By Tom Handford MRICS
As building fabric standards improve, the way we think about heat loss, and about where surfaces run cold, is changing.
For many years, the focus of energy-efficient design has understandably been on improving the thermal performance of the main building elements: better insulated walls, roofs and floors, higher-performing windows, and improved airtightness.
But as these elements become more efficient, another part of the building fabric becomes increasingly significant: the junctions between them.
This is where thermal bridging comes into the picture.
What is a thermal bridge?
A thermal bridge occurs where heat can pass through part of the building envelope more readily than through the surrounding construction.
Typical examples include wall-to-floor junctions, roof-to-wall junctions, window and door reveals, balconies, parapets, structural penetrations and changes in construction.
While a U-value describes heat transfer through a building element, such as a wall or roof, linear thermal transmittance (ψ-value, W/mK) describes the additional heat flow associated with a junction.
The overall transmission heat loss of a building therefore depends not only on the areas and U-values of its elements, but also on the lengths and ψ-values of its junctions.
In simplified terms:
Fabric heat loss = planar heat loss + junction heat loss
H = Σ(U × A) + Σ(ψ × L) + Σχ
where U is thermal transmittance, A is element area, ψ is linear thermal transmittance, L is junction length and χ represents point thermal bridges.
This means that specifying a highly insulated wall does not, by itself, guarantee a thermally efficient envelope. How that wall connects to the floor, roof, windows and structure matters too.
Why is thermal bridging becoming more important?
There is a simple reason.
As we reduce heat loss through the main building elements, junction heat losses become proportionally more important.
Consider a wall whose U-value has been substantially improved through additional insulation. Heat flow through the centre of that wall may now be relatively small. But if the insulation layer is interrupted at a slab edge, balcony connection or structural element, the heat flow through that junction has not necessarily reduced at the same rate.
The better the surrounding fabric becomes, the harder it is to ignore poorly resolved junctions.
This is one reason thermal bridge design has long been an important part of the Passive House Institute Passivhaus methodology. The objective is not simply to specify low U-values, but to create a continuous thermal envelope, resolving junctions carefully enough that avoidable thermal bridges are designed out wherever practical.
And the consequences are not limited to energy consumption.
Poorly designed thermal bridges can create locally reduced internal surface temperatures. This can increase the risk of surface condensation and mould growth, making thermal bridge analysis relevant to building durability, moisture risk and occupant comfort, as well as energy performance.
Surface condensation risk and the temperature factor
Surface condensation and mould growth tend to receive most attention once they have already appeared in an occupied building. They are much less often verified at design stage, when they can still be designed out.
The check that does this is the temperature factor, fRsi. It expresses the lowest internal surface temperature at a junction relative to the internal and external air temperatures, and it is assessed against a critical value for the room type, typically 0.75 for habitable rooms, with higher values required for wetter environments such as kitchens and bathrooms.
Because fRsi is a measure of a single coldest point rather than an average, a junction can present a reasonable ψ-value and still fail the check in one localised position. A window sill, a reveal at a corner, or a point where insulation is pinched around a structural element will often be that position.
A junction can look acceptable overall and still fail the fRsi check in one specific spot.
This is why the same heat-flow model used to derive a ψ-value is worth interrogating for surface temperature as well. The useful output is not only whether a junction passes, but where along the junction the minimum occurs and which change to the detail actually lifts it: moving insulation, adjusting the window position within the thermal envelope, or reconsidering how the frame is supported.
Concern about condensation risk is common. A calculation that identifies where it will occur, and why, is what separates guessing at a fix from specifying the right one.
From Passivhaus principle to mainstream calculation
What makes this particularly relevant now is the direction of UK energy regulation.
Under SAP, thermal bridging already forms part of the dwelling heat-loss calculation. Linear thermal bridges can be accounted for using their lengths and ψ-values, while point thermal bridges can be represented through point thermal transmittance.
The Future Homes Standard is pushing fabric and building-services performance further, while the methodology used to assess homes is also evolving.
As of 2026, SAP 10.3 is the approved methodology for demonstrating compliance with the Future Homes Standard. The government's new Home Energy Model (HEM), intended ultimately to replace SAP, has been delayed and is expected to be introduced alongside SAP 10.3 before eventually becoming the primary methodology.
Importantly, thermal bridging is not disappearing within this transition.
Quite the opposite.
The government's HEM methodology explicitly incorporates thermal bridges into the heat balance of the dwelling. Junction heat losses are combined for each zone and can be accounted for within the model's half-hourly calculation timestep.
This reflects a broader direction of travel in building-performance modelling: moving towards a more detailed representation of how buildings actually exchange heat.
The value of calculating project-specific ψ-values
This also raises an important question for design teams.
Should we rely on generic junction assumptions, or calculate what has actually been designed?
Generic values have an important role in compliance methodologies, particularly during early design. But they cannot necessarily capture the performance of a particular construction detail.
A project-specific thermal bridge calculation allows the actual geometry, materials, insulation continuity and structural arrangement of a junction to be assessed.
Using two-dimensional or three-dimensional heat-flow modelling, it is possible to quantify the ψ-value of a junction and investigate its internal surface temperatures.
More importantly, the analysis can become part of the design process rather than simply a compliance calculation.
A slab edge can be adjusted.
Insulation can be repositioned.
A window can move within the thermal envelope.
A structural penetration can be reconsidered.
The model can then be recalculated to understand whether the proposed change actually improves performance.
That feedback loop is where thermal bridge modelling becomes particularly valuable.
Earlier analysis leads to better details
Thermal bridging is much easier to resolve on a drawing than it is on site.
If analysis only takes place once construction details are effectively fixed, there may be limited opportunity to improve a problematic junction without affecting structure, architecture, cost or programme.
Bringing thermal bridge analysis into the design process earlier allows architects, structural engineers, façade designers and energy consultants to work together around the same problem:
How do we maintain the thermal envelope through this junction?
For Passivhaus projects, this thinking is already fundamental.
But as regulatory calculations become more sophisticated and fabric performance continues to improve, the same approach is becoming increasingly relevant to mainstream construction.
Thermal bridging at Develeco
At Develeco, we support project teams with thermal bridge modelling and ψ-value calculations, alongside our wider building-performance and Passivhaus consultancy services. We are certified by Elmhurst on Psi-Value Competency (2D & 3D TRISCO Thermal Bridging).
Our approach is not simply to produce a ψ-value at the end of the design process. Where possible, thermal modelling should help inform the detail itself, identifying where heat is escaping, understanding surface-temperature risks and testing practical improvements with the wider design team.
Because ultimately, high-performance building fabric is not just about achieving a good U-value.
It is about making the thermal envelope work as a complete system.
