Understanding Thermal Bridging: The Hidden Cause of Condensation

You've inspected the property, the heating works, humidity levels seem reasonable—yet mould keeps appearing in the same spots. The likely culprit is thermal bridging: localised cold areas in the building fabric where condensation forms regardless of overall room conditions.

What Is a Thermal Bridge?

The Basic Concept

A thermal bridge (or cold bridge) is an area where heat transfers through the building envelope more easily than the surrounding structure. This creates a cold spot on the internal surface—cold enough to reach dew point and cause condensation.

Even if the room is adequately heated and ventilated, these localised cold spots will still experience condensation because their surface temperature is below the dew point of the room air.

Why It Matters

Thermal bridges cause:

• Localised condensation and mould growth
• Increased heat loss (energy inefficiency)
• Tenant discomfort near cold areas
• Recurring problems that don't respond to ventilation advice

Common Types of Thermal Bridges

Geometric Bridges

Created by building geometry, particularly corners where external walls meet:

• External corners: Two cold surfaces meeting, creating a particularly cold zone
• Wall/floor junctions: Where ground floor meets external wall
• Wall/roof junctions: Eaves and verges

These are why mould often appears in ceiling corners of external walls.

Material Bridges

Where materials with different thermal properties meet:

• Concrete lintels: Above windows and doors, bridging the insulated cavity
• Steel beams: Structural elements that conduct heat
• Window frames: Particularly metal frames in older installations
• Balcony connections: Concrete balconies penetrating the wall

Insulation Gaps

Where insulation is missing or inadequate:

• Partial cavity fill: Cavities not fully insulated
• Gaps around services: Holes for pipes and cables
• Loft insulation edges: Where insulation doesn't reach the eaves

Identifying Thermal Bridges

Visual Indicators

Look for patterns that suggest thermal bridging:

• Mould or discolouration in consistent locations
• Straight lines of mould (following structural elements)
• Corner condensation when walls are generally dry
• Cold areas near windows, doors, or structural features

Thermal Imaging

Infrared cameras reveal temperature differences:

• Cold spots show clearly against warmer surroundings
• Can identify bridges not visible to the eye
• Best conducted in cold weather with heating on
• Requires trained interpretation

Surface Temperature Measurement

Direct measurement can confirm thermal bridging:

• Compare surface temperature to room air temperature
• Calculate if surface is below dew point
• Document the temperature differential

Assessing Severity

Temperature Factor (f-factor)

Building professionals use the temperature factor to assess thermal bridge severity:

A temperature factor below 0.75 indicates high condensation risk. Modern building regulations require f-factor of at least 0.75 for new construction, but many older buildings fall below this threshold.

Practical Assessment

Without detailed calculations, consider:

• Is mould recurring despite adequate heating and ventilation?
• Is it in locations typical of thermal bridges?
• Does the area feel noticeably cold to touch?
• Are similar properties experiencing the same issue?

Remediation Options

Internal Insulation

Adding insulation to the internal surface:

• Insulated plasterboard or independent insulation
• Must extend far enough to address the bridge zone
• Reduces room size slightly
• Care needed to avoid creating new condensation points

External Insulation

Wrapping the outside of the building:

• Most effective but most expensive and disruptive
• Changes building appearance (planning considerations)
• Best done as part of whole-building retrofit
• Addresses bridges comprehensively

Targeted Treatments

For specific bridge locations:

• Thermal break products: For lintels and structural elements
• Anti-condensation paint: Raises surface temperature slightly
• Improved heating: Radiators positioned near cold spots

Window and Door Upgrades

For frame-related bridges:

• Thermally broken frames
• Better performing glazing
• Improved installation details

When Thermal Bridging Isn't the Cause

Differential Diagnosis

Before attributing issues to thermal bridging, rule out:

• Penetrating damp (moisture from outside)
• Rising damp (ground moisture)
• Leaks from plumbing or rainwater goods
• General under-heating or under-ventilation

Multiple Factors

Often, thermal bridging is one factor among several:

• Bridge location is vulnerable, but high humidity makes it worse
• Improving ventilation reduces humidity, giving more margin at the bridge
• Combined approach may be needed

Stock Management Implications

Archetype Analysis

Thermal bridges are often construction-specific:

• Certain build types have predictable bridge locations
• 1960s-70s system-built properties often have significant bridges
• Identifying patterns helps prioritise intervention

Investment Planning

Addressing thermal bridges at scale requires:

• Survey to identify extent of bridging
• Cost-benefit analysis of remediation options
• Coordination with other planned works (windows, EWI)