Sustainable Retrofit

Courtesy of STBA and Warm Up Bristol


Sustainable Retrofit is a growing area of work in Wales and thankfully the Welsh Government is fully onboard with a sustainable approach to it. This is partially based on their requirement to follow the Wellbeing of Future Generations Act (2015) and its seven aims and five ways of working. but is also driven by the recognition that we need to do things better in order to save carbon, heritage and our uniquely Welsh communities.

The success of retrofit is the adherence to the great advice in the BSI White Paper and by following the STBA Whole House Approach. This has mostly been transposed into the new PAS2035 Retrofitting dwellings for improved energy efficiency – Specification and guidance document.

One of the key messages is that of the 4 C’s (from BSI White Paper)

Context (inc. compatibility and condition)




The understanding and assessment of location, orientation and local conditions are essential to understanding moisture risks from driven rain, flooding, wind and solar effects.

The condition of the fabric and services is an essential part of any moisture risk assessment of an existing building. Buildings in good condition are, on the whole, much more able to deal with interventions and abnormal events (such as freak flooding) than buildings already under stress or not in equilibrium (i.e. buildings in good condition have more capacity). Understanding the building condition and moisture performance of a building and, importantly, what constitutes good or poor conditions and the consequences of these, is essential prior to altering a building.

The building materials and construction method have a considerable effect on moisture safety in a number of ways. In retrofit, this is a given context for new work. In new-build it is part of the design process. Certain materials and construction methods will be more or less appropriate in different contexts  (geographical, form, use, condition).

Designers should understand not only their buildings but how the building users interact with the building and the likely future use of the building.

Having understood the context of a project and made sure that the general project aims and criteria are compatible with this context, the next stage is to ensure that there is coherence in detailing and in the process of design, construction and use. Coherence is an essential concept in the assessment and mitigation of moisture risk.

There are two fundamentally different approaches possible to moisture control in regard to the fabric of buildings: moisture closed and moisture open approaches. A moisture closed approach, which is used in many modern buildings, attempts to eliminate moisture risk in the fabric (particularly external walls, floors and roofs) by excluding moisture as a vapour or liquid by the installation of impermeable materials and systems.
A moisture open approach allows both moisture ingress and egress through the many hygrothermal mechanisms. The moisture carrying capacity and decay mechanisms of the building materials and systems as well as the use of the building will determine how much moisture ingress is acceptable without detrimental effects to fabric or occupants. A moisture open approach is found in most traditional buildings, which use naturally moisture open materials. Correctly designed, a moisture open approach can
have greater capacity to deal with construction moisture and ongoing building faults as it allows for some moisture ingress and has physical mechanisms for drying. This is not to say however that moisture open approaches of any type are always safer or better than moisture closed approaches. This depends on context.

Where there is uncertainty about the moisture performance of a building, capacity should be built into the processes of assessment, construction and use. Capacity should take into account not only current but future uncertainties, such as potential building use and occupancy patterns as well as the effects of increased driven rain or wind because of possible climate change.

Do not over-optimize. Building design that pushes the capacity of a building to deal with moisture to the limit is likely to fail. Ensure that design for moisture risk is not overwhelmed by energy strategies to increase performance. Particularly in retrofit a balance needs to be struck between energy reduction and moisture safety.

Design for the most severe internal and external conditions likely to be experienced by the building. Consider this both generally and where particular conditions, such as exposure, building form, condition or materials raise the risk level, and design accordingly. For example in areas of high driven rain consider increasing rainwater capacity, increasing roof overhangs, removing vulnerable junctions in the design, extra attention to detail at openings, etc. Where the qualities of certain fabric materials are unknown (such as the porosity of a brick wall), design for the worst type. Where there are risks internally due to unknown moisture levels and air permeability ensure capacity for full ventilation design and installation.

The many uncertainties, complexities and unknowns in regard to moisture risks, both at the point of design and over the life of the building, necessitate caution in design and construction and ongoing watchfulness in use in order
to mitigate possible problems. In particular many serious moisture problems may be hidden from view for some time prior to their effects being felt (for example when there is moisture build-up in the middle of walls, behind linings or in joist or rafter ends; or in slowly accumulating levels of mould or dust mites). Because of the many
complex interactions in walls and the difficulty in understanding fully their consequences, in addition to building in additional capacity, measures of caution should also be built into construction and particularly user information and
maintenance programmes.

Usability of services (particularly ventilation and heating) is essential. The context of the user should be prioritised in the specification of services.

Both services (particularly ventilation and heating) and fabric (particularly rainwater goods and drains) must be easy to maintain.

Where there is uncertainty about the moisture safety of a measure or part of the building, allow for simple checks and testing to be undertaken with clear instructions on to how to assess or feedback information.

Great resources to help with any decision making include the STBA, especially the STBA Guidance Wheel