BLOG: Should letting agents approve insulation retrofits for older rental homes?

Insulation is arriving in older rented homes faster than most agents have had time to think about it.

Grant-funded schemes are making this all happen quickly and, meanwhile, tenants are asking for warmer flats and landlords are being offered work that sounds straightforward.

But the Public Accounts Committee recently warned that more than 30,000 homes have picked up defective installs under the government’s own schemes, which is a useful prompt to look closely at what is being proposed rather than a reason to avoid the work.

Solid wall insulation done properly is transformative, and the questions that separate a strong job from a weak one are not technical. An agent can ask all of them without knowing anything about building physics.

Wall to wall

Most pre-1919 housing has solid walls. In Scotland that usually means sandstone; elsewhere it is often brick. Either way there is no cavity and no damp proof course, and the wall has stayed dry for a century by absorbing moisture and releasing it again in both directions. It breathes.

That single fact drives everything else. Insulation that lets the wall carry on drying works with the building. Insulation that seals the inside face changes how the wall behaves. So, the first question to any contractor is simply: is this build-up vapour open, and can you show me why?

A confident answer will describe the layers and what each is for. On the work we do that is a lime parge coat applied straight onto the stone, wood fibre board bedded in lime, and a lime plaster finish, with no vapour control layer anywhere in it.

How was the thickness decided?

This is the question that tells you most about who you are dealing with.

There is a real difference between a thickness taken off a product datasheet and one produced by modelling the actual property.

On our jobs, the specification comes out of a 30-year hygrothermal simulation run with a building physicist who co-authors PAS standards, using a real local climate file. Every elevation is modelled separately, because the weather does not treat them equally.

Western Scotland takes some of the heaviest wind-driven rain exposure in Europe. A build-up that performs comfortably on a sheltered rear wall can come back with a different answer on an exposed west-facing gable. Often the modelled answer on that gable is a thinner board than the landlord expected, which surprises people until they see the reasoning.

If a contractor can explain how thickness was arrived at for each wall, that is a good sign. If the answer is a single number for the whole building, ask what it was based on.

Edges, not middle

The middle of a wall is the easy part. Where jobs are won or lost is at the junctions.

Window ‘reveals’ and lintels need attention because they are the coldest parts of the elevation. On a well-detailed job, they get a thinner board so they stay as warm as the wall around them. Joist ends buried in the masonry need thinking about too.

Then there is ventilation, which is the item most often overlooked. The original air bricks and sub-floor vents were put there for a reason, and after insulation the void below the floor needs that airflow more, not less. Ask directly whether the air bricks will be preserved and kept clear. It is a small question that reveals a lot.

Paperwork before scaffold

Finally, ask what the handover file will contain. On a compliant retrofit there should be a design document, the specification and the reasoning behind it, and a record of what was actually installed.

Coordination under PAS 2035 should sit with a named person, and an agent is entitled to know who that is.

None of this requires an agent to become a building specialist. It requires four questions, asked early: is the build-up breathable, how was the thickness decided for each wall, what happens at the reveals and the air bricks, and what documentation do I get at the end.

Ask those, and the job that follows tends to be the one nobody has to revisit.

Calum Farquhar is a PAS 2030 Retrofit Coordinator and director of Insulation Installers Scotland, which specialises in breathable internal wall insulation for pre-1919 stone buildings across Glasgow and Central Scotland, with a directly employed NVQ/SVQ-trained crew.