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Thermal bridges — the gallery

Detailer's solver is one-dimensional: it assumes heat crosses your assembly in parallel straight paths, layer by layer. These five junctions are where that assumption breaks — the ACTUAL path runs sideways through connected structure, and the loss is measured per meter of junction as a Ψ (psi) value no stack-up can see.

Illustrative lessons, NOT a 2-D solver. The Ψ ranges below are handbook magnitudes (cited per detail) so you know which bridges are rounding errors and which dominate a heat-loss budget — they are not THERM output and not usable for compliance. A junction you'll actually build deserves a THERM model.

Rim joist

joist bay (beyond)heat exits through the solid rimEXTINTrim joist: R≈1.25/in wood where the wall wants R-20+

The 1-D stack assumes every path through the wall crosses the batts. At the floor line the actual path runs through the solid rim board — wood at R-1.25/inch standing in for a cavity the wall model credits at R-20+. The stack-up above and below is honest; the band between floors is a different wall entirely.

Magnitude (illustrative): Ψ ≈ 0.05–0.25 W/(m·K) for an intermediate-floor junction in a framed wall — toward the low end once continuous exterior insulation crosses the floor line.

Source: ISO 14683 default-value catalogues; BRE IP 1/06 junction tables.

What fixes it: Run continuous exterior insulation past the floor band so the rim sits inside the thermal line; insulate the rim bay (spray foam or batt + sealed poly) from inside as the retrofit.

Slab edge

gradeearthinterior floorexposed edgethe slab is a concrete fin planted in outdoor air

In the floor tool the slab field can carry sub-slab foam and look respectable — but the actual path exits sideways: the slab edge is bare concrete planted in outdoor air, and the ground under the middle of the slab was never the problem. This is why slab code is written as an EDGE requirement (an F-factor per foot of perimeter), not a field R.

Magnitude (illustrative): Ψ ≈ 0.4–0.8 W/(m·K) uninsulated; ≈ 0.05–0.15 with R-10 vertical edge insulation.

Source: ISO 14683 default values; ASHRAE 90.1 slab F-factor tables (the code's own framing of this bridge).

What fixes it: Vertical rigid insulation on the slab edge (R-10, 2–4 ft down or wrapping the footing in CZ5) — with a protective skirt where it meets grade.

Cantilevered balcony

balconyinterior slabinsulationone continuous pour crosses the whole insulation plane

The textbook worst case: the interior floor slab and the balcony are one continuous pour, so the actual path is a highly conductive concrete blade crossing the entire insulation plane. Parallel-path thinking sees an insulated wall with a small interruption; the 2-D truth is a radiator fin bolted to your heated slab.

Magnitude (illustrative): Ψ ≈ 0.5–1.0 W/(m·K) for an unbroken concrete cantilever; ≈ 0.1–0.3 with a structural thermal break.

Source: Schöck Isokorb design guides; Passive House Institute thermal-bridge guidance.

What fixes it: A structural thermal-break module (insulation block with stainless rebar crossing it) between slab and balcony — or a self-supported balcony on its own posts, touching the building only at pinned points.

Window jamb

window frameEXT · claddingINTthe jamb return skips the insulation layer entirely

Around every opening the wall turns a corner to meet the frame, and the insulation layer usually stops at the rough opening. The actual path sneaks around the frame through that uninsulated return — multiplied by the entire perimeter of every window. Installation position matters more than frame R: a great frame in the wrong plane still bridges.

Magnitude (illustrative): Ψ ≈ 0.03–0.15 W/(m·K) of installation Ψ per meter of perimeter — lowest when the frame aligns with the wall's insulation layer ("over-insulated" jambs approach zero).

Source: ISO 14683 installation-Ψ defaults; Passive House window-installation guidance.

What fixes it: Set the window in the insulation plane (not flush with the structure) and wrap insulation onto the frame — cover 20–40 mm of frame with the jamb return.

Parapet

parapet — cold on BOTH facesroof insulationwallins.wall and roof insulation never meet — the structure runs through

The roof stack and the wall stack can each pass their own 1-D check while the junction between them leaks: the wall structure continues past the roof plane into a fin washed by outdoor air on BOTH faces, and the roof insulation never touches the wall insulation. The actual path climbs the structure and radiates from the fin.

Magnitude (illustrative): Ψ ≈ 0.3–0.7 W/(m·K) uninsulated; ≈ 0.05–0.2 with the parapet fully wrapped or structurally broken.

Source: ISO 14683 default-value catalogues; flat-roof detailing handbooks (BRE / PHI).

What fixes it: Wrap the parapet — insulation up both faces and over the top under the coping — so the fin sits inside the envelope; or break the parapet structurally and let a lower-conductivity element carry the guard.

The pattern across all five: keep the insulation layer CONTINUOUS — trace it around your section with a pen and every place the pen lifts is a bridge. Where structure must cross it, cross with the least conductive thing you can (wood beats steel beats concrete) over the smallest area, or buy the engineered break.