Thermal bridging is one of those things that catches builders out long after the walls are up and the inspector has moved on. You can have perfectly specified insulation, tidy blockwork and decent airtightness, and still end up with a property that haemorrhages heat, shows condensation rings on the plasterboard and fails its SAP calculation. The problem is usually in the details, not the main fabric. Lintels, reveals, ground floor junctions, roof abutments, wall ties, padstones, even the sole plates of studwork can all act as shortcuts for heat to bypass your insulation and escape. That is what thermal bridging is, and under Part L of the Building Regulations it matters more than ever in 2026.
I have seen brand new extensions with patches of surface condensation appearing within the first winter. The homeowner thinks there is a damp problem. There is not. It is cold bridging, and it was designed in from the first brick course. Getting on top of this early, before the plasterer arrives, is the only sensible approach.
What thermal bridging actually means on site
A thermal bridge is any part of the building envelope where heat moves through the structure significantly faster than it does through the surrounding insulated wall, floor or roof. Materials with high thermal conductivity, typically steel, concrete and masonry, conduct heat readily. When any of these run continuously from the warm inside to the cold outside, without a break of low-conductivity material, you have a bridge.
There are two types worth knowing. Linear thermal bridges are the ones at junctions: where the ground floor meets the wall, where a flat roof abutment hits the external leaf, where a cavity closer runs around a window opening. Point thermal bridges are smaller but still count: wall ties, fixings and structural brackets that penetrate insulation layers. Both are quantified in the Psi (Ψ) values and Chi (χ) values used in your SAP or SBEM energy calculations. The lower those values, the better the detail.
The Building Regulations Approved Document L sets out the framework: the notional dwelling specification, the limiting U-values, and the requirement to account for thermal bridging in the overall heat loss calculation. Getting the details right is not optional; it is part of demonstrating compliance.
Where thermal bridges are most likely to occur on UK builds
The usual suspects on a typical domestic extension or new build are pretty consistent once you know what to look for.
Lintels over openings
A standard steel boot lintel spanning a cavity wall opening creates a cold bridge unless you specify a thermally broken version. The steel flange sits directly in the outer leaf and transfers cold straight through to the inner skin. Suppliers like Catnic and IG Lintels both produce thermally broken lintels with a polyamide break that drops the linear Ψ value substantially. On a house with ten or twelve windows and doors, that adds up in the SAP calculation. I always specify thermal lintels as standard now. The cost difference per opening is negligible compared to the compliance headache later.
Window and door reveals
Where the cavity closer meets the window frame is another common weak point. Proprietary insulated cavity closers from companies like Cavity Trays Ltd or Thermabate deal with this properly, providing a continuous insulated return around the reveal. Standard PVC-u cavity closers with no insulation core are fine structurally but do very little thermally. On a tight SAP, the difference between a good reveal detail and a bad one can push you over the compliance threshold.
Ground floor and wall junctions
The junction between the insulated ground floor and the cavity wall is one of the most commonly underspecified details on smaller residential jobs. Where the inner leaf blockwork bears directly onto the edge of the floor slab, or where a concrete oversite runs hard up against the cavity, heat can track through the masonry below the insulation layer. Returning the floor insulation up the face of the inner leaf, or using a thermally broken edge insulation strip, addresses this. The BRE’s Accredited Construction Details library has worked junction details that carry known Ψ values and are accepted by most BCOs without further justification.
Roof-to-wall junctions and eaves
On a flat roof extension, the junction between the roof build-up and the external wall is a thermal bridging hotspot. If you are building to a warm roof specification (and you should be), the insulation needs to be continuous from the wall insulation into the roof, with no masonry bridging that layer. I wrote more on getting the insulation position right in our piece on cold roof vs warm roof flat roof construction, but the short version is: keep the insulation layer unbroken and you greatly reduce the bridging risk at the abutment.
Pitched roof eaves on extensions need the same attention. Where the ceiling insulation in a loft ends, you need to maintain a minimum 50mm air gap for ventilation on a cold roof, but you also need to push that insulation right to the eaves edge. Proprietary eaves vents help with airflow without leaving an insulation gap that becomes a cold bridge.
Structural elements penetrating insulation
Padstones, steel beam bearing points, restraint straps and hurricane ties all create point bridges if they pass through or sit against the insulation layer. On steel frame extensions and garden rooms, this is particularly relevant. Every fixing bracket on an EWI system is a potential point bridge, which is why external wall insulation systems need specific fixing schedules that account for the number and type of fixings used.
How to specify details that satisfy Part L
The practical approach is to use the BRE’s Accredited Construction Details where you can. These are pre-assessed junction details with published Ψ values, available as a free download from the BRE website. If your construction details match what is shown, you plug those Ψ values into your SAP and the calculation works. Simple.
Where you are doing something non-standard, a timber frame wall tied into a masonry party wall for instance, or a structural insulated panel (SIP) system meeting a traditional blockwork gable, you will need bespoke Psi values calculated using thermal modelling software. This is a job for a building physics specialist, not your architect’s back-of-the-envelope estimate. The cost for junction modelling is usually a few hundred pounds per detail, and it is money well spent versus a failed SAP or a building control hold.
Airtightness ties in here too. Thermal bridging and air leakage often occur at the same junctions, and a draft at a window reveal or a roof abutment amplifies the heat loss beyond what the Ψ value alone suggests. If you are already detailing correctly for thermal bridging, you are halfway to a decent airtightness result at those points.
For anyone keeping tabs on the compliance paperwork side of things, it is worth getting your head around how building control actually assesses this. There is a reasonable amount of crossover between thermal performance evidence and other regulatory documentation, and checking in with a trusted what is AEO resource can help clarify where different compliance roles sit within the wider approvals process.
Materials that reduce thermal bridging by design
Some block and brick products help significantly. Thin-joint blockwork systems using Celcon or Ytong aerated blocks have inherently lower conductivity than dense aggregate blocks, which means even the uninsulated inner leaf contributes less to bridging. Insulated concrete formwork (ICF) systems have insulation integral to both faces, which largely eliminates the standard masonry bridging risks at the cost of a different set of details to get right.
For partition walls meeting external walls, choosing low-density blockwork or implementing a staggered thermal break at the junction reduces the cold bridge where the internal partition ties into the outer envelope. The choice between stud walls and blockwork partitions also affects this; a timber stud wall meeting an external wall has a lower conductivity at the junction than a dense block partition, which matters at exposed external corners especially.
Getting sign-off without hassle
Building control officers are looking for evidence, not promises. A SAP calculation that uses Ψ values from accredited details, a lintel specification sheet showing a thermally broken product, and a marked-up drawing showing the insulation continuity at each junction is usually enough. Some BCOs will ask for a pre-completion SAP on anything more than a modest extension. Get that prepared by a competent assessor before the inspection, not the morning of.
On site, photograph your insulation continuity at key junctions before covering them. A picture of the insulated cavity closer in the reveal, or the floor edge insulation returning up the inner leaf, takes ten seconds and gives you hard evidence if a question arises later. I have bailed a subbie out of a dispute with exactly that kind of photo more than once.
Thermal bridging is not complicated to deal with when you catch it at the specification stage. It is expensive, embarrassing and very difficult to fix once the plasterboard is on.