Energy Improvement Retrofit discussions

Energy Improvement Retrofit discussions

Calum Maclean, an architect based in the Highlands, who is an AHSS member and cases panel member, as well as holding RIAS Conservation Accreditation at Advanced Level and being a PAS 2035 Retrofit Co-ordinator, recently prepared this guidance note in relation to an enquiry to the AHSS.

Retrofit is inherently risky, especially so when dealing with any building of traditional construction, regardless of whether or not it is listed. Getting it wrong can result in problems that put occupants’ health at risk and are very expensive to resolve.

In terms of how to approach retrofit, PAS 2035 offers a robust methodology that has been specifically developed to address the inherent problems and risks associated with retrofit, which gave rise to well documented problems with energy improvement schemes in the past. Please refer to the UK government’s 2016 report – Each Home Counts.

One of the first issues that needs to be addressed in any retrofit programme is the issue of maintenance and the condition of the building fabric.

Many buildings of traditional construction suffer from a lack of maintenance, resulting in water ingress and saturated masonry. This is a particular problem with houses that are linked either as terraces or as apartments.

In addition to a general lack of maintenance, many of these properties have flashings and rainwater goods that were not designed for the current levels of rainfall that are a result of climate change.

Another problems is that many will have been altered since they were first built, often with the introduction of modern non-breathable materials, such as the installation of bitumen roofing felts below slates, without any consideration for ventilation of the structure.

A great many walls have also been repointed with cement or harled with a cement render that prevents the walls from breathing.

These initial repairs can be a significant cost that is over and above the cost of the energy improvement measures.

This will also introduce a time challenge.

Saturated stonemasonry can take many months – if not years – to dry out. It must be allowed to dry out before installing insulation. Placing insulation onto damp walls will certainly result in failure.

Every building will present a unique set of issues and it is really important that the condition and fabric buildup of each property is assessed on an individual basis by somebody who has a lot of knowledge and experience in dealing with traditionally built structures. A regular building surveyor or architect is not going to have the level of experience necessary and I would recommend that this is carried out by conservation accredited professionals – as required by PAS 2035.

A fabric first approach is to be commended – and will align with the new EPC methodology being developed by the Scottish Government.

Using breathing construction materials is critical, but in itself does not guarantee that condensation will not occur and a very careful analysis of the fabric build up will be required.

There is a risk in trying to push the performance of masonry, and current guidance is that a U-Value of 0.6 is the limit. I would refer you to the UK government’s current guidance on internal wall insulation (“Retrofit Internal Wall Insulation – Guide to Best Practice” – see pages 30 and 31 in particular).  See also the Bristolian Guide to wall insulation referred to in the government guidance.

Different stone types, details and wall thicknesses will perform differently, both in terms of thermal performance and risk of water ingress. Granite will perform very differently to sandstone. A historic building built from granite with a crow-stepped gable will represent an extremely high risk of moisture penetration.

A one size fits all approach is therefore out of the question.

Given the risks involved, I would thoroughly recommend the established conservation approach, whereby whatever improvements are carried out, if there are any problems in the future, they can be removed and the building returned to its original state. This may limit the use of some building materials, such as spray foam insulations which physically penetrate and adhere to original fabric and cannot be removed. I would also point out that the use of these materials in a retrofit situation does not comply with their BBA certification.

In addition to building control and planning, there are various other legal requirements that need to be complied with.

Under CDM regulations, a risk assessment will need to be carried out – which will require, at the very least, an asbestos survey before any work is carried out.

There is environmental legislation that will require bat and other ecological surveys to be carried out – this will impact on most rural houses of traditional construction. It is a criminal offence to carry out work that would disturb protected species – such as installing roof insulation. Note – bat surveys can only be carried out between May and August and most ecologists in rural Scotland are already working at full capacity.

Retrofit requires a whole house approach, and to reduce risks, it is important that there are no gaps around the thermal envelope. This means getting into awkward corners. Physically gaining access to some spaces within an existing building can be challenging, particularly the coombs, where upper rooms are built within the roof structure. This may require extensive and costly opening up works followed by re-instatement on completion. If you are dealing with a listed building, or one with ornate decorative plaster cornices, this will require additional care and cost. There is often a legacy of redundant wiring and pipework that has been left behind from work carried out many decades ago, this often gets in the way and needs to be removed. Sub floor spaces often have insufficient ventilation or depth and are again filled with decades of redundant wiring and pipework.

Addressing these issues will require a whole new approach to construction from tradesmen. Very few of our current joiners, electricians and plumbers are familiar with the need to get challenging details right on site. Even when there is a clear strategy and a good set of drawings, the physical challenges of delivering these on-site must not be underestimated.

Supply chain issues to be considered are:

Availability of architects/surveyors with conservation expertise.

Availability of ecologists

Availability of suitably skilled and motivated tradesmen.


These are first thoughts and no doubt many more points will arise.


PAS 2035 : 2023 Retrofitting dwellings for improved energy efficiency – Specification and guidance

EPC – Energy Performance Certificate

BBA – British Board of Agrément Construction Product Certification