Dean Firth, Senior Technical Manager, Salix
Since the introduction of PAS2035 in 2019 the mantra of domestic retrofit has been fabric first.
With space heating accounting for around two thirds of a typical home’s energy load (https://www.gov.uk/government/statistics/energy-consumption-in-the-uk-2024), reducing that load by trapping as much heat inside the building as possible makes good sense; especially when it has the bonus of increasing thermal comfort, tackling chronic cold, and reducing the risk of damp and mould.
In the fabric first approach only once the building’s thermal efficiency is optimised should grid connected technologies such as solar PV, heat pumps, and batteries be considered.
Take a scenario of a single dwelling undergoing a programme of deep retrofit. External wall insulation (EWI) is installed along with roof and floor insulation in the first phase, completely insulating the thermal envelope of the building.
Here we might have our first interaction with the distribution network operator (DNO). In an example where best practice is followed, wall-mounted meter boxes would be relocated at the start of the first phase to avoid thermal bridging in the wall insulation. This is often a thorny issue, as moving meter boxes can cost upwards of £1,000, and the waiting list can be several months.
Also, the cost and response time varies from distribution network operator to distribution network operator with no standardised approach. As a result, meter boxes are often left in situ and uninsulated, which can cause a thermal bridge leading to patches of black mould on the internal wall.
In a second phase of retrofit, zero/low carbon technologies such as air source heat pump (ASHP), solar PV, and a battery array might be installed. The interactions with the distribution network operator here would be minimal: assuming an inverter of <3.68kW has been installed, a single G98 notification for the PV, heat pump, and batteries would be submitted by the installer up to 28 days after the commissioning date.
However, this picture changes when we move to retrofitting at scale, as is often the case in social housing schemes like the Social Housing Decarbonisation Fund (SHDF).
At Salix we act as delivery agent for the fund on behalf of the Department for Energy Security and Net Zero. Much of my work is involved in supporting grant recipients in the journey to net zero.
My job allows me to use my experience in retrofit to support colleagues at the Department and the delivery partner in understanding the real-world challenges of delivering retrofit. I find that my technical knowledge and expertise in industry practices goes a long way in providing context and insight to key decision makers.
In these schemes a social housing provider will apply to the government grant funding scheme to deliver retrofit at scale across their housing stock. The social landlord grant recipient might own all the houses on a street. In this instance it would make very good sense, due to the economies of scale, to retrofit the whole street at the same time.
Moving the heating load of a whole street from gas to electricity might very well overburden the local grid, which is likely to be old if the houses it serves are being retrofitted. In the same way, the unpredictable power fluctuations from a whole street of small solar arrays might be too much for the existing grid to handle. That is to say that the local substation and the electrical cables running to the houses would have to be radically overhauled to facilitate this upgrade.
Complicated, expensive, and time-consuming applications would have to be negotiated ahead of any large-scale install, and the cost of submitting the applications, as well as designing and delivering the grid upgrades will usually fall to the social housing provider. This is an externality which is not factored into the grant funding structure.
The result is that large-scale retrofit schemes at a single geographical location often radically scale back or forego entirety grid connected technologies.
There is no one clear solution to this multifaceted issue, but joined up thinking on the issue is certainly required. Dedicated domestic retrofit teams at distribution network operators might help. Another consideration might be adapting the structure of grant funding policy to allow for the cost of local grid reinforcement to be covered. Maybe the Great Grid Upgrade (The Great Grid Upgrade | Powering The Things You Love) should take large-scale domestic retrofit into account, as it upgrades the grid to accommodate large scale renewables onto the UK grid.
I’m passionate about retrofit, and especially in social housing, which I see as a chance to deliver real change at real scale.
Through my work at Salix, I hope to be a meaningful part of that change. This is a fascinating industry to work in, where every day we help to lift people out of fuel poverty as well as to drive down carbon emissions.
This article appeared in the May 2025 issue of Energy Manager magazine. Subscribe here.