Retrofit and Mould: What the Assessment Must Address

Mould growth in buildings is a significant health and performance issue that retrofit projects must actively address. Poor assessment of mould risk during the retrofit planning stage can undermine the entire project and create liability for all parties involved. This guide sets out the key areas that any retrofit assessment must examine.

Why Mould Assessment Matters in Retrofit

Retrofit work fundamentally changes how a building performs. Improved insulation, air-tightness measures and ventilation modifications all affect moisture movement and surface temperatures. Without proper mould risk assessment, these improvements can inadvertently create conditions where mould thrives.

Mould presents genuine health risks, particularly for vulnerable occupants including children, elderly residents and those with respiratory conditions. Housing associations and retrofit providers have both legal and moral obligations to identify and mitigate these risks before work begins.

Key Assessment Areas

Existing Moisture and Condensation Patterns

The assessment must establish baseline conditions:

• Visual evidence of existing mould, dampness or discolouration
• Moisture readings using calibrated meters on suspect areas
• Occupant reports of where condensation occurs seasonally
• Evidence of previous remedial work or repainting over mould
• Patterns of mould growth related to specific rooms or building features

Do not rely solely on visual inspection. Moisture can be present behind surfaces and in concealed cavities. Thermal imaging can reveal cold bridges where condensation risk is highest.

Thermal Performance and Cold Spots

Mould grows where surface temperatures remain below the dew point for extended periods. The assessment must identify:

• Thermal bridging through structural elements, fixings and junctions
• External corners and window reveals
• Areas where insulation thickness is compromised or absent
• Junctions between different building elements
• Sections where external air temperatures fall below interior dew point

Thermal imaging should be conducted during cold weather when temperature differentials are greatest. Models and calculations can predict surface temperatures post-retrofit and identify whether proposed measures will adequately raise them.

Ventilation and Moisture Generation

The assessment must examine:

• Current ventilation provision and how air circulates through the building
• Extraction from kitchens and bathrooms
• Sources of moisture generation (cooking, washing, drying clothes)
• Whether occupants can control ventilation (openable windows)
• Existing condensation problems that suggest inadequate air change rates

Where air-tightness works are planned, the assessment must ensure that mechanical ventilation systems are specified and will be installed to replace natural ventilation losses.

Building Materials and Vapour Movement

The assessment should consider:

• Vapour permeability of existing walls and proposed insulation materials
• Whether the building has breathable or non-breathable construction
• Risk of interstitial condensation within wall cavities or above ceilings
• Compatibility of new materials with existing construction
• Potential for moisture trapping at junctions between old and new work

Different retrofit strategies suit different building types. A solid stone wall may require different approaches than a cavity wall, and vapour control strategy must reflect this.

Building Use and Occupancy Patterns

Mould risk depends partly on how the building is used:

• Number of occupants and their daily routines
• Whether the property is occupied continuously or intermittently
• Special moisture-generating activities (hobby workshops, laundry services)
• Whether occupants are likely to open windows in winter
• Age and health profile of occupants

An assessment must acknowledge real occupancy patterns, not theoretical ones. A family with multiple young children generating high moisture loads requires different mitigation than empty-nesters.

What the Assessment Must Deliver

A thorough mould risk assessment should produce:

1. Clear documentation of baseline conditions with photographs and measurements
2. Identification of all areas at elevated mould risk post-retrofit
3. Specification of design measures to mitigate identified risks
4. Details of ventilation provision and how occupants will be advised to use it
5. Maintenance requirements and occupant guidance documentation
6. Post-completion monitoring plan if residual risk remains

Integration with Retrofit Design

Mould assessment findings must directly inform retrofit specification. If assessment identifies cold corners, the retrofit design must include sufficient insulation thickness at those locations. If moisture generation is high, ventilation capacity must be increased accordingly.

The assessment should be completed early enough that its findings genuinely shape the retrofit approach, not used retrospectively to justify decisions already made.

Professional Standards and Responsibility

Those conducting mould risk assessment should understand building physics, moisture behaviour, and PAS 2035 requirements. Assessment is not a role for those with only surface-level retrofit knowledge.

All findings and recommendations should be clearly documented and communicated to occupants. Where risk cannot be fully eliminated, occupants must understand what they need to do to manage moisture in their home.