What is a U-value?

The definition

A u-value (thermal transmittance) measures how readily heat passes through a building element — wall, floor, roof, window or door — in W/m²K: watts of heat lost per square metre, per degree of temperature difference between inside and out. Lower is better. A solid Victorian brick wall might be around 2.0; a modern insulated cavity wall 0.18; a Passivhaus wall 0.10–0.15.

Put another way: a 1 m² wall with a u-value of 0.2 W/m²K, with 20°C inside and 0°C outside, loses 0.2 × 20 = 4 watts through that square metre. Scale that across the whole envelope and you can see why the figure matters so much for both energy bills and the SAP calculation that proves compliance.

How a U-value is actually calculated

U-values are calculated from the build-up of the element — not measured on a wall. The calculation, normally to BS EN ISO 6946 (the “combined method” for building elements), adds up the thermal resistance of every layer:

1. Each material layer contributes its thickness (m) divided by its thermal conductivity λ (W/m·K) — the R-value. The lower the λ, the better the insulator: mineral wool is around 0.032–0.044, PIR ~0.022, brickwork ~0.77, concrete ~1.3.
2. Surface resistances — the still-air films on the inside and outside faces (these differ for walls, roofs and floors).
3. Corrections — for repeating thermal bridges like timber studs or mortar joints, air gaps between layers, and mechanical fixings that pierce the insulation.

The total resistance R_T = R_si + R₁ + R₂ + … + R_se, and the u-value is simply U = 1 / R_T. Floors have extra ground-heat-loss terms (BS EN ISO 13370), and the whole thing needs the conventions of BR 443 to be consistent between one assessor’s software and another’s.

That’s exactly the method our free U-Monkey calculator uses — build up your element from real UK products and it computes the u-value, the Part L and Future Homes Standard targets, and a condensation screening check, with the PDF report included free.

A worked example

Take a modern insulated cavity wall, outside → inside:

Total resistance R_T = 0.04 + 0.13 + 2.86 + 0.91 + 0.05 + 0.13 = 4.12, so U = 1 / 4.12 ≈ 0.24 W/m²K (before corrections for mortar joints and wall ties, which push it a touch higher — the full-fill cavity wall page shows the real-world figures). Change the insulation to 150 mm and U drops to roughly 0.18; switch to PIR (λ 0.022) and it’s better still. That sensitivity is why the build-up — not the wall “type” — is what your SAP assessor needs.

R-value vs U-value

They’re often confused. R-value is a material property of a single layer or assembly (m²K/W) — resistance, higher is better. U-value is the overall element performance including surface films (W/m²K) — transmittance, lower is better. You can’t sensibly compare an R-value to a U-value directly; the industry standard is to design in U-values because that’s what Part L and SAP use.

Where the numbers come from

Layer conductivities come from BR 443 conventions, manufacturer declared λ values (for insulation, the declared figure is more pessimistic than the lab value, and SAP/SBEM expect it), or standard tables for masonry. The same build-up can therefore give slightly different U-values between software packages if one uses declared and another uses tabulated λ — another reason to trust a single consistent method (like U-Monkey’s) when comparing options.

Typical U-values — a reality check

There’s a page of typical values and editable build-ups for the most common constructions — cavity walls, solid walls, timber-frame walls, pitched roofs, flat roofs (including how much flat-roof insulation you need), loft insulation, concrete ground floors, suspended timber floors and beam & block floors.

What Part L requires

Part L 2021 works on two levels. The notional dwelling that sets your target uses good u-values (walls 0.18, floor 0.13, roof 0.11, windows 1.2 — see SAP 10: what’s new), and there are backstop limiting values you must never exceed (walls 0.26, floor 0.18, roof 0.16, windows/doors 1.6). Build between the two and the shortfall must be offset elsewhere in the SAP calculation.

For extensions and work to existing dwellings, new thermal elements have their own minimum u-value standards — which is why the same insulated build-up appears whether you’re renovating a Victorian terrace or building new.

How to improve a U-value

Almost always in this order:

1. Add insulation — the single biggest lever. Where it goes (cavity fill, external wall insulation, internal wall insulation, loft, floor overlay) depends on the element and the building.
2. Insulate the whole element, not part of it — a thermal bridge at the edge undoes good work elsewhere (see below).
3. Upgrade the windows — low-e glass, argon fill and warm-edge spacers take a typical double-glazed unit from ~1.6 to ~1.2.
4. Don’t chase the last 0.02 — beyond ~0.15, each extra millimetre of insulation buys very little; the energy hierarchy and fabric energy efficiency guides put this in perspective.

U-values aren’t the whole story

An element’s u-value covers its flat area — but heat also escapes where elements meet. Those junction losses are measured separately as Ψ-values, and at modern u-value levels they can be 20–30% of total fabric heat loss. See thermal bridging explained, or calculate junction values with ΨMonkey.

U-values also feed everything downstream: SAP and SBEM calculations, energy statements, Part O overheating checks (better fabric often means more careful glazing design) and Part G hot-water demand. Get the fabric right and the rest of the compliance picture gets easier.