What is a thermal bridge?

A thermal bridge (cold bridge) is a localised area of the building envelope where heat flows at a higher rate than the surrounding construction. Common examples include steel lintels, window reveals, balcony connections, and wall-to-floor junctions. Thermal bridges can account for 20-30% of total fabric heat loss in well-insulated buildings.

A psi-value (ψ-value) measures the additional heat loss per metre length of a linear thermal bridge, expressed in W/mK. A lower psi-value means less heat loss. Typical values range from 0.04 W/mK for a well-detailed window sill to 0.50 W/mK for an uninsulated balcony connection. Psi-values are determined by thermal modelling software or from published tables like BRE IP1/06.

What is the y-factor in SAP?

The y-factor represents the overall thermal bridging in a building as a proportion of the total envelope area. The SAP default y-factor is 0.15 W/m²K. If you can demonstrate better detailing through calculated psi-values (Accredited Construction Details or bespoke modelling), you can achieve a lower y-factor, improving the SAP rating. Values of 0.04-0.08 are achievable with good detailing.

How do I reduce thermal bridging?

Key strategies include: using insulated lintels instead of bare steel, insulating window reveals, using thermal breaks at balcony connections, ensuring continuous insulation at junctions, using Accredited Construction Details (ACDs), and avoiding metal fixings through insulation. Passivhaus construction aims for virtually zero thermal bridging through careful detailing.

What are Accredited Construction Details?

ACDs are a set of standardised junction details published by the UK government that provide pre-calculated psi-values. Using ACDs allows builders to claim a y-factor of 0.05 in SAP without bespoke thermal modelling. The details cover common junctions in masonry, timber frame, and steel frame construction. They're free to download from the Planning Portal.

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Thermal Bridge Calculator — Psi Values & y-Factor

Calculate thermal bridging heat loss from junction psi-values. Compare calculated y-factor against SAP default for accurate energy assessments.

Total Envelope Area

For y-factor calculation

Total Floor Area

For context

Add Junctions

Junction Type

ψ = 0.16 W/mK

Junction Length

How We Calculate This

This calculator sums the thermal bridging heat loss from individual building junctions using their psi-values and lengths, then calculates the y-factor for SAP comparison.

The formula

Junction loss (W/K) = Σ(Psi-value × Junction length)

Heat loss (W) = Total junction loss × ΔT

y-factor = Total junction loss ÷ Envelope area

For example, 40m wall-floor junction (ψ = 0.16) + 30m eaves (ψ = 0.06) + 20m window jambs (ψ = 0.05): Total = 40 × 0.16 + 30 × 0.06 + 20 × 0.05 = 9.2 W/K. At ΔT = 33K: heat loss = 303.6 W.

Frequently Asked Questions

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Last updated: March 2026

All calculations are estimates. Verify with your supplier.