Calculated U-Value
0.00 W/m²K
Check building regulations
Professional calculator for walls and roofs thermal transmittance
Calculate U-values, R-values, and insulation requirements for building regulations compliance and energy efficiency in 2026.
Calculate thermal transmittance for building element compliance
Calculate thermal transmittance (U-value) for walls, roofs, and floors with multi-layer construction. Determine insulation requirements for UK Building Regulations Part L, NCC Section J, and international energy codes. Lower U-values indicate better thermal performance and reduced heat loss.
Access comprehensive material thermal properties including conductivity (λ), specific heat capacity, and density. Calculate R-values for brickwork, blockwork, timber, insulation materials, plasterboard, and finishes based on standardized test data.
Verify compliance with current energy efficiency standards. UK Building Regulations 2021 require wall U-values ≤0.18 W/m²K, roofs ≤0.15 W/m²K for new dwellings. Our calculator shows if your construction meets minimum thermal performance requirements for 2026 standards.
Build your construction layers and calculate thermal performance
U-value, measured in watts per square metre kelvin (W/m²K), quantifies the rate of heat transfer through a building element. Lower U-values indicate better insulation - a wall with U-value 0.15 W/m²K loses heat half as fast as one with 0.30 W/m²K. Building regulations specify maximum U-values to ensure energy efficiency and reduce heating/cooling costs. The UK Building Regulations Part L mandates progressively lower U-values to meet climate targets and reduce carbon emissions from buildings.
U-value calculation involves determining the thermal resistance (R-value) of each material layer, plus internal and external surface resistances. R-value equals material thickness divided by thermal conductivity (λ). Total R-value is the sum of all layer R-values plus surface resistances. U-value is simply the reciprocal: U = 1/R_total. This fundamental relationship means increasing insulation thickness (raising R-value) proportionally decreases U-value and heat loss through the building fabric.
Each material layer's resistance to heat flow
Sum of all resistances including surfaces
Thermal transmittance (W/m²K) - lower is better insulation
Assuming 21°C internal, 5°C external (ΔT = 16K)
| Building Element | UK 2021 (W/m²K) | UK 2025 Future Homes (W/m²K) | Passivhaus (W/m²K) |
|---|---|---|---|
| External Walls | ≤0.18 | ≤0.15 | ≤0.15 |
| Pitched Roof (insulated at ceiling) | ≤0.15 | ≤0.11 | ≤0.12 |
| Flat Roof | ≤0.15 | ≤0.11 | ≤0.12 |
| Ground Floor | ≤0.18 | ≤0.13 | ≤0.15 |
| Windows (whole unit) | ≤1.40 | ≤1.20 | ≤0.80 |
| Doors (external) | ≤1.40 | ≤1.20 | ≤0.80 |
Construction: 105mm brick + 100mm cavity with 80mm PIR insulation + 100mm concrete block + 12.5mm plasterboard. U-value: 0.18 W/m²K - meets current UK Building Regulations. Common specification for new build residential construction providing good thermal performance at reasonable cost.
Construction: Brick outer leaf + cavity + breather membrane + 150mm timber frame with mineral wool + vapour barrier + 12.5mm plasterboard. U-value: 0.15-0.17 W/m²K depending on insulation specification. Popular for self-build and volume house builders.
Construction: Tiles on battens + underlay + ventilated void + 270mm mineral wool between/over ceiling joists + 12.5mm plasterboard. U-value: 0.13-0.15 W/m²K - standard loft insulation achieving current standards. Easy to retrofit in existing buildings.
Construction: Paving/green roof + waterproof membrane + 150mm PIR insulation + vapour control layer + deck. U-value: 0.11-0.13 W/m²K - exceeds minimum requirements. Warm deck construction avoids interstitial condensation issues common in cold deck designs.
| Material | Conductivity λ (W/mK) | Typical Use | Notes |
|---|---|---|---|
| Phenolic Foam (PIR/PUR) | 0.020-0.022 | High-performance insulation | Best performance per mm thickness |
| XPS Insulation | 0.030-0.035 | Below-ground, roofs | Good compressive strength |
| Mineral Wool (Glass/Rock) | 0.032-0.040 | Cavity walls, roofs, timber frame | Fire resistant, breathable |
| EPS Insulation | 0.035-0.040 | Floor insulation, cavity walls | Cost-effective option |
| Lightweight Block (AAC) | 0.11-0.19 | Inner leaf masonry | Some insulating value |
| Softwood Timber | 0.13 | Structural framing | Moderate insulation |
| Air Cavity (still) | R=0.18 m²K/W | Cavity walls | Fixed resistance value |
| Plasterboard | 0.21-0.25 | Internal lining | Minimal insulation |
| Concrete Block (Dense) | 0.51-1.13 | Structural walls | Poor insulator |
| Brick (Outer Leaf) | 0.77-0.84 | External cladding | High conductivity |
| Dense Concrete | 1.13-1.40 | Floors, structural elements | Very high conductivity |
Existing solid walls typically have U-values of 1.5-2.1 W/m²K - far below current standards. Internal wall insulation (IWI) adds insulation board (50-100mm) on internal surface, reducing room size but avoiding external appearance changes. External wall insulation (EWI) applies 80-150mm insulation board to external walls with render/cladding finish. EWI is more thermally efficient as it treats thermal bridges and protects the wall structure, but changes building appearance and may require planning permission.
Loft insulation is the most cost-effective energy efficiency measure. UK Building Regulations originally required 100mm loft insulation (1980s), increased to 200mm (1990s), 270mm (2002), and current 300mm+ recommendation. Each thickness increase reduces U-value: 100mm ≈ 0.40 W/m²K, 200mm ≈ 0.20 W/m²K, 270mm ≈ 0.15 W/m²K, 300mm ≈ 0.13 W/m²K using mineral wool (λ=0.040).
Thermal bridges (cold bridges) are areas where insulation continuity is broken, creating paths of high heat flow. Common thermal bridges include wall corners, junctions between walls and floors/roofs, lintels above windows, steel frame members, and wall ties in cavity construction. A well-insulated wall achieving U=0.18 W/m²K in field area may have overall U-value 0.25-0.30 W/m²K when thermal bridges are accounted for - increasing heat loss 40-60%.
Psi-values (Ψ) quantify linear thermal bridging in watts per metre kelvin (W/mK). Typical values: wall-floor junction 0.05-0.30 W/mK, wall-roof junction 0.05-0.15 W/mK, lintel 0.10-0.30 W/mK. SAP (Standard Assessment Procedure) calculations include default y-value (heat loss factor) of 0.15 W/m²K for thermal bridging unless detailed calculations prove lower values. Reducing thermal bridging through careful detailing significantly improves actual thermal performance.
Impact: Poor thermal bridge detailing can increase whole-building heat loss by 30-50% compared to clear-field U-value calculations, significantly reducing actual energy performance and causing local cold spots prone to condensation.
Interstitial condensation occurs when warm moist air from inside the building penetrates the construction and reaches a cold layer where water vapour condenses. This moisture can damage insulation (reducing thermal performance), cause timber decay, and corrode metal components. Risk is highest in highly insulated walls with internal vapour sources (kitchens, bathrooms) and cold external climates. Condensation risk analysis (Glaser method per BS EN ISO 13788) should be performed for all insulated constructions.
Prevent interstitial condensation through: vapour control layers on warm side of insulation restricting moisture ingress, breathable external layers allowing outward drying, balanced construction with external resistance >> internal resistance, and adequate ventilation reducing internal humidity. Wall constructions should be assessed for both winter condensation risk (moisture moving outward) and summer drying potential (moisture moving inward in humid climates).
Scenario: Detached house, 150m² wall area, upgrading from uninsulated cavity (U=1.5) to insulated cavity (U=0.30).
Annual Heat Loss Reduction:
Cost Savings: At £0.07/kWh (gas) = £746/year, or £0.24/kWh (electric) = £2,558/year. Cavity insulation typical cost £600-1,200, payback 1-2 years for gas heating. Carbon savings: 2,131 kg CO₂/year (gas) or 2,877 kg CO₂/year (electric).
For new dwellings in England/Wales (Building Regulations 2021), maximum U-values are: external walls ≤0.18 W/m²K, roofs ≤0.15 W/m²K, ground floors ≤0.18 W/m²K, windows ≤1.40 W/m²K. Scotland and Northern Ireland have similar but slightly different requirements. Extensions and renovations have more relaxed standards - typically walls 0.28 W/m²K, roofs 0.16 W/m²K. Future Homes Standard (2025+) will tighten requirements further to walls 0.15, roofs 0.11 W/m²K. Always check current regulations with local building control before construction.
Calculate R-value for each layer: R = thickness(m) / conductivity λ(W/mK). Sum all layer R-values plus surface resistances (typically R_se=0.04 external, R_si=0.13 internal for walls). U-value = 1 / R_total. Example: 105mm brick (λ=0.77), 100mm PIR insulation (λ=0.022), 100mm block (λ=0.51), 12.5mm plasterboard (λ=0.25): R = 0.04 + 0.105/0.77 + 0.100/0.022 + 0.100/0.51 + 0.0125/0.25 + 0.13 = 5.13 m²K/W. U = 1/5.13 = 0.19 W/m²K. Use our calculator above for automatic calculations with material database.
Depends on insulation type and wall construction. For cavity wall to achieve U=0.18 W/m²K: 80-100mm PIR/PUR (λ=0.022), or 100-120mm mineral wool (λ=0.035). For loft insulation to achieve U=0.15 W/m²K: 270-300mm mineral wool between/over joists. For external wall insulation achieving U=0.18: 100-120mm EPS (λ=0.035), or 80-100mm PIR. Better insulation materials (lower λ) require less thickness. Always calculate precisely for your specific construction - thickness alone doesn't determine U-value, it's thickness divided by conductivity that matters.
R-value (thermal resistance) measures how well a material resists heat flow - higher R-value means better insulation. Units are m²K/W. U-value (thermal transmittance) measures rate of heat transfer through entire construction - lower U-value means better insulation. Units are W/m²K. They are reciprocals: U = 1/R. USA/Canada primarily use R-values, UK/Europe/Australia use U-values. 300mm mineral wool has R≈7.5 m²K/W, giving U≈0.13 W/m²K for a complete construction. Think: R for resistance (bigger=better), U for heat loss (smaller=better).
Yes, if you know the construction layers. For cavity walls, check if cavity is insulated - probe through air vent or remove electrical socket to inspect. Uninsulated cavity walls typically U=1.2-1.7 W/m²K, insulated cavity 0.35-0.55 W/m²K depending on insulation type and thickness. Solid brick walls: 220mm single brick U≈2.1, 330mm double brick U≈1.7 W/m²K. For precise assessment, professional thermal imaging or heat flux measurements can measure actual U-value in-situ. Government EPC assessments use standard construction assumptions from building age if actual details unknown.
Thermal bridges are breaks in insulation continuity creating preferential heat loss paths. Common examples: steel lintels over windows, wall ties in cavity construction, junctions between walls/floors/roofs, corners, penetrations. A wall with clear-field U=0.18 W/m²K may have effective U=0.25-0.30 W/m²K when thermal bridges included - 40-65% increase in heat loss. SAP calculations add y-value of 0.15 W/m²K for thermal bridging unless detailed calculations prove otherwise. Reduce thermal bridges through: continuous external insulation, thermally broken components, careful junction detailing. Thermal modeling software (THERM, PSI-Therm) quantifies thermal bridge effects.
Savings depend on current U-value, improved U-value, area, climate, and heating system. General rule: improving 100m² wall from U=1.5 to U=0.3 saves approximately 7,000-9,000 kWh/year (£500-2,200 depending on fuel). Loft insulation upgrade 100mm to 300mm saves 1,500-2,500 kWh/year (£100-600). Payback periods: loft insulation 2-5 years, cavity fill 1-3 years, external wall insulation 8-15 years, internal wall insulation 10-20 years. Also reduces carbon emissions 30-50% for heating. Government funding (ECO4, Green Homes Grant successors) can reduce upfront costs significantly.
Depends on construction type and climate. Warm-side vapour control layers prevent interstitial condensation in cold climates with internal moisture sources. Required for: timber frame walls (installed behind plasterboard), cold deck flat roofs (below insulation), some internal wall insulation systems. Not needed for: cavity wall insulation (cavity provides ventilated gap), warm deck roofs (insulation above deck), external wall insulation (breathable construction). Use condensation risk analysis (Glaser method BS EN ISO 13788) to determine requirement. Over-specification can trap moisture preventing drying - breathable construction often preferable to impermeable barriers in mild UK climate.
Official guidance on Part L (Conservation of Fuel and Power) including approved U-values, calculation methods, and compliance routes for new builds and renovations.
View Part L →Building Research Establishment resources on thermal bridging, U-value calculation conventions (BR 443), and condensation risk assessment methodologies.
Access BRE Resources →Practical guidance on improving home insulation, grants and funding schemes, and energy performance improvements for existing buildings.
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