Thermal Bridging: The Hidden Cause of Damp and Mould

You've seen it before: mould appearing in corners, around windows, or in specific spots on walls while the rest of the room stays dry. Often, the culprit isn't moisture production or inadequate ventilation—it's thermal bridging, a building physics problem that creates localised cold spots where condensation inevitably forms.

What Is Thermal Bridging?

A thermal bridge (sometimes called a cold bridge) is an area of the building envelope where heat transfers more readily than through surrounding materials. This creates a cold spot on the internal surface.

Think of it like a shortcut for heat to escape. While insulation slows heat loss through most of the wall, the thermal bridge allows heat to bypass the insulation and escape quickly—leaving the internal surface cold.

The Condensation Connection

Cold surfaces cause condensation. When warm, moist indoor air meets a surface below its dew point temperature, water vapour condenses into liquid water. This moisture creates the conditions mould needs to grow.

A wall might be generally well-insulated, but if a thermal bridge creates a cold spot just 3-4°C colder than the surrounding surface, that spot will consistently attract condensation while the rest of the wall stays dry.

Common Types of Thermal Bridges

Structural Thermal Bridges

Where structural elements penetrate the insulation layer:

• Steel lintels: Above windows and doors, conducting heat directly outside
• Concrete floor edges: Where floor slabs meet external walls
• Balcony connections: Steel or concrete extending through the wall
• Roof-wall junctions: Where rafters meet the wall plate

Geometric Thermal Bridges

Where building geometry concentrates heat loss:

• External corners: More external surface area per unit of internal volume
• Window reveals: The return where wall meets window frame
• Recessed features: Alcoves, bay windows, dormers

Material Thermal Bridges

Where high-conductivity materials interrupt insulation:

• Metal wall ties: In cavity walls, conducting heat across the cavity
• Window frames: Especially older aluminium frames without thermal breaks
• Fixings and fasteners: Screws through insulation boards

Identifying Thermal Bridges

Visual Indicators

Thermal bridges often reveal themselves through:

• Mould patterns: Growth in corners, around windows, along floor edges
• Condensation patterns: Water consistently appearing in specific locations
• Dust marking: Dark staining where air deposits particles on cold surfaces
• Paint deterioration: Peeling or bubbling in localised areas

Thermal Imaging

Infrared cameras make thermal bridges clearly visible:

• External surveys show heat escaping through bridges
• Internal surveys show cold spots on wall surfaces
• Best conducted during heating season with 10°C+ temperature difference

Environmental Monitoring

Temperature and humidity data reveals patterns:

• Surface temperatures in problem areas vs. general wall
• Correlation between external temperature and condensation
• Humidity spikes in specific locations

Common Problem Locations

External Corners

Perhaps the most common thermal bridge in UK housing. An external corner has more external wall area than internal floor area, so heat loss is concentrated. The corner surface is always colder than adjacent flat wall surfaces.

Window Reveals

Where the wall thickness is exposed around windows. Often poorly insulated, especially in older properties where windows sit close to the external face of thick walls.

Lintels

Steel lintels above windows and doors conduct heat rapidly. Older properties may have uninsulated lintels; even newer properties often have thermal bridges at this junction.

Ground Floor Edges

Where solid ground floors meet external walls. The floor edge and wall foundation often form an unbroken path for heat loss.

Party Wall Junctions

Where external walls meet party walls. If the party wall cavity isn't properly sealed, cold air can circulate and chill the junction.

Solutions and Remediation

Internal Insulation

Adding insulation to internal surfaces can address thermal bridges:

• Insulated plasterboard on cold walls
• Corner fillets to increase surface temperature at junctions
• Reveal linings around windows

Caution: Internal insulation can create new problems if not properly designed—including interstitial condensation within the wall construction.

External Insulation

Wrapping the building externally is often more effective:

• Covers thermal bridges comprehensively
• Keeps structure warm, reducing condensation risk
• Maintains internal room dimensions

However, it changes the building's appearance and may not be permitted on listed buildings or in conservation areas.

Targeted Interventions

Addressing specific thermal bridges:

• Thermally-broken lintels for replacement windows
• Insulated reveals during window installation
• Aerogel or vacuum insulation for thin applications

Managing the Consequences

Where remediation isn't feasible, manage the symptoms:

• Improved ventilation to reduce humidity levels
• Heating adjustments to raise surface temperatures
• Anti-condensation paint (reduces condensation but doesn't fix the bridge)
• Regular monitoring to catch mould early

Design Considerations for New Build

Thermal bridging is much easier to prevent than to cure:

• Continuous insulation: Avoid breaks in the thermal envelope
• Thermal break components: Insulated lintels, thermally-broken fixings
• Junction detailing: Careful attention to corners, edges, openings
• Psi values: Calculate linear thermal transmittance at junctions

Why This Matters for Housing Providers

Understanding thermal bridging helps you:

• Diagnose recurring mould problems correctly
• Avoid blaming tenants for physics problems
• Specify effective remediation (not just mould wash)
• Plan capital works that address root causes
• Predict which properties will have problems