Module 2 · Buildings & Moisture Basics
2.7 Thermal Bridges
7 min read
A thermal bridge is a place where heat escapes faster than through the surrounding fabric, leaving a locally cold surface. It is a design and construction characteristic, not a defect — but it is one of the most common causes of mould you will meet, and telling a client their wall is fine while it grows mould requires you to understand exactly why.
Learning objectives
By the end of this lesson you will be able to:
- Explain what a thermal bridge is and why it produces a cold surface.
- Identify the locations where thermal bridges typically occur.
- Explain why insulation and airtightness improvements can make bridges worse.
- Distinguish thermal-bridge mould from a fabric defect.
Key terms
- Thermal bridge
- A localised path of higher heat loss through the envelope, producing a colder internal surface than the surrounding area.
- Geometric bridge
- A bridge caused by shape rather than materials — an external corner loses heat across more outside area than inside.
- Repeating bridge
- Regularly spaced bridges such as timber studs or joists interrupting an insulation layer.
- Surface temperature factor
- How well a junction resists cold, expressed as a ratio. Building standards use it to set minimum acceptable junction performance.
Read the rest of Buildings & Moisture Basics
Module 1 is open to everyone. The remaining six modules — building science, each damp mechanism in depth, meters and thermal imaging, the full inspection method, and report writing — are free too. We just ask for an email so you can pick up where you left off, and so we can send your certificate when you finish.