Psi values and thermal bridging explained

You can hit every U-value target in Building Regulations and still build a cold, damp building. That's because U-values only describe one-dimensional heat loss through the middle of a wall, roof, or floor — they say nothing about what happens at the junctions: corners, eaves, window reveals, lintels, and floor-to-wall edges.

Those junctions are thermal bridges. Heat takes a shortcut through the structure, cold spots appear on the inside surface, and condensation, mould, and compliance failures follow.

The measure for thermal bridging is the Psi value (Ψ, written "psi" and pronounced "sigh"). This guide covers what it is, how it differs from U-values, the current UK approach via SAP 2021 and the Accredited Construction Details, and what to do on real projects.

U-value vs Psi value: the one-sentence difference

• A U-value (W/m²K) describes heat loss per square metre of a building element in one dimension.
• A Psi value (W/mK) describes heat loss per linear metre of a junction in two dimensions.

Psi values are multiplied by the length of the junction, not its area. A 1m long corner with a Psi value of 0.08 W/mK loses 0.08 × 1 = 0.08 W per kelvin of temperature difference.

Both are additive in the overall building heat loss calculation. U-values give you the main fabric loss; Psi values give you the junction loss. Add them together and you have the total fabric heat loss.

What's a thermal bridge?

A thermal bridge is any part of the building envelope where heat flow is significantly higher than through the surrounding element. In practice that means anywhere:

• Insulation is interrupted or reduced (lintels, cavity trays, window frames in heavy masonry reveals)
• A structural element conducts heat between inside and outside (steel beams, concrete floor slabs continuing through the external wall)
• Geometry concentrates heat loss (external corners, eaves, parapet junctions)
• Fixings penetrate the insulation (repeating steel ties, structural fixings)

Even a perfectly insulated wall with Passivhaus-grade U-values can lose 20–30% of its notional heat budget through junctions if they're detailed badly. That's why regulators now require thermal bridging to be accounted for in compliance calculations.

Linear, point, and repeating thermal bridges

Three flavours, handled differently in the calculation:

1. Repeating thermal bridges — things like timber studs, metal rails, or mortar joints that occur systematically through the build-up. These are folded into the U-value calculation itself (via the combined method in ISO 6946) — see our U-value calculation guide.
2. Linear thermal bridges — junctions between elements (wall-floor, wall-roof, jamb, sill, lintel, corner). Captured by Psi values per metre of junction length.
3. Point thermal bridges — individual penetrations like wall ties or fixings. Captured by Chi values (Χ) in W/K, per fixing.

In normal compliance work on UK housing, you worry mostly about linear thermal bridges. Point bridges are usually corrected with a small ΔU adjustment to the U-value itself.

How Psi values are calculated

You can't hand-calculate a Psi value. It's the output of a 2D finite-element thermal analysis through a junction, which needs software like THERM, Physibel BISCO, or Honeybee/THERM.

The calculation logic:

1. Model the junction in 2D with every layer and material
2. Apply internal and external temperatures and surface resistances
3. Compute the total heat flow per metre of junction, Lψ (W/mK)
4. Subtract the one-dimensional heat loss through the adjoining U-value elements:

Ψ = L_junction – Σ(U_i × l_i)

Where U_i is the U-value of each adjoining element and l_i is the length from the junction centreline to the modelled boundary.

The result is the additional heat loss caused by the junction over and above the U-values of the elements either side. A good junction has Psi near zero; a bad one can be 0.5 W/mK or more.

UK regulatory context — SAP 2021 and Part L 2021

In England and Wales, the compliance calculation for new-build dwellings is done through SAP 2021 (Standard Assessment Procedure, the national energy calculation method). SAP calls for thermal bridging to be accounted for in one of three ways:

Option 1 — Accredited Construction Details (ACDs)

The simplest route. The UK government publishes a set of pre-calculated junction details with approved Psi values. If you build to those details, you use the published Psi values directly — no thermal modelling required. Each ACD covers a specific junction type: corner, eaves, sill, jamb, lintel, ground floor perimeter, and so on.

The catch: you have to build it exactly as drawn. Deviations void the approved Psi value and push you into Option 2 or 3.

Option 2 — Calculated Psi values

You commission a thermal modeller (or do it yourself with THERM) to compute Psi for every junction type in your building. Time-consuming but defensible and potentially better than ACD values if your details are well-designed.

Option 3 — Default y-value

If you do nothing, SAP applies a penalty y-value of 0.15 W/m²K to the entire building envelope. This is deliberately pessimistic — designed to make Option 1 or 2 worth the effort. A y-value of 0.15 can blow your compliance budget on its own.

The y-value is a per-m² multiplier applied to the whole fabric area. Think of it as a blanket thermal-bridging surcharge:

Heat loss from junctions = y × A_envelope × ΔT

Where y is derived from the sum of Psi × length, divided by the envelope area. A good building with proper detailing achieves y ≈ 0.04–0.08. A default (unassessed) building sits at y = 0.15.

Typical Psi values for common junctions

These are indicative figures from the ACDs for a well-detailed modern cavity-wall build-up. Real values depend on your specific construction.

Junction	Typical Ψ (W/mK)
External corner (insulated cavity)	0.04 – 0.09
Intermediate floor	0.00 – 0.06 (with a proper insulation upstand)
Eaves (cold roof, insulation at ceiling)	0.04 – 0.08
Eaves (warm roof, insulation at rafter)	0.05 – 0.09
Ground floor / wall junction	0.10 – 0.18 (depends on wall-foundation detail)
Window jamb (cavity closer, insulation to frame)	0.04 – 0.08
Window sill	0.04 – 0.10
Lintel (insulated steel or composite lintel)	0.06 – 0.15
Lintel (uninsulated concrete lintel)	0.30 – 0.65

The difference between a good and a bad lintel detail is nearly an order of magnitude. This is why modern cavity walls use insulated box lintels — anything uninsulated in the thickness of the wall is a thermal disaster zone.

The real-world impact: a quick worked example

Consider a two-storey, 90 m² new-build semi-detached house. Envelope area (walls + roof + floor): roughly 240 m². Total length of linear thermal bridges: around 180m (all junction types combined).

Scenario A — Assessed building with ACD details: average Ψ = 0.05 W/mK.

Junction heat loss = 0.05 × 180 = 9.0 W/K
Equivalent y-value = 9.0 / 240 = 0.0375 W/m²K

Scenario B — Default y = 0.15 (no assessment done):

Junction heat loss = 0.15 × 240 = 36.0 W/K

That's four times more heat loss through junctions in the unassessed case. Over a heating season that translates to roughly 150–250 kWh extra energy per year, which at current gas prices is £15–30 annually — not huge, but the compliance cost of being over the target on SAP is bigger: you either have to compensate with better insulation elsewhere, or you fail Part L.

Surface condensation risk — the hidden consequence

Thermal bridging doesn't just waste energy. It also creates cold internal surfaces at junctions, which become condensation and mould hotspots. The relevant regulatory metric is the temperature factor (f_Rsi), which must exceed 0.75 for all junctions in dwellings (BRE IP 1/06) to avoid surface condensation at typical internal humidity.

A temperature factor of 0.75 means the internal surface temperature stays above 75% of the way between external and internal temperatures. Cold corners sit below 0.75; mould appears within a few winters.

Our condensation risk guide covers this in detail, including interstitial condensation which is the hidden twin problem.

Common design mistakes

Things I see routinely on drawings that compromise thermal bridging performance:

1. Insulated cavity not continuous at window reveals. The reveal is left as dense masonry — that's a textbook cold bridge straight to the window frame.
2. Insulation stopping short of the damp-proof course. Leaves a cold strip at floor perimeter.
3. Uninsulated concrete lintels. Always specify an insulated lintel. The cost difference is negligible compared to the Psi value impact.
4. Balcony cantilevers in concrete. A concrete slab continuing through the external wall is one of the worst thermal bridges possible. Use a thermal break (Schöck Isokorb or equivalent) or change the detail.
5. Steel beam noggins or purlins in external walls. Steel's conductivity is 50× timber. A single continuous steel element through the insulation zone ruins the calculation.
6. "Optimistic" Psi assumptions. Using ACD values when the actual detail drifts from the ACD drawing. Site detailing matters.

What's new under Part L 2021 and SAP 2021

Part L 2021 didn't introduce new Psi values but did tighten the compliance route. Under the 2021 regs:

• Default y-value remains 0.15 W/m²K — increasingly painful to accept given tightening U-value targets
• Notional y-value (in the notional dwelling used to set the target) is 0.05 for new dwellings — so you basically need to assess junctions to hit compliance
• Accredited Construction Details were re-published in 2023 with updated Psi values aligned to modern detailing
• Junctions must be drawn on Building Control submissions, with Psi values stated

In Scotland, Section 6 requires either ACDs or calculated Psi values with no default route — you can't just accept the penalty y-value.

In Northern Ireland, Technical Booklet F follows the same pattern as Part L 2021 with its own minor variations on target U-values.

How to get Psi values without hiring a thermal modeller

For a standard housing build with typical junctions:

1. Use the ACDs. Build to the drawings, use the published values. Free, defensible, well-supported by Building Control.
2. Use manufacturer-calculated bundles. Many insulation manufacturers (Kingspan, Celotex, Rockwool) publish calculated Psi values for junctions using their specific products. Free with the product.
3. Use a design tool. Some SAP software packages include a junctions library and can compute y-values automatically.

For bespoke or high-performance projects (Passivhaus, complex geometries, unusual structural detailing):

1. Commission a thermal modeller. Expect £800–2,500 for a full junction set for one project.
2. Do it yourself with THERM or Physibel. Steep learning curve but essentially free once you know it.

Frequently asked questions

Do I have to do Psi values for an extension? Yes, if the extension is over 50 m² or involves new thermal elements. Smaller extensions may fall under the elemental compliance route where default y-values are acceptable, but you'll pay for it in tighter U-value targets elsewhere.

Are ACDs still current in 2026? Yes. The ACDs were last republished in late 2023. Check https://www.gov.uk/government/publications/accredited-construction-details-acds for the current version. Use the most recent edition for new compliance submissions.

What's the difference between Psi value and Chi value? Psi (Ψ) is per metre of junction (linear bridge). Chi (Χ) is per fixing (point bridge). Chi values are rarely used in UK housing compliance — point bridges are usually absorbed into the U-value via the fixing correction in ISO 6946.

Does our AI U-value calculator handle Psi values? The calculator handles U-values and the repeating thermal bridges inside them (timber studs, metal rails). Linear Psi values at junctions are a separate calculation and require 2D modelling — we don't attempt to compute those automatically. We do flag when a construction is likely to have problematic junctions based on the build-up.

Next steps

• What is a U-value? — start with the basics
• How to calculate U-values — the ISO 6946 method
• UK and Scottish U-value regulations — the targets you're hitting
• Condensation risk in construction — the hidden cost of thermal bridging

Need a U-value for a build-up? Upload a drawing to the AI calculator and get a compliant answer in seconds.