U-Value Calculator 2026 | Thermal Transmittance for Walls, Floors & Roofs

U-Value Calculator 2026

Calculate Thermal Transmittance for Walls, Floors & Roofs

Building Regulations Part L Compliance Tool

U-values measure thermal transmittance – the rate of heat loss through building elements such as walls, floors, roofs, windows, and doors. Expressed in W/m²K (Watts per square meter per Kelvin), lower U-values indicate better insulation and reduced heat loss. Our U-value calculator helps determine thermal performance to meet Building Regulations Part L 2021 (with 2023 amendments) requirements for energy efficiency in 2026.

Calculating accurate U-values is essential for SAP assessments, planning applications, building control approval, and achieving net-zero targets. This calculator uses BR 443 conventions and industry-standard methods to evaluate wall, floor, and roof constructions with various insulation materials.

🌡️ U-Value Calculator

Calculate thermal transmittance for building elements

Element Type

Select Building Element

Construction Layers

Enter layers from outside to inside

Outer Layer Material

Insulation Material

Insulation Thickness (mm)

Inner Layer Material

Cavity Width (mm) - if applicable

Target U-value (W/m²K) - Optional

Building Regulations Part L U-Value Standards 2026

Building Regulations Approved Document L (Conservation of fuel and power), updated in 2021 with 2023 amendments, sets maximum U-value limits for new and existing buildings. Our U-value calculator helps verify compliance with these stringent thermal performance requirements.

Maximum U-Values for New Dwellings (2026)

Building Element	Maximum U-Value (W/m²K)	Common Insulation	Typical Thickness
External Walls	0.18	PIR/PUR board in cavity	100-125mm
Party Walls (between dwellings)	0.20	Mineral wool cavity fill	Full cavity fill
Ground Floors	0.13	PIR under slab	100-150mm
Exposed Floors (over unheated space)	0.13	PIR or mineral wool	150-200mm
Pitched Roof - Insulation at ceiling	0.15	Mineral wool between/over joists	270-300mm
Pitched Roof - Insulation at rafter	0.15	PIR between rafters	140-170mm
Flat Roof	0.15	PIR warm deck	150-180mm
Windows & Roof Windows	1.40	Double/triple glazing	Argon-filled units
Doors (with >60% glazing)	1.40	Glazed door unit	Double glazed
Doors (with ≤60% glazing)	1.00	Insulated door core	44-54mm composite
Rooflights	1.70	Double glazed units	Low-E coating

External Walls

Max U-Value 0.18 W/m²K

Insulation PIR 100-125mm

Ground Floors

Max U-Value 0.13 W/m²K

Insulation PIR 100-150mm

Pitched Roof (Ceiling)

Max U-Value 0.15 W/m²K

Insulation Mineral wool 270-300mm

Windows

Max U-Value 1.40 W/m²K

Glazing Double/triple glazed

Existing Dwellings - Renovation & Extensions

Building Element	Renovation/Replacement	Extensions/New Elements	Upgrade Trigger
External Walls	0.30 W/m²K	0.18 W/m²K	When >25% renovated
Ground Floors	0.25 W/m²K	0.13 W/m²K	When floor replaced
Roofs	0.16 W/m²K	0.15 W/m²K	When roof covering replaced
Windows	1.60 W/m²K	1.40 W/m²K	When windows replaced
Doors	1.40 W/m²K	1.00 W/m²K	When doors replaced

External Walls (Existing)

Renovation 0.30 W/m²K

New Extension 0.18 W/m²K

Roofs (Existing)

Renovation 0.16 W/m²K

New Extension 0.15 W/m²K

Understanding U-Values and R-Values

U-values and R-values are inversely related measures of thermal performance. While U-values measure heat transmission (lower is better), R-values measure thermal resistance (higher is better). The relationship is expressed as: U-value = 1 / Total R-value

U-Value (Thermal Transmittance)

Definition: Rate of heat transfer through 1m² with 1°C temperature difference

Units: W/m²K (Watts per square meter per Kelvin)

Interpretation: Lower values = better insulation

Example: U = 0.18 W/m²K means 0.18 Watts heat loss per m² per °C difference

Usage: Building Regulations specify maximum U-values

R-Value (Thermal Resistance)

Definition: Ability of material to resist heat flow

Units: m²K/W (square meters Kelvin per Watt)

Interpretation: Higher values = better insulation

Calculation: R = thickness (m) / thermal conductivity (λ)

Additive: Total R = sum of all layer R-values plus surface resistances

Thermal Conductivity (Lambda λ)

Definition: Material property - rate heat passes through homogeneous material

Units: W/mK (Watts per meter per Kelvin)

Interpretation: Lower values = better insulating material

Examples: PIR λ=0.022; Mineral wool λ=0.035; Brick λ=0.77

Fixed: Material property independent of thickness

Surface Resistances

External Surface (Rse): 0.04 m²K/W (standard walls/roofs)

Internal Surface (Rsi): 0.13 m²K/W (walls/roofs)

Floor Surface: Different values for ground contact

Important: Always include in U-value calculations

Standard: Values from BS EN ISO 6946

Cavity Resistance

Unventilated Cavity: 0.18 m²K/W (typical 50mm air gap)

Width Effect: Resistance increases with width up to ~25mm, then plateaus

Ventilated Cavity: Reduced resistance; treated as external surface

Partially Filled: Residual cavity provides additional R-value

Fully Filled: Cavity completely replaced by insulation R-value

Thermal Bridging

Definition: Areas where insulation continuity is broken (studs, lintels, etc.)

Effect: Increases actual U-value above calculated value

PSI Value (Ψ): Linear thermal transmittance W/mK

Accredited Details: Use approved construction details to minimize

Impact: Can increase U-value by 10-50% if not addressed

Insulation Materials and Thermal Conductivity

Different insulation materials have varying thermal conductivities (λ values). Our U-value calculator includes common insulation types used in UK construction. Lower λ values require less thickness to achieve target U-values.

Common Insulation Materials (2026)

Insulation Type	λ Value (W/mK)	For U=0.18 Wall	For U=0.15 Roof	Cost/Performance
Phenolic Foam Board	0.020	90-100mm	120-140mm	Premium/Thinnest
PIR/PUR (Polyisocyanurate)	0.022-0.023	100-115mm	140-160mm	Mid-High/Popular
Spray Polyurethane Foam	0.026	115-125mm	150-170mm	High/Specialist
Extruded Polystyrene (XPS)	0.032-0.035	140-155mm	185-210mm	Mid/Good moisture resistance
Mineral Wool (Glass/Stone)	0.035-0.040	150-170mm	250-300mm	Low-Mid/Fire resistant
Expanded Polystyrene (EPS)	0.038	160-175mm	220-250mm	Low/Budget option
Cellulose Fibre (Recycled)	0.038-0.040	165-180mm	240-280mm	Low-Mid/Eco-friendly
Sheep's Wool	0.038	165-180mm	240-270mm	High/Natural & sustainable
Wood Fibre Board	0.038-0.050	170-200mm	250-300mm	Mid-High/Breathable
Hemp Insulation	0.040	170-185mm	250-280mm	Mid-High/Natural

PIR/PUR Board

λ Value 0.022 W/mK

Wall (U=0.18) 100-115mm

Cost Mid-High/Popular

Mineral Wool

λ Value 0.035-0.040 W/mK

Roof (U=0.15) 250-300mm

Cost Low-Mid/Fire resistant

Phenolic Foam

λ Value 0.020 W/mK

Thickness Thinnest option

Cost Premium

Typical Wall Construction U-Values

Standard UK wall constructions achieve different U-values depending on insulation type and thickness. Use our U-value calculator to model specific build-ups and optimize insulation specifications.

Full-Fill Cavity Wall (Traditional)

Build-up: 105mm brick + 100mm cavity fully filled mineral wool + 100mm block + 13mm plaster

U-Value: Approximately 0.30 W/m²K

Compliance: Suitable for renovations; not new builds

Advantages: Simple retrofit; good fire performance

Disadvantages: Higher heat loss than current standards

Partial-Fill Cavity Wall (PIR)

Build-up: 105mm brick + 100mm PIR + 50mm residual cavity + 100mm block + 13mm plaster

U-Value: Approximately 0.18 W/m²K

Compliance: Meets Part L 2021 for new dwellings

Advantages: Residual cavity prevents moisture bridging

Disadvantages: Requires wider cavity (minimum 150mm total)

Enhanced Cavity Wall (Hybrid)

Build-up: 105mm brick + 125mm PIR + 25mm cavity + 100mm insulated block + plaster

U-Value: Approximately 0.15 W/m²K

Compliance: Exceeds current standards; future-proof

Advantages: Excellent thermal performance

Disadvantages: Wider wall construction; higher cost

External Wall Insulation (EWI)

Build-up: Existing wall + 100-150mm EPS/mineral wool + render system

U-Value: 0.18-0.25 W/m²K (depends on existing wall)

Compliance: Ideal for solid wall retrofits

Advantages: No internal space loss; continuous insulation

Disadvantages: Changes external appearance; planning considerations

Internal Wall Insulation (IWI)

Build-up: Existing wall + 60-100mm PIR + 12.5mm plasterboard

U-Value: 0.25-0.35 W/m²K (depends on existing wall)

Compliance: Retrofit option where EWI not possible

Advantages: No external changes; room-by-room upgrade

Disadvantages: Reduces room size; thermal bridging risks

Timber Frame Wall

Build-up: Brick + breather membrane + 140mm timber frame with mineral wool + VCL + plasterboard

U-Value: Approximately 0.18-0.20 W/m²K

Compliance: Meets Part L with careful detailing

Advantages: Lightweight; fast construction

Disadvantages: Requires vapor control; thermal bridging at studs

Floor and Roof U-Value Constructions

Floors and roofs require different approaches to insulation. Ground floors benefit from earth's thermal mass, while roofs face the greatest temperature differentials and typically require the highest insulation levels.

✅ Typical Floor Constructions:

Ground Floor (Solid): 100mm PIR under 150mm concrete slab achieves U=0.13 W/m²K
Ground Floor (Beam & Block): 150mm PIR between beams + 100mm screed achieves U=0.12 W/m²K
Suspended Timber Floor: 200mm mineral wool between joists achieves U=0.15 W/m²K
Floor over Unheated Space: 150-200mm PIR required for U=0.13 W/m²K
Underfloor Heating: Enhanced insulation (U=0.10-0.12) improves efficiency

✅ Typical Roof Constructions:

Pitched Roof (Ceiling Level): 270-300mm mineral wool achieves U=0.15 W/m²K - most economical
Pitched Roof (Rafter Level): 140-170mm PIR between/under rafters achieves U=0.15 W/m²K
Flat Roof (Warm Deck): 150-180mm PIR above deck achieves U=0.15 W/m²K
Flat Roof (Inverted): 180-200mm XPS above waterproofing achieves U=0.15 W/m²K
Green Roof: Enhanced insulation (160-200mm PIR) compensates for substrate weight

SAP Calculations and U-Values

The Standard Assessment Procedure (SAP) is the UK government's methodology for assessing dwelling energy performance. Accurate U-values are essential inputs for SAP calculations, which determine Energy Performance Certificate (EPC) ratings and Building Regulations compliance.

SAP Input Requirements

Element U-Values: All walls, floors, roofs, windows, doors must be specified

Thermal Bridging: Y-value (whole building) or PSI values for junctions

Area Calculations: Accurate measurements of each element type

Improvement: Lower U-values directly improve SAP score and EPC rating

Default vs Calculated U-Values

Default U-Values: Conservative assumptions if calculations not provided (typically worse)

Calculated U-Values: Based on actual construction specification

As-Built U-Values: Verified through site testing or approved construction details

Recommendation: Always provide calculated U-values for better SAP scores

Thermal Bridging in SAP

Y-Value: Average heat loss per m² of envelope area (W/m²K)

Accredited Details: Y=0.08 when using approved construction details

Default Y-Value: 0.15 W/m²K if details not accredited (penalty)

PSI Values: Junction-specific linear thermal transmittance (advanced)

Target Fabric Energy Efficiency (TFEE)

Definition: Maximum energy demand for space heating per m² per year

Compliance: Must not exceed TFEE for dwelling type and size

Calculation: Determined by U-values, thermal bridging, and air tightness

Strategy: Lower U-values reduce TFEE, allowing flexibility elsewhere

Windows and Doors U-Value Performance

Glazed elements typically have significantly higher U-values than solid construction. Window and door U-values depend on frame material, glazing specification, and overall size. Use whole-window U-value calculations per BS EN ISO 10077.

Window U-Value Performance

Window Type	Typical U-Value	Glazing Specification	Frame Material
Single Glazed (Historic)	4.8-5.7 W/m²K	4mm glass	Timber or metal
Double Glazed (Air-filled)	2.7-3.0 W/m²K	4-16-4mm air gap	Aluminum or basic uPVC
Double Glazed (Argon, Standard)	1.6-2.0 W/m²K	4-16-4mm argon-filled	uPVC or timber
Double Glazed (Low-E, Argon)	1.2-1.4 W/m²K	4-16-4mm argon + Low-E coating	uPVC with thermal breaks
Triple Glazed (Standard)	1.0-1.2 W/m²K	4-12-4-12-4mm argon	uPVC or timber
Triple Glazed (Low-E, Argon)	0.8-1.0 W/m²K	4-14-4-14-4mm argon + Low-E	uPVC or composite
Passive House Standard	0.6-0.8 W/m²K	Triple glazing + warm edge spacers	Multi-chamber uPVC or timber-alu

Double Glazed (Low-E)

U-Value 1.2-1.4 W/m²K

Glazing Argon + Low-E

Compliance ✅ Meets Part L

Triple Glazed

U-Value 0.8-1.0 W/m²K

Glazing Triple + Low-E

Performance Premium efficiency

U-Value Testing and Verification

While calculated U-values are standard for design and compliance, in-situ testing can verify actual thermal performance. This is particularly important for innovative constructions, retrofits, or where building fabric quality is questioned.

⚠️ In-Situ U-Value Testing:

Method: Heat flux sensors measure actual heat flow through element over 2+ weeks
Standard: ISO 9869 provides methodology for in-situ measurement
Conditions: Requires stable temperature difference (≥10°C) and multiple test locations
Accuracy: ±20% typical; affected by weather, thermal mass, and measurement duration
Applications: Retrofit verification, Listed buildings, defect investigation
Cost: £500-1500 per element depending on duration and reporting requirements
When Required: Sometimes specified by Building Control for unusual constructions

U-Value Calculator FAQs

What is a good U-value for walls in 2026?

For new dwellings in 2026, the maximum U-value for external walls is 0.18 W/m²K under Building Regulations Part L 2021 (with 2023 amendments). For existing dwellings undergoing renovation, 0.30 W/m²K is acceptable, but new extensions must meet the 0.18 standard. Lower U-values (0.15 or below) provide better energy efficiency and future-proof performance.

How do I calculate the U-value of my wall?

To calculate a U-value: (1) List all layers from outside to inside; (2) Find thermal conductivity (λ) for each material; (3) Calculate R-value for each layer (thickness in meters ÷ λ); (4) Add external surface resistance (0.04) and internal surface resistance (0.13); (5) Sum all R-values; (6) U-value = 1 ÷ total R-value. Use our calculator above for quick results.

What insulation thickness do I need for U-value 0.18?

For a typical cavity wall to achieve U=0.18 W/m²K: PIR insulation requires 100-125mm; mineral wool requires 150-170mm; EPS requires 160-180mm. The exact thickness depends on the complete wall construction including brick, blocks, cavity, and finishes. Thinner high-performance insulation like phenolic foam (90-100mm) can achieve the same U-value.

Can I use different U-values for different walls?

Yes, Building Regulations allow different U-values for different elements, provided the overall building performance meets requirements through SAP or SBEM calculations. For example, you might use U=0.15 on north-facing walls and U=0.18 on south-facing walls. However, each element must not exceed its specified maximum U-value for that element type.

What's the difference between U-value and R-value?

U-values measure heat transmission (W/m²K) – how easily heat passes through – where lower is better. R-values measure thermal resistance (m²K/W) – how well something resists heat flow – where higher is better. They are mathematical inverses: U-value = 1 / Total R-value. R-values are additive (you sum layers), while U-values are not.

Do windows need the same U-value as walls?

No, windows have separate U-value requirements because they cannot achieve the same performance as solid walls. For new dwellings in 2026, windows must not exceed U=1.40 W/m²K, while walls must not exceed U=0.18 W/m²K. Triple-glazed windows can achieve U=0.8-1.0 W/m²K, significantly better than the minimum standard but still higher than walls.

How does thermal bridging affect U-values?

Thermal bridges (e.g., wall ties, lintels, mortar joints, timber studs) create paths for heat to bypass insulation, increasing the actual U-value above the calculated clear-field value. In SAP calculations, this is accounted for through Y-values (whole building average) or PSI values (junction-specific). Using accredited construction details minimizes thermal bridging penalties.

Can I improve the U-value of an existing wall?

Yes, existing walls can be upgraded through: (1) External Wall Insulation (EWI) – adding insulation and render externally; (2) Internal Wall Insulation (IWI) – adding insulation and plasterboard internally; (3) Cavity wall insulation – injecting insulation into unfilled cavities. EWI typically achieves better performance (U=0.18-0.25) as it minimizes thermal bridging and protects the wall structure.

What U-value do I need for a conservatory roof?

If the conservatory is separated from the dwelling by external-quality walls, windows and doors, the conservatory roof doesn't need to meet specific U-values. However, if it's thermally integrated (no separating elements), the roof must meet Building Regulations: U=0.15 W/m²K for new installations or U=0.16 W/m²K for replacements. Solid insulated roofs are increasingly popular for comfort.

How much does better U-value save on energy bills?

Improving wall U-value from 0.30 to 0.18 W/m²K reduces heat loss through walls by 40%. For a typical semi-detached house with 100m² wall area and 20°C temperature difference, this saves approximately 480 kWh/year (£145-240 annually at 2026 energy prices). Whole-house improvements (walls, roof, floor, windows) typically save 30-50% on heating costs.