll specify one). An architect's drawing or spec sheet is ideal; a clear written description with photos works for simpler projects.
Single-element calculations are usually returned within a couple of working days of receiving the full build-up. Tell us your building control deadline and we'll confirm timing with your quote.
We'll show you the options: a higher-performance insulation in the same thickness, a small increase in depth, or an alternative build-up — ranked by cost and buildability. Then we issue the final calculation for the version you choose.
Manufacturers publish figures for idealised constructions. Your real wall has timber studs, mortar joints, air gaps and fixings that bridge the insulation, and building control knows it — which is why they ask for a project-specific calculation to the BR 443 conventions.
Tell us the property type and postcode — we'll come back with a fixed quote and the earliest available date.