A U-value measures how much heat passes through a building element such as a wall, roof or floor. It is given in W/m²K, the watts lost per square metre for each degree of temperature difference. Lower is better.

What is the maximum U-value for a new wall under Part L?

Approved Document L Volume 1 (2021) sets a limiting U-value of 0.26 W/m²K for walls in new dwellings, with 0.16 for roofs and 0.18 for floors. In practice the notional dwelling uses about 0.18 for walls, so most new-build walls are designed well below the limit.

What U-values do I need for an extension?

Approved Document L Volume 1 Table 4.2 gives limiting U-values for new elements in existing dwellings: 0.18 W/m²K for walls, 0.15 for roofs, 0.18 for floors, and 1.4 for windows (or Window Energy Rating band B).

How is a U-value calculated?

Under BS EN ISO 6946, add up the thermal resistance of each layer (thickness divided by thermal conductivity) plus the inside and outside surface resistances, then take the reciprocal: U = 1 ÷ (Rsi + R1 + R2 + … + Rse).

What is the difference between a U-value and an R-value?

An R-value is the thermal resistance of a single layer. A U-value is the overall heat transfer through the whole build-up, and equals 1 divided by the total R-value. R-values add together; U-values don't.

Does a cavity add to the insulation?

An unventilated cavity of 25mm or more adds a fixed resistance of 0.18 m²K/W under BS EN ISO 6946. A well-ventilated cavity adds nothing, and the layers outside it are ignored in the calculation.

Pinned

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Indicative onlyThese tools provide indicative calculations only. No warranty is given as to the accuracy, completeness or fitness for purpose of any output. Using this site does not create a professional relationship or duty of care between brickdims and the user. Results must not be used as a substitute for professional advice, site-specific assessment, or formal compliance sign-off. Building regulations are subject to change, so always verify against the current editions of the relevant Approved Documents and consult a building control officer where required. brickdims accepts no liability for any loss, damage or non-compliance arising from reliance on these tools, to the fullest extent permitted by law. If any part of these terms is found unenforceable, the remaining terms continue to apply. These terms are governed by the laws of England and Wales. By using this site you accept these terms.

U-value
calculatorBETA

Calculate the U-value of a wall, roof, or floor from individual material layers.

Results from brickdims.com tools are indicative only. Using this site does not create a professional relationship or duty of care. Not a substitute for professional advice or formal compliance sign-off.

Wall U-value

0.222

W/m²K

Rse (exterior surface)0.04

Σ R (layers)4.32

Rsi (interior surface)0.13

Rtotal4.49

This result is indicative only. Accuracy depends on inputs provided. Results are not a substitute for a certified assessment using approved methods.

Wall layers, exterior to interior

MaterialThicknessR-value

0.013

0.133

0.18 (fixed)

3.030

0.909

0.060

Total377.5 mm4.495

Drag to reorder. R = thickness ÷ λ

Material library

Material	λ W/(mK)
Clay brick, outer leaf	0.77
Clay brick, inner leaf	0.56
Mortar, outer leaf	0.94
Mortar, inner leaf	0.88
Engineering brick (1.0–1.7 by type)	1.3
Concrete (dense, 2200 kg/m³)	1.65
Concrete (lightweight)	0.38
Limestone	1.5
Granite	2.9
Dense aggregate block	1.13
Aerated concrete block	0.11
Softwood timber	0.13
Hardwood timber	0.18
Plywood	0.13
OSB / chipboard	0.13
Chipboard flooring	0.13
Timber floorboards	0.14
Mineral wool (0.033–0.044)	0.033
Glass wool (0.033–0.044)	0.033
Rigid PIR / PUR	0.022
Rigid EPS	0.038
Rigid XPS	0.034
Phenolic foam	0.019
Wood fibre (rigid)	0.042
Wood fibre (flexible)	0.05
Cellulose (blown)	0.04
Sheep wool (0.035–0.040)	0.038
Hemp / flax batts	0.04
Cork board	0.04
Aerogel blanket (0.013–0.018)	0.015
Vacuum insulation panel (0.007–0.010)	0.008
Unventilated air cavity (R = 0.18 fixed)	R 0.18 fixed
Well-ventilated cavity (R = 0, see note)	R 0 fixed
Plasterboard (standard, ≤700 kg/m³)	0.21
Plasterboard (high density, ≤900 kg/m³)	0.25
Sand / cement render	1
Dense gypsum plaster	0.57
Lightweight / perlite plaster	0.18
Ceramic tiles	1.3
Sand / cement screed	0.41
Carpet and underlay	0.06
Vinyl / linoleum	0.17
Clay roof tiles	1
Concrete roof tiles	1.5
Natural slate	2
Roofing felt / underlay	0.23
Sarking board	0.13
Profiled metal deck	50
Built-up felt (3 layer)	0.23
Vapour control layer	0.23
Hardcore / gravel	0.36
Sand blinding	0.3
DPM (polythene)	0.23
Reinforced concrete slab (2% steel)	2.3
Beam and block floor	1.13
Clay / subsoil (0.9–1.8)	1.5

Click any row to add as a layer. R = thickness ÷ λ for standard materials. Cavity R-values are fixed per BS EN ISO 6946 Table 2: unventilated cavity ≥25mm gives R = 0.18 m²K/W. Well-ventilated cavities eliminate the resistance of the cavity and all layers between it and the external surface; model only the layers on the room side.

λ values shown are typical design values based on BS EN ISO 10456. Manufacturer declared values (from product test certificates) will often differ and should be used where available for accurate results, particularly for insulation products.

R and U converter

R-value (m²K/W)

U-value (W/m²K)

0.4000Calculated

U = 1/R. Surface resistances not included in this converter.

Benchmark comparison: indicative reference only

Project type (for Part L reference only)

Part L limits shown for reference only. Always verify against the full Approved Document for your project.

Passivhaus ClassicEnerPHitAECB SilverAECB Gold

Passivhaus ClassicReview
0.222 W/m²K is above the 0.15 W/m²K figure. Passivhaus Institut, voluntary.
AECB SilverReview
0.222 W/m²K is above the 0.2 W/m²K figure. AECB, voluntary.

Heat loss calculation

Internal design temperature (°C)

External design temperature (°C)

Temperature inputs affect the heat loss calculation only. The U-value and all benchmark comparisons are independent of temperature. Default values based on CIBSE Guide A UK design conditions for dwellings. Adjust for your project as required.

Element area (m²)

Heat flux

5.3 W/m²

Heat loss per square metre of this element

Enter element area above to calculate total heat loss.

ΔT = 24.0 K (21°C − -3°C)

This is a simplified steady-state calculation for a single element. It does not account for thermal bridging, air infiltration, or intermittent heating. For whole-building heat loss calculations, approved methodologies include SAP (for dwellings) and PHPP.

Lambda values from BS EN ISO 10456:2007. Surface resistances per BS EN ISO 6946:2017. Cavity air gap R-value is fixed at 0.18 m²K/W per BS EN ISO 6946 Table 2 (unventilated, ≥25mm); equivalent lambda shown is calibrated for 50mm; vary thickness with caution. AD L (para 4.1) states that U-values should be assessed using the methods and conventions in BRE BR 443.

U-valuecalculatorBETA