Thermal bridging: what it is and how to avoid it

What a thermal bridge is

Heat moves from warm to cold by the easiest route. If part of the insulation is interrupted by something that conducts heat well, such as concrete, steel or even timber, heat moves through it faster. That place is a thermal bridge, also called a cold bridge.

Where they happen

• Wall and floor junctions, where a slab edge passes through the wall.
• Balconies and canopies that continue from inside to outside.
• Lintels, window and door reveals and cills.
• Roof junctions, such as eaves and parapets.
• Fixings, such as wall ties, brackets and anchors that pass through insulation.
• Structural elements, such as steel beams and columns that pierce the envelope.
• Studs and rafters, which are less insulating than the insulation between them.

Heat moves from warm to cold by the easiest route.

Why they matter

• Energy. More heat is lost, so more is needed to keep the building warm.
• Comfort. Cold surfaces make rooms feel colder, and people feel drafts from radiant cold.
• Condensation and mould. If a surface inside gets cold enough, moisture in the air condenses on it. Mould grows in these cold, damp corners. See Damp, condensation and mould.
• Performance gap. If thermal bridges were ignored in the design, the finished building will use more energy than predicted.

How to reduce them

1. Keep the insulation continuous. Draw a continuous line of insulation around the building, and make sure the line is not broken at junctions.
2. Move the insulation outside the structure where possible, so that the structure is warm and not a bridge.
3. Break the bridge. Use thermal breaks, such as insulating blocks or special connectors, where a structural element must pass through.
4. Avoid projecting elements. Detach balconies or support them separately, using thermally broken connections.
5. Use less conductive materials for fixings, or fewer fixings.
6. Detail window positions so that frames overlap the insulation.
7. Draw the junctions. If you cannot draw how the insulation goes round a corner, it will not be built properly.

Checking

Designers can assess thermal bridges by calculation or modelling and by using tested, recognised details. Site checks and, in some cases, thermal imaging help to confirm that the building has been built as drawn. The methods and the details accepted by regulations differ between countries.

In short

Thermal bridges are weak spots in the insulation. Find them in the design, keep the insulation continuous, and detail every junction. See also Cavity walls explained and Airtightness and ventilation.

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