Improving the energy efficiency of an existing building changes how heat, air and moisture move through it.
This does not mean that insulation should be avoided. It means that retrofit measures should be selected and designed with an understanding of the whole building.
A property is not a collection of separate components. Its walls, roof, floors, windows, heating, ventilation, drainage, materials and occupants interact. A change to one part of the building can affect the performance of another.
Moisture risk should therefore be considered from the beginning of the retrofit process rather than addressed after visible damp or mould develops.
Begin with the existing condition
Before proposing measures, the existing condition of the building should be understood.
Visible moisture may be associated with several factors, including:
- Rainwater penetration
- Defective gutters or downpipes
- Plumbing leaks
- Ground moisture
- Damaged pointing or finishes
- Inadequate ventilation
- Low internal temperatures
- Cold surfaces
- Occupant moisture generation
- A combination of several causes
Installing insulation without understanding the source of an existing problem can conceal the evidence while allowing the underlying issue to continue.
Repairs and maintenance may therefore be an essential part of retrofit readiness.
Understand the construction
Traditional and older buildings may contain materials and construction forms that behave differently from modern cavity wall construction.
The designer should understand the wall, roof and floor build ups, including later alterations and finishes. Materials should not be selected only on the basis of thermal performance.
The design should consider:
- Moisture movement
- Drying potential
- Exposure to wind driven rain
- Existing internal and external finishes
- Junctions and penetrations
- Thermal bridging
- Local climate and orientation
- The condition of the existing fabric
BS 5250:2021 provides a code of practice for the management of moisture in buildings. Its scope reflects the need to manage moisture across the building rather than treating condensation as an isolated calculation.
Insulation changes surface temperatures
Insulation is intended to reduce heat loss, but the position and continuity of the insulation affect temperatures within the construction.
Gaps, poorly treated junctions and incomplete reveals can create locally colder surfaces. These locations may be more vulnerable to surface condensation and mould where internal humidity is high.
The design should consider continuity at:
- Windows and doors
- Roof and wall junctions
- Ground and intermediate floors
- Party walls
- Internal partitions
- Service penetrations
- Balconies and structural elements
- Changes in wall thickness
A drawing that shows only the main wall area does not fully address the performance of the building.
Ventilation must be considered with fabric improvements
Fabric improvements can reduce uncontrolled air leakage. This may improve comfort and energy performance, but it can also increase reliance on the designed ventilation system.
Ventilation should not be treated as a separate item considered at the end of the project.
The existing provision should be assessed, and the proposed strategy should reflect the intended fabric improvements, property layout and occupancy needs.
Approved Document F provides statutory guidance for meeting ventilation requirements in England. The 2021 edition took effect in June 2022 for work subject to those standards.
The practical questions include:
- Is background ventilation adequate?
- Is extract ventilation present and operational?
- Are internal doors and transfer paths suitable?
- Will new windows alter the existing ventilation provision?
- Can the resident understand and operate the system?
- Has the completed system been commissioned and checked?
A ventilation system provides limited value if it is noisy, inaccessible, incorrectly commissioned or not understood by the occupant.
Moisture risk continues during construction
Temporary exposure and construction sequencing can create risks even where the final design is sound.
Materials may become wet before installation. Existing walls may be covered before repairs have dried. Internal insulation may be installed before rainwater defects have been resolved. Ventilation systems may be completed after airtightness and internal finishes.
The method and sequence should therefore protect the building during the work.
Site teams should know which conditions require work to stop and which areas must be inspected before they are concealed.
Occupants influence performance
Building performance is affected by how the home is heated, ventilated and used.
This should not be used to place responsibility for technical defects onto residents. The system should be designed to be practical and understandable.
Handover information should explain:
- How the ventilation operates
- Which controls should remain active
- How filters or components are maintained
- How heating controls should be used
- What signs of moisture or system failure should be reported
- Who to contact when support is needed
Clear guidance can help the completed measures perform as intended.
Monitor and learn
Where appropriate, post completion review can help confirm whether the intended outcomes are being achieved.
This may involve resident feedback, internal environmental monitoring, ventilation checks, thermographic surveys or targeted investigation of recurring concerns.
Monitoring should have a clear purpose. Data should be collected only where the team knows what question it is trying to answer and how the results will inform action.
The AISK perspective
AISK approaches building performance as a system.
Insulation, ventilation, heating, airtightness, moisture and occupant use should be considered together. This does not require every project to undertake complex modelling. It requires the level of assessment and analysis to reflect the property, proposed measures and identified risk.
Early consideration of moisture is more effective than investigating failure after completion.
