Scale model of a steel-framed house corner cut away to show heat escaping through the studs, slab edge and window frame.

Thermal bridging in buildings: why it keeps failing compliance and what designers need to know

In short

  • Thermal bridging is heat flowing through framing, fixings, slab edges and window frames instead of through the insulation beside them.
  • For houses, the Deemed-to-Satisfy thermal bridging rules sit in Part 13.2 of the ABCB Housing Provisions. They apply to repeating metal framing in roofs, walls and floors, not to timber framing.
  • The ABCB says the rules aim to bring metal-framed elements to at least 90 to 95% of the performance of their timber-framed equivalents.
  • NCC 2025 did not change the thermal bridging clauses. They read the same under NCC 2022 and NCC 2025.
  • Most failures come from documentation: a batt R-Value on the drawings, no junction details, and an assessor left to guess.

Correct as at September 2026.

A wall can be full of R2.5 batts and still lose heat like a sieve. The batts are fine. The heat is simply taking the easier route, straight through the studs, the top plate, the lintel and the window frame.

That is thermal bridging, and it is why a house that rated well on paper can feel cold, cost more to heat and grow mould on the plasterboard at the corners. It keeps failing compliance for a simple reason: documentation treats the insulation R-Value as the answer, when the NCC asks for the performance of the whole assembly.

What thermal bridging is

The ABCB describes thermal bridging, in practical NCC terms, as an unintended path of heat flow between the outside and inside of a building. It happens where insulation is interrupted, or where a highly conductive material such as metal passes through the insulated layer.

The ABCB also names the two costs. Bridges bypass the insulation, so rooms lose heat in winter and gain it in summer, and energy use goes up. And an unaddressed bridge can become a cold spot where warm, moist air condenses, which leads to mould and durability problems.

Metal framing is the main target of the code because steel conducts heat so well. Timber, concrete and masonry bridge too, but the house Deemed-to-Satisfy provisions do not ask you to calculate them.

What the NCC actually requires

The thermal bridging Deemed-to-Satisfy provisions for Class 1 buildings (and Class 10a buildings with a conditioned space) sit in Part 13.2 of the ABCB Housing Provisions:

  • Roofs and ceilings, 13.2.3(3). For a metal-framed pitched roof with a flat ceiling, you can meet a minimum Total R-Value, add R0.5 to the ceiling insulation, add a continuous layer of at least R0.13 above or below the joists, or stack two layers so the top one (at least R0.5) covers the joists or bottom chords.
  • Walls, 13.2.5(4). For metal-framed walls, meet the minimum Total R-Value tables calculated to AS/NZS 4859.2, or pick an option from the mitigation tables, such as a reflective airspace of at least 20 mm outside the frame, extra insulation between the studs, or continuous insulation of at least R0.30.
  • Floors, 13.2.6(3). Metal-framed suspended floors have their own Total R-Value table and mitigation options.

There are also two thermal break rules. Clause 13.2.3(7) requires a thermal break of at least R0.2 between metal roof sheeting and metal purlins, rafters or battens where there is no ceiling lining, or the lining is fixed straight to that metal. Clause 13.2.5(5) requires an R0.2 thermal break at every point where lightweight cladding meets a metal wall frame, where the wall has no lining or the lining is fixed directly to the frame. The explanatory notes say 12 mm expanded polystyrene strips or 20 mm timber are deemed to achieve R0.2.

Watch the condensation link. Continuous insulation added outside the frame to deal with bridging can also be a control layer under the condensation rules in Part 10.8. Many foils and rigid boards have low vapour permeance, so check the product against the permeance limit for your climate zone before you specify it.

For Class 2 apartments, the equivalent requirements are in NCC Volume One, and the ABCB notes that Total R-Value calculations for apartment external walls must account for bridging whatever the frame material.

Did NCC 2025 change anything?

No, not for thermal bridging. The ABCB list of amendments for the NCC 2025 Housing Provisions shows no change to 13.2.3, and the only change to 13.2.5 relates to overhangs. Building Ministers also agreed in October 2025 that residential energy efficiency changes would not be introduced in NCC 2025, and further residential changes are paused until mid 2029 other than essential safety and quality measures.

On timing, NCC 2025 applies in Victoria and, alongside NCC 2022 until 1 May 2027, in the ACT and WA, while the other states and the NT are on NCC 2022 for now. Our NCC 2025 vs NCC 2022 guide has the state by state detail. NSW, the NT, Tasmania and WA also have their own variations to Part 13.2, so check the one for your state.

Why it keeps failing compliance

The theory is not new. What keeps failing is the practice. Five patterns cover most of it.

  1. R-Value only. The drawings name a batt and stop there. The framing material, the mitigation option and the thermal breaks are never stated.
  2. No junction details. Insulation is drawn as a continuous hatch that wraps neatly round every corner. Without a detail, the builder takes the easiest route, and that route has the bridge in it.
  3. Designer and assessor on different assumptions. If the frame type and junction treatment are not agreed in writing, the rating reflects one building and the site delivers another.
  4. Steel treated like timber. A single R-Value is applied to a steel-framed wall as if the studs were not there. Under the NCC they must be accounted for.
  5. Late substitution. A thermal break or continuous insulation board is swapped out on site to save money, and nobody tells the designer or the assessor before the occupancy certificate or permit is issued.

Five junctions to detail deliberately

Most residential bridging sits in five places. Detail these on purpose and you remove most of the risk.

  • Wall to floor. Slab edges and bottom plates. Show how edge insulation and the wall insulation meet, and where slab edge insulation is required, show it.
  • Wall to ceiling. Top plates often leave a gap in the ceiling insulation. Detail how the ceiling insulation runs over or meets the plate, and remember the 450 mm edge concession in 13.2.3(6) when setting depths.
  • Window to wall. Window frames, particularly aluminium ones, usually conduct heat more readily than the insulated wall around them. Address head, jamb and sill separately, and specify thermally broken frames where the energy brief calls for them.
  • Balcony slab. A concrete slab running through the envelope of an apartment is a serious bridge. Specify a structural thermal break product, or document the condition so the assessor can model it and the certifier can check it.
  • Service penetrations. Each one is small. Clustered in kitchens, bathrooms and plant areas, they add up. Show how penetrations are sealed and insulated around.

What a defensible documentation set includes

  • The framing material for every roof, wall and floor, and the 13.2 mitigation option chosen for each metal-framed element.
  • Thermal breaks shown and specified by product where 13.2.3(7) or 13.2.5(5) applies.
  • Junction details for wall to floor, wall to ceiling, window head, jamb and sill, balcony and service zones.
  • Vapour permeance of any continuous insulation or foil outside the frame, checked against Part 10.8.
  • A specification clause that stops thermal bridging products being substituted without designer and assessor sign-off.

If a certifier asks how you dealt with thermal bridging, point to a drawing and a clause. If you cannot, that gap is where a defect claim starts.

Your next steps

  1. Audit one live project against the five junctions and add a detail wherever you cannot point to one.
  2. Before the next rating, email your energy assessor the frame type, thermal breaks and junction assumptions, and ask them to confirm in writing.
  3. If you use steel framing, write the chosen 13.2 mitigation option into the drawings, not just the specification.

CPD on thermal bridging

Each of these live courses runs for one hour:

For the related moisture risks, read how to manage condensation in residential buildings, and see the CPD requirements guide and the CPD deadlines calendar.

Frequently asked questions

What is thermal bridging in simple terms?

Thermal bridging is heat taking a shortcut through the building fabric. Framing, fixings, slab edges and window frames conduct heat more readily than the insulation beside them, so heat bypasses the insulation. The ABCB describes it as an unintended path of heat flow between the outside and inside of a building.

Does the NCC require thermal bridging to be addressed?

Yes, for metal framing. Part 13.2 of the ABCB Housing Provisions sets Deemed-to-Satisfy rules for thermal bridging in metal-framed roofs, walls and floors of houses, including R0.2 thermal breaks in some roof and wall types. Timber framing is not covered by those house provisions.

Did NCC 2025 change the thermal bridging rules?

No. The NCC 2025 list of amendments for the Housing Provisions shows no change to clause 13.2.3, and the only change to 13.2.5 relates to overhangs. Building Ministers also agreed in October 2025 that residential energy efficiency changes would not be introduced in NCC 2025.

How much performance does thermal bridging cost?

It depends on the frame, spacing and insulation, so there is no single figure. The ABCB says the NCC thermal bridging provisions aim to bring metal-framed walls, roofs, ceilings and floors to at least 90 to 95% of the performance of their timber-framed equivalents, which tells you unmitigated steel falls short of that.

Will higher R-Value insulation fix a thermal bridge?

Not on its own. More insulation between the studs does not change how much heat the studs conduct. The NCC options work by adding insulation that covers the frame, creating a reflective airspace outside it, or adding a thermal break between the cladding and the frame.

Who is responsible for thermal bridging on a project?

It is shared. The designer specifies the assembly and details the junctions, the energy assessor models what is documented, and the certifier checks that what was built matches. If any step is left implicit, the bridge gets built in.

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