What is a U-value?

Updated 20 September 2026 — reviewed against the regs as they stand.

Quick answer: a U-value measures how fast heat passes through a building element, in watts per square metre per degree (W/m²K) — so lower is better. New-build walls in England must not exceed 0.26 W/m²K under Part L 2021, and typically need to reach about 0.18 to pass the overall SAP. Those limits do not change under the Future Homes Standard in 2027.

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.

If you have come from an American source, U-factor is the same thing in Btu/(h·ft²·°F); multiply a U-factor by 5.68 to get W/m²K, or a U-value by 0.176 to go the other way.

Which U-value do I need?

This is the question most people arrive with, and the answer depends on which of three situations you are in. All figures W/m²K, from Approved Document L Volume 1, 2021 edition.

Three things to take from the table. New build: the limiting column is a backstop, not a target — a dwelling built to 0.26 walls passes that row and fails the SAP by a distance, because the notional dwelling it is measured against has walls at 0.18 and solar panels on the roof. Build between the two columns and the shortfall has to be made up elsewhere in the calculation. Existing dwellings: the “new element” column is what an extension wall, a new roof or a replacement window has to hit, and it is tighter than the new-build backstop. Renovation: a retained element worse than the threshold has to be improved to the second figure where that is technically and functionally feasible and pays back within 15 years — the clause that keeps solid-wall houses insurable and barn conversions possible. The last two columns have a guide each: extensions and renovation.

The Part L guide explains which edition applies to your project; U-Monkey checks a build-up against every column live.

Area-weighting — and the window trap

The limiting values for walls, floors and roofs apply to the area-weighted average of all elements of that type: multiply each section’s U-value by its area, add them up, divide by the total area. A short run of weaker wall can be carried by better wall elsewhere, and that same arithmetic is one of the two routes to compliance for an over-glazed extension.

The exception catches people every week. In an existing dwelling, windows, doors, roof windows and rooflights are excluded from area-weighting — every single one has to meet 1.4 on its own. You cannot average a poor window against a good one. Where a building’s character genuinely requires the original windows, the Approved Document accepts a centre-pane U-value of 1.2 or better, or single glazing with low-emissivity secondary glazing behind it.

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, to BS EN ISO 6946, 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) — its R-value. The lower the λ, the better the insulator.
2. Surface resistances — the still-air films on the inside and outside faces. Inside: 0.13 m²K/W for a wall, 0.10 for a roof (heat flowing up), 0.17 for a floor (heat flowing down). Outside: 0.04 for all of them.
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 — the U-value to R-value converter does that arithmetic either way, with the surface resistances built in. Ground floors use BS EN ISO 13370 instead, because heat loss through the ground depends on the floor’s perimeter-to-area ratio — the same slab gets a different U-value in a different floor plan. And the whole thing follows the conventions of BR 443 (third edition, 2019) so that one assessor’s software agrees with another’s: default bridging fractions, air-gap corrections, and — critically — which conductivity values you are allowed to use.

That is 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 checks, a condensation screening, and the thermal mass — decrement factor, time lag and areal heat capacity to ISO 13786 — with the PDF report free. How to use U-Monkey walks through a build-up layer by layer, bridging fractions and cavities included.

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 is better still. That sensitivity is why the build-up — not the wall “type” — is what your SAP assessor needs.

Hand-summing like this only works for a homogeneous element. Anything with timber studs, wall ties, metal fixings or a ground floor needs the bridging and correction methods — which is what the software is for.

R-value vs U-value

They are often confused. R-value is a 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 cannot sensibly compare one with the other; Part L and SAP work in U-values.

Materials do not have U-values — they have conductivities

People search for “the U-value of wood” or “the U-value of steel”. A material has a thermal conductivity, λ, in W/m·K; it only acquires a U-value once it is a layer of a certain thickness in an element with surfaces. The conductivities that matter, from the BR 443 conventions and the manufacturers’ declared values:

The distinction in the right-hand column matters. BR 443 tabulates defaults for masonry, timber and metals but not for blocks or insulation — for those it requires the manufacturer’s declared λ, which is a certified figure a little more pessimistic than the lab value. A calculation built on a generic insulation figure instead of the declared one is a common reason for a U-value being rejected, and why the same build-up can give different answers in different software. U-Monkey uses declared values throughout, and every λ it works with — 418 figures across 53 generic materials and 365 named products from 108 manufacturers, each carrying the document it was read from and the date — is listed in the thermal conductivity library.

Windows: whole-window, not centre-pane

Part L uses the whole-window U-value — glass, frame and spacer together — not the centre-pane figure a glazing brochure leads with. A unit with a centre-pane value of 1.0 can sit in a window whose whole-window value is 1.4 to 1.6, so the glass-only figure is not a compliance figure. In an existing dwelling the Approved Document accepts a Window Energy Rating as an alternative to a U-value — band B or better for windows — which can reward a window that admits useful solar gain in a way the U-value alone cannot.

Passivhaus U-values

Passivhaus does not actually set a maximum U-value for walls, roofs or floors. The standard is performance-based — a space heating demand of 15 kWh/m² a year or less, an airtightness of 0.6 air changes per hour at 50 Pa, and a primary energy limit — and the fabric is whatever it takes to get there, which in the UK’s cool-temperate climate typically means opaque elements at 0.10 to 0.15 W/m²K. Windows are certified as components: a whole-window U-value of 0.80 or better and 0.85 installed, with glazing at 0.70 or better. That is about half the Part L limit for a new dwelling, and it is where triple glazing stops being optional.

Measuring a U-value in an existing wall

When the build-up is unknown — a rendered wall that might or might not have a cavity, a stone wall of uncertain thickness — the U-value can be measured in situ with a heat flux plate to BS ISO 9869-1. A sensor on the inside face and temperature probes inside and out log for a minimum of 72 hours in the heating season, and the test ends only when the result has stopped moving — within 5% over the last 24 hours. Heavy masonry walls often need considerably longer; lightweight walls are analysed on night-time data only, to exclude the sun. It is the honest answer to “what is my wall actually doing?” for a retrofit design or a suspected workmanship problem — but BRE’s guidance is that it is not suitable for producing an individual dwelling’s EPC. An assessment of an existing home takes a different route entirely: where the build-up cannot be evidenced, RdSAP assigns a default from the dwelling’s age band, and all of those defaults are tabulated here.

Typical U-values — a reality check

There is a page of typical values and editable build-ups for the most common constructions — cavity walls, solid walls (including the one-and-a-half-brick, 317.5mm solid wall), 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.

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.

What changes in 2027 — and what does not

Approved Document L 2026, the Future Homes Standard, takes effect on 24 March 2027. The limiting U-values do not change: walls 0.26, floors 0.18, roofs 0.16, windows and doors 1.6, rooflights 2.2 — the same numbers, renumbered into new tables — and the notional dwelling’s fabric values are unchanged too. What tightens is the notional dwelling’s air permeability, the switch to a heat-pump baseline, and a new legal requirement for on-site renewables. If you arrived here expecting a new set of U-value limits for 2027, there isn’t one; the fabric you design to Part L 2021 is the fabric the Future Homes Standard expects.

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, calculate junction values with the free psi value calculator, or build to Accredited and Recognised Construction Details and use their published values. The Table K1 defaults are what you are measured against if you do neither.

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.