Wall, Roof, Floor, Window & Door Thermal Transmittance – Updated 2026
Build up your construction element layer by layer to calculate the overall U-value (W/m²K). Select each material and enter its thickness — the calculator applies the correct thermal conductivity (λ value) and surface resistances. Check compliance against UK Building Regulations Part L (England) 2021 limiting U-values for new build and retained elements.
A U-value (thermal transmittance) is a measure of how much heat flows through a building element — a wall, roof, floor, window, or door — per unit area per unit temperature difference between the inside and outside. It is expressed in W/m²K (watts per square metre per kelvin of temperature difference). A lower U-value means better thermal insulation — less heat escapes through the element. A U-value of 0.20 W/m²K means that for every square metre of the element, 0.20 watts of heat flows through it for every degree Celsius difference between inside and outside temperatures.
U-values are calculated using the method set out in BS EN ISO 6946:2017 (Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods). The basic calculation adds up the thermal resistances (R-values) of each layer in the construction — calculated as layer thickness (metres) divided by thermal conductivity (λ, in W/mK) — plus standard surface resistances for the inside and outside surfaces, then takes the reciprocal: U = 1 / (Rsi + R1 + R2 + ... + Rn + Rse).
In the UK, U-values are regulated by Building Regulations Approved Document L (Conservation of Fuel and Power), which was significantly updated in England in 2021 to tighten the thermal performance requirements for new buildings and extensions. From June 2022, the Part L 2021 limiting U-values apply to all new dwellings in England, requiring external walls of new homes to achieve no worse than 0.26 W/m²K, roofs no worse than 0.16 W/m²K, and ground floors no worse than 0.18 W/m²K. Scotland, Wales, and Northern Ireland have their own building regulations with separate U-value requirements.
To calculate the U-value of a construction element, follow these steps:
The thermal conductivity (λ) of a material determines how quickly heat flows through it. Lower λ = better insulator. The values below are design values from BR 443 / BS EN ISO 10456 used in UK U-value calculations:
The table below gives the limiting U-values (maximum permitted U-values) for common building elements under UK Building Regulations in 2026. These are the maximum values — better performance (lower U-value) is encouraged and required by SAP assessment to achieve compliance with the overall energy efficiency standard.
| Element | Part L 2021 England – New Dwelling | Part L 2021 England – Extension | Part L 2021 England – Retrofit / Renovation | Scotland Section 6 (new build) | Notes |
|---|---|---|---|---|---|
| External wall | 0.26 W/m²K | 0.28 W/m²K | 0.55 W/m²K | 0.22 W/m²K | Cavity wall: typically achieved with 100mm full-fill cavity insulation + lightweight blockwork inner leaf |
| Party / separating wall | 0.20 W/m²K | 0.20 W/m²K | — | 0.20 W/m²K | Resistance to heat loss to adjacent dwelling |
| Pitched roof (at ceiling) | 0.16 W/m²K | 0.16 W/m²K | 0.35 W/m²K | 0.15 W/m²K | 100mm between joists + 200mm above (total ~300mm) mineral wool meets 0.16 |
| Pitched roof (at rafter – room-in-roof) | 0.16 W/m²K | 0.16 W/m²K | 0.35 W/m²K | 0.15 W/m²K | Typically requires PIR between and below rafters |
| Flat roof / roof room | 0.16 W/m²K | 0.18 W/m²K | 0.35 W/m²K | 0.15 W/m²K | Warm roof construction preferred; cold roof requires ventilation gap |
| Ground floor (new build) | 0.18 W/m²K | 0.22 W/m²K | 0.25 W/m²K | 0.17 W/m²K | Exposed floor (above external): 0.15 W/m²K; depends on floor perimeter/area ratio |
| Exposed floor (above external air) | 0.15 W/m²K | 0.22 W/m²K | 0.25 W/m²K | 0.15 W/m²K | Floor over unheated garage or over open air |
| Windows (whole-unit) | 1.40 W/m²K | 1.40 W/m²K (centre-pane 1.2) | 1.60 W/m²K | 1.20 W/m²K | Whole-unit U-value from BFRC certificate; double glazed argon-filled low-E typically 1.2–1.4 |
| Rooflights | 1.80 W/m²K (centre-pane) | 1.80 W/m²K | 2.20 W/m²K | 1.40 W/m²K | Whole-unit values typically higher than vertical windows due to geometry |
| Doors (solid) – opaque | 1.40 W/m²K | 1.40 W/m²K | 1.80 W/m²K | 1.20 W/m²K | Whole-unit U-value including frame; composite insulated doors typically 0.8–1.2 W/m²K |
| Curtain walling | 1.50 W/m²K | 1.50 W/m²K | — | 1.20 W/m²K | Commercial building façade systems; thermally broken aluminium frame required |
Part L 2021 (Approved Document L for Dwellings, England) values effective from June 2022. These are limiting (maximum) values — a dwelling must also meet the overall energy efficiency standard via SAP assessment. Scotland uses Section 6 of Technical Handbook (Domestic). Wales uses Part L of Building Regulations (Wales). Northern Ireland uses Part F of Technical Booklets. Always verify current requirements with your local Building Control authority.
| Material | λ (W/mK) | Typical Thickness | R-value at typical thickness (m²K/W) | Common Application |
|---|---|---|---|---|
| Dense concrete (2000 kg/m³) | 1.13 | 100mm | 0.09 | Ground floor slab, structural elements |
| Brickwork outer leaf (1800 kg/m³) | 0.77 | 102.5mm | 0.13 | External cavity wall outer leaf |
| Dense concrete block (1800 kg/m³) | 0.99 | 100mm | 0.10 | Inner leaf (uncommon in new build) |
| Lightweight agg. block (600 kg/m³, Thermalite) | 0.15 | 100mm | 0.67 | Most common inner leaf block in UK new build |
| Aerated concrete block (Aircrete, 475 kg/m³) | 0.11 | 100mm | 0.91 | Inner leaf, below ground walls, high-performance |
| Sand:cement render (1800 kg/m³) | 0.88 | 13mm | 0.01 | Outer face render on cavity wall |
| Plasterboard (900 kg/m³) | 0.21 | 12.5mm | 0.06 | Internal finish / dot-and-dab lining |
| Plaster (gypsum, 1200 kg/m³) | 0.46 | 13mm | 0.03 | Traditional plaster finish |
| Softwood timber (500 kg/m³) | 0.13 | 47mm (rafter) | 0.36 | Rafters, joists, stud walls |
| Mineral wool batts (glass or rock wool, 25 kg/m³) | 0.038 | 100mm | 2.63 | Cavity fill, loft, between rafters/joists |
| Mineral wool quilt (12–15 kg/m³) | 0.044 | 100mm | 2.27 | Loft insulation laid on ceiling |
| PIR board (rigid, polyisocyanurate) | 0.022 | 70mm | 3.18 | Warm roof, floor insulation, dot-and-dab systems |
| PIR board (rigid, higher performance) | 0.022 | 100mm | 4.55 | Best performing common rigid insulation type |
| EPS board (expanded polystyrene, 20 kg/m³) | 0.038 | 100mm | 2.63 | Floor insulation, EWI systems |
| XPS board (extruded polystyrene) | 0.034 | 100mm | 2.94 | Below-slab insulation, inverted roofs |
| Phenolic foam board | 0.020 | 50mm | 2.50 | High-performance; thinnest option per R-value |
| Cellulose / loose-fill insulation | 0.040 | 300mm | 7.50 | Blown into loft or wall cavities; eco insulation |
| Spray polyurethane foam (closed cell) | 0.028 | 100mm | 3.57 | Roof slope insulation; specialist application |
| Woodfibre board (240 kg/m³) | 0.038 | 100mm | 2.63 | Sustainable insulation; walls, roofs |
| Glass (double-glazed unit) | — | 4+16+4mm unit | Centre-pane 0.9–1.1 | Use whole-unit U-value for Building Regs |
| Unventilated air cavity (any orientation) | — | 25mm+ | 0.18 | Cavity in wall construction; BS EN ISO 6946 Table D.3 |
U-value and energy efficiency resources UK 2026: GOV.UK – Approved Document L (Part L) | BRE – BR 443 Thermal Conductivity Values | BFRC – Window Energy Ratings | SAP Assessor Directory | AECB – Low Energy Building Standards
| Construction Type | Typical U-Value (W/m²K) | Part L 2021 Limit | Compliant? | Notes |
|---|---|---|---|---|
| Solid brick wall (225mm), no insulation | ~2.00 | 0.26 | No | Typical pre-1920s solid wall; requires EWI or IWI for improvement |
| Uninsulated cavity wall (brick/cavity/dense block) | ~1.50 | 0.26 | No | Pre-1976 construction; cavity fill insulation improves to ~0.5 |
| Cavity wall: brick / 100mm MW / aerated block / plasterboard | ~0.24 | 0.26 | Yes | Typical UK new build 2022+ (Part L 2021 compliant) |
| Cavity wall: brick / 100mm PIR / aerated block / plasterboard | ~0.19 | 0.26 | Yes (good margin) | Enhanced new build spec; often used to allow bridging margins |
| Timber frame wall: plasterboard / 140mm MW in studs / sheathing / brick | ~0.22 | 0.26 | Yes | Standard UK timber frame new build; briging factor applied for studs |
| Pitched roof: 100mm MW between joists + 270mm above | ~0.11 | 0.16 | Yes | Exceeds minimum; equivalent to ~370mm total mineral wool |
| Pitched roof: 100mm between joists + 200mm above (total 300mm MW) | ~0.15 | 0.16 | Yes (minimal margin) | Minimum typically recommended for Part L 2021 compliance |
| Flat warm roof: 120mm PIR on deck | ~0.17 | 0.16 | Marginal (check) | Increase to 140mm PIR for comfortable compliance |
| Solid concrete ground floor + 100mm EPS | ~0.19 | 0.18 | Marginal | Ground floor U-value depends on perimeter:area ratio; edge insulation important |
| Solid concrete ground floor + 100mm PIR | ~0.14 | 0.18 | Yes | PIR provides higher R-value per mm than EPS |
| Double glazing (argon, low-E, aluminium frame) | ~1.6–2.0 | 1.40 | No (frame dependent) | Non-thermally broken aluminium frame significantly worsens whole-unit U-value |
| Double glazing (argon, low-E, thermally broken frame) | ~1.2–1.4 | 1.40 | Yes (borderline) | Standard UK new build window; uPVC or thermally broken aluminium |
| Triple glazing (argon/krypton, low-E, TBr frame) | ~0.7–1.0 | 1.40 | Yes | Passive House / Fabric First standard; significant cost premium |
U-value is the rate of heat transfer — a lower U-value means less heat escapes. A Passivhaus-standard wall at 0.10 W/m²K loses heat at one-twentieth the rate of an uninsulated solid brick wall (2.0 W/m²K). The UK Building Regulations Part L 2021 limiting external wall U-value is 0.26 W/m²K for new dwellings — representing a major improvement over the 2002 limit of 0.35 W/m²K.
The calculated U-value assumes uniform material layers. In practice, thermal bridges — structural elements, wall ties, window reveals, and junctions — conduct heat more rapidly than the surrounding insulation, increasing the actual heat loss above the calculated U-value. The correction factor for thermal bridging is expressed as the linear thermal transmittance (Ψ, psi) and should be calculated for all significant bridges and added to the SAP assessment.
The 2021 update to Approved Document L (England) introduced the Future Homes Standard uplift, tightening the notional dwelling specification. New dwellings must achieve a primary energy rate approximately 31% lower than 2013 Part L, with limiting U-values of 0.26 W/m²K (walls), 0.16 W/m²K (roofs), and 0.18 W/m²K (floors) — with a further uplift to the Future Homes Standard expected around 2025.
The internal and external surface resistances (Rsi and Rse) are standardised values that account for the convective and radiative heat transfer at building surfaces. For external walls: Rsi = 0.13 m²K/W, Rse = 0.04 m²K/W. For roofs: Rsi = 0.10, Rse = 0.04. For ground floors: Rsi = 0.17, Rse = 0.04. These add approximately 0.17 m²K/W to the total resistance of any vertical external element.
In timber frame walls and roofs with insulation between studs or rafters, the timber framing conducts heat approximately 3× more than mineral wool insulation (λ = 0.13 vs 0.038 W/mK). This reduces the effective U-value calculation through a bridging correction (often 20–25% of the wall area is timber). For studs at 600mm centres in a 90mm stud wall, the bridging factor reduces the effective R-value by approximately 5–10%.
To achieve R = 3.0 m²K/W requires approximately 66mm of PIR (λ = 0.022) or 114mm of mineral wool (λ = 0.038). This nearly 2× difference in required thickness makes PIR the preferred option where space is constrained — in flat roofs, floors under beam-and-block, and internal wall insulation upgrades. Mineral wool is preferred where fire resistance, acoustic performance, or vapour permeability are important.
Window U-values are expressed as either whole-unit (including frame and edge effects) or centre-pane values. Building Regulations use whole-unit U-values — always specified as UW on BFRC certificates. A standard argon-filled low-E double-glazed unit has a centre-pane U-value of approximately 1.0–1.2 W/m²K, but the whole-unit value (including frame) is typically 1.2–1.6 W/m²K depending on frame material and design.
Scotland's Technical Handbook Section 6 has historically maintained stricter fabric performance requirements than England's Approved Document L. Scotland requires new-build walls to achieve 0.22 W/m²K, roofs 0.15 W/m²K, and windows 1.20 W/m²K — compared to 0.26, 0.16, and 1.40 W/m²K respectively for new dwellings in England. Wales and Northern Ireland have separate but broadly similar requirements to England.
The Passivhaus (Passive House) standard requires a much higher fabric performance than UK Building Regulations: external walls typically at 0.10–0.12 W/m²K, roofs at 0.09–0.12 W/m²K, floors at 0.10–0.13 W/m²K, and windows at 0.70–0.85 W/m²K (triple-glazed). Passivhaus heating demand is limited to 15 kWh/m²/year, achievable only with ultra-low U-values, thermal bridge-free construction, and mechanical ventilation with heat recovery (MVHR).
All new dwellings in England require a Standard Assessment Procedure (SAP) assessment to demonstrate compliance with Part L. SAP calculates the dwelling's energy performance using U-values for all elements, thermal bridging, air permeability, heating system efficiency, and renewable energy generation. U-values must be confirmed by a qualified energy assessor — the calculator on this page is a planning tool and does not constitute an official SAP assessment.
When upgrading existing building elements (renovation or change of use), Part L sets threshold U-values — the maximum permitted U-value after improvement. For existing walls: 0.55 W/m²K; roofs: 0.35 W/m²K; floors: 0.25 W/m²K. If an existing element already achieves better than the threshold, no improvement is required. Where technically or economically unfeasible (thin cavity, listed building, protected façade), exemptions may apply — always confirm with Building Control.
Steady-state heat loss through a building element = U × A × ΔT, where U is the U-value (W/m²K), A is the element area (m²), and ΔT is the temperature difference (K or °C). For a 25m² external wall at U = 0.24 W/m²K with ΔT = 25°C: heat loss = 0.24 × 25 × 25 = 150 W. This represents the continuous heat loss through that element alone under peak winter conditions. Annual energy loss is typically calculated using degree-day data.
A U-value (thermal transmittance) measures the rate at which heat passes through a building element (wall, roof, floor, window, or door) per unit area per unit temperature difference. It is expressed in W/m²K — watts per square metre per kelvin of temperature difference. A U-value of 0.25 W/m²K means that for each square metre of the element, 0.25 watts of heat flows through it for every degree Celsius difference between inside and outside. The lower the U-value, the better the thermal insulation — a Passivhaus wall at 0.10 W/m²K loses heat at one-twentieth the rate of an uninsulated solid brick wall at approximately 2.0 W/m²K.
Under Approved Document L 2021 (England), the limiting (maximum permitted) U-value for external walls of new dwellings is 0.26 W/m²K. However, this is a limiting value, not a target — the SAP assessment may require the wall to achieve a lower U-value as part of a package to meet the overall energy efficiency standard. In Scotland, the equivalent limit under Section 6 of the Technical Handbook is 0.22 W/m²K. For extensions to existing dwellings in England, the limiting U-value is 0.28 W/m²K. For retained walls in renovation works, the threshold is 0.55 W/m²K. Always confirm with your Building Control officer or SAP assessor.
A U-value is calculated by summing the thermal resistances (R-values) of all layers in the construction, adding the standard surface resistances, and taking the reciprocal: U = 1 / RT, where RT = Rsi + R1 + R2 + ... + Rn + Rse. Each solid layer's resistance is R = d/λ (thickness in metres ÷ thermal conductivity in W/mK). Air cavities have fixed tabulated R-values. Surface resistances are standardised: for a vertical wall, Rsi = 0.13 m²K/W and Rse = 0.04 m²K/W. This calculator performs the full calculation automatically — select each layer, enter its thickness, and the U-value is calculated instantly.
U-value and R-value are reciprocals of each other: U = 1/R and R = 1/U. The R-value (thermal resistance) of an individual layer is R = d/λ (thickness/conductivity). The total R-value (RT) of the complete element (including surface resistances) is the sum of all individual layer R-values. The U-value is then U = 1/RT. U-values are used in Building Regulations and heat loss calculations. R-values are used in the intermediate calculation — insulation products are sometimes marketed with their R-value (e.g. "R3.0 PIR board"), which is a fixed property of that specific product thickness and is useful for comparing insulation products directly.
A standard UK cavity wall construction with 102.5mm brick outer leaf, 100mm unventilated cavity, 100mm aerated concrete (Aircrete) inner leaf, and 12.5mm plasterboard achieves approximately 0.24 W/m²K when the cavity is fully filled with mineral wool insulation (λ = 0.038 W/mK) — comfortably meeting the 0.26 W/m²K Part L 2021 limit. With partial-fill PIR boards (50mm PIR + 50mm clear cavity), the U-value is approximately 0.22 W/m²K — providing more margin. The exact U-value depends on the precise material specifications and the thermal bridging correction for wall ties. Always use your specific material λ values and include a thermal bridging allowance in a formal SAP calculation.
To achieve the Part L 2021 limiting U-value of 0.16 W/m²K for a pitched roof insulated at ceiling level, approximately 300mm of mineral wool is required — typically 100mm between the joists and 200mm laid crossways above. To achieve a better (lower) U-value: 350mm mineral wool achieves approximately 0.13 W/m²K; 400mm achieves approximately 0.11 W/m²K. The exact value depends on the material's λ — higher-performance (lower λ) mineral wool allows the same U-value with less thickness. The government recommends loft insulation as one of the most cost-effective home energy efficiency improvements, with typical payback periods of 2–5 years for top-up insulation in under-insulated homes.
Under Approved Document L 2021 (England), new windows in new dwellings must achieve a whole-unit U-value of no worse than 1.40 W/m²K. Standard argon-filled low-E double-glazed windows with uPVC or thermally broken aluminium frames typically achieve 1.2–1.4 W/m²K — just meeting the requirement. To achieve significantly better performance, triple glazing (0.7–1.0 W/m²K) is required, at a significant cost premium. The whole-unit U-value (UW, including the frame) must be used for Building Regulations compliance — not the centre-pane value. Window U-values are certified by the BFRC (British Fenestration Rating Council) and should be confirmed from the manufacturer's BFRC certificate.
Solid brick walls (typically 225mm or 327mm thick) have U-values of approximately 1.5–2.0 W/m²K — far above modern Building Regulations limits. The options for improvement are: External Wall Insulation (EWI) — insulation boards fixed to the outside face, covered with render or cladding. 100mm EPS (λ = 0.038) reduces the wall U-value to approximately 0.3 W/m²K. 100mm PIR (λ = 0.022) achieves approximately 0.17 W/m²K. EWI changes the external appearance (planning permission may be needed). Internal Wall Insulation (IWI) — insulation fixed to the inside face, reducing room dimensions but not affecting external appearance. 80mm PIR with plasterboard achieves approximately 0.20 W/m²K. Both methods require attention to the DPC line, window reveals, and services to avoid creating cold bridges or condensation risks.
A thermal bridge is a localised area of a building envelope where heat flow is significantly higher than through the adjacent construction — typically at junctions between elements, at structural connections, or where insulation continuity is broken. Examples include: wall ties in cavity walls (metal ties can increase effective U-value by 5–15%); window reveals; structural beams penetrating insulation layers; concrete floor edges at external walls; and column-to-wall junctions. Thermal bridges are not captured in a simple layer-by-layer U-value calculation — they must be assessed separately using linear thermal transmittance values (Ψ, psi values) and included in the SAP assessment as additional heat loss. In practice, thermal bridging can add 20–50% to the heat loss from well-insulated walls.
For new dwellings and major renovations in England, a SAP (Standard Assessment Procedure) assessment is required as part of the Building Regulations approval process, and this assessment uses U-values for all building elements. The SAP assessor will calculate the U-values from the construction specification or verify manufacturer-certified values. For extensions to dwellings, you must demonstrate that each new element meets the limiting U-value, either by providing a U-value calculation or by specifying a pre-approved standard construction. The U-value calculation on this page uses standard BS EN ISO 6946 methodology and provides a useful planning estimate, but a formal SAP assessment by an accredited assessor is required for Building Regulations compliance documentation.
Additional U-value and thermal performance resources UK 2026: GOV.UK – Approved Document L | BRE – Thermal Performance Guidance | BFRC – Window Energy Ratings | Energy Saving Trust | Passivhaus Trust UK