What is Thermal Bridging?

Heat doesn't just escape through walls and roofs — it leaks through the junctions between them. Here's why thermal bridges matter, which junctions cost you most, and why Ψ-values can make or break a SAP pass.

Thermal bridges: the weak points in the fabric

A thermal bridge is a localised path through the building envelope where heat escapes more readily than through the surrounding fabric — typically at junctions: wall-to-floor, wall-to-roof, around windows and doors, at corners. Even with superb u-values everywhere else, poorly detailed junctions can account for 20–30% of a dwelling’s fabric heat loss.

There are two kinds. Geometric bridges — like an external corner, which simply has more exposed surface area than the flat wall. And constructional bridges — where a high-conductivity material (a concrete slab edge, a steel lintel, an uninsulated cavity closer) pierces the insulation line and carries heat straight through.

Section through a wall-to-floor junction showing the perimeter heat-loss path around the slab edge
Wall-to-floor: heat escapes around the slab edge.
Section through an eaves junction where the wall meets the roof, showing the thermal bridge
Eaves: where the wall meets the roof.
Plan through a window jamb junction showing heat loss around the window reveal
Window jamb: around the opening.
Plan through an external corner junction showing the thermal bridge at the corner
External corner: heat loss at the corner.

Ψ-values (psi values)

Each junction type has a linear thermal transmittance, or Ψ-value, measured in W/m·K — the heat loss per metre of junction per degree of temperature difference. In SAP and HEM, every junction length is multiplied by its Ψ-value and summed into the heat-loss calculation.

A typical well-detailed junction sits around 0.02–0.08 W/m·K; a poor one can be 0.15–0.30. Because the lengths run for metres (a wall-to-floor junction runs the whole perimeter), small differences multiply quickly — which is why a SAP calculation with default Ψ-values can need noticeably more insulation or PV than one with calculated values.

The maths in SAP (why a few hundredths matter)

In SAP, junction heat loss is simply length × Ψ-value. A detached house has roughly 50 m of wall-to-floor junction around its perimeter. At a calculated 0.05 W/m·K that’s 50 × 0.05 = 2.5 W/K. With the punitive SAP default for that junction it’s more like 50 × 0.15 = 7.5 W/K — a 5 W/K difference that must be made up in better walls, more insulation or bigger PV. Across all your junctions, that gap is often the difference between a comfortable pass and a design rework. It’s also why “measure the junction lengths, not just the wall area” is the first thing an assessor checks.

What a good detail looks like

The golden rules for every junction: keep the insulation line continuous, avoid bridging it with high-conductivity material, and close cavities with insulated closers. Concretely:

Each junction type has a page showing how to model it in ΨMonkey — from wall-to-floor and window jambs to balconies and exposed floors.

Which junctions cost you most

SAP / Approved Document L labels each junction with an “E-number” reference, and they’re all modelled in our free ΨMonkey psi calculator:

Your three options in SAP

  1. Default values — punitive, and they make passing harder and more expensive.
  2. Accredited/standard details — better, if your details exactly match the published ones (more on ACDs). Note that SAP 10 removed Accredited Construction Details over accuracy concerns, leaving manufacturer schemes or calculated values.
  3. Calculated Ψ-values — modelled to BS EN ISO 10211. Almost always the best result, often the cheapest route to compliance.

Calculate your own Ψ-values

Our free in-browser tool, ΨMonkey, models junctions with a finite-difference solver to BS EN ISO 10211 and gives you the Ψ-value and f_Rsi temperature factor. Use it to test your details — PDF reports for Building Control are £25 a junction. The process:

  1. Model the junction — paint the build-up (walls, floors, roof, insulation lines, cavities, DPC) on the grid.
  2. Solve — ΨMonkey runs the 2-D heat-flow calculation and returns the Ψ-value and the coldest-surface temperature factor.
  3. Iterate — move the insulation line, change the closer, compare details until the number is good.
  4. Export — the branded PDF carries the parameters, the result and a unique Report ID that Building Control can verify.
  5. Hand to your SAP assessor — calculated values slot straight into the SAP calculation as an alternative to defaults.

Condensation risk: f_Rsi

Thermal bridges aren’t just about heat loss. A cold internal surface at a junction invites condensation and mould. The temperature factor f_Rsi (0 to 1) measures how cold the worst internal surface gets; Building Regulations require f_Rsi ≥ 0.75 for dwellings. ΨMonkey reports it automatically — and the same junction that leaks heat is usually the one that risks mould, so fixing the detail fixes both.

The bottom line

Get the U-values right, then get the junctions right — calculated Ψ-values are frequently the cheapest single route to a SAP 10 pass, and they de-risk your compliance pack. Model yours free with ΨMonkey — no subscription, no sign-up.

Got a project? Let’s take a look.

Hundreds of clients a year trust us with theirs — big, small, and gloriously complicated. Tell us what you’re up against and we’ll tell you how we’d sort it.

Get a Quote or call 01202 623236