Joe Ragdale, Technical Director at Wetherby Wall Systems Ltd, outlines why architects must put building fabric at the centre of retrofit design to support low carbon heating and long term energy performance.
The UK faces a demanding task as it works to lower emissions from its 29 million homes. Much of this stock predates modern insulation practices, leaving architects with a retrofit challenge that cannot be solved by heating technology alone. While national policy promotes wider use of heat pumps, these systems can only deliver stable performance when the building fabric supports them. Without first reducing heat loss, even advanced units may fall short of their expected efficiency.
Parliament’s Environmental Audit Committee states that around 80% of the homes that will be lived in by 2050 already stand today. Many contain solid walls with limited insulation, creating unstable indoor conditions and high energy demand. This places emphasis on the long term improvement rather than quick mechanical upgrades. A Fabric First approach offers a clear structure for architects planning retrofits across varied and ageing housing types.
Heat loss through solid walls highlights the scale of the challenge, with the Energy Saving Trust estimating that up to a third of a home’s heat may escape through uninsulated walls. This directly shapes how a heat pump must work. Such systems perform well when supplying steady warmth at low flow temperatures. In a home with high heat loss, however, they must raise output, which increases energy use and heightens the chance of discomfort during colder weather.
Evidence from modelling highlights how insulation changes a building’s thermal behaviour. In an uninsulated solid walled home, switching from a gas boiler to a heat pump may still cut heating demand by around 60 to 70%. Once robust solid wall insulation is added, that reduction may rise to roughly 90% annually. This shows the influence of the building fabric itself. For architects, it confirms that insulation is not an optional extra but the basis upon which heating performance depends.
The UK has around 7.7 million homes with solid walls and most do not currently contain insulation. These dwellings are often among the least efficient in Europe, placing architects at the centre of a large national programme of improvement.
Joe Ragdale stresses this point, stating, “The high quality retrofit of the UK’s solid wall stock is non negotiable for Net Zero. Without certified EWI or IWI, we are essentially asking homeowners to fit expensive, high tech systems into thermal sieves. This leads to poor homeowner experience, high electricity bills, and a widespread failure to achieve the low flow temperatures required for heat pumps to run efficiently.”
There is also a wider implication. When heat pumps run in homes with high heat loss, they may place greater demand on the electricity grid during winter peaks. In contrast, well insulated homes produce steadier and more manageable load patterns. Fabric improvement therefore supports national planning as well as household comfort.
Past retrofit schemes have shown the problems that arise when measures are installed without an ordered plan. Issues such as damp, mould or poor indoor conditions have often been linked to work that did not address ventilation or airtightness alongside insulation. PAS 2035 was introduced to reduce these risks by giving a structured approach to domestic retrofit, with its Whole House method requiring a clear understanding of the building’s condition before any heating system is selected.
The Retrofit Coordinator guides this process, beginning with a risk assessment followed by a medium term improvement plan that sets out the sequence of measures. For architects, this creates a consistent route for developing safe, durable retrofit strategies and supports joined up design thinking, lowering the chance that one element may conflict with another or lead to later maintenance issues.
A recent retrofit programme involving a group of solid walled homes shows how fabric upgrades shape overall performance. After internal or external wall insulation was installed to a high standard, the buildings held heat more steadily. When low carbon heating was added later, the systems were able to run at lower, more consistent temperatures. Although the insulation products differed across the homes, the outcome was clear, mechanical systems perform as intended only when the building fabric has been improved first.
For architects, this reinforces the value of addressing the envelope early in the design process. Early consideration of insulation, ventilation and airtightness helps set realistic expectations about future heating choices. It also supports accurate system sizing and reduces the likelihood of redesign or cost changes later in the programme. When the structure is improved at the outset, future upgrades become simpler and ongoing energy use becomes easier to predict.
As the UK works towards its climate goals, the performance of existing homes remains central to the wider transition. A Fabric First approach offers architects a steady framework for improving the country’s ageing housing stock and better thermal behaviour brings more consistent comfort, reduced energy use and a lighter load on the electricity grid. It also provides the setting needed for low carbon heating to function well.
For a sector that shapes buildings intended to last for decades, the direction is clear. Retrofit begins with the fabric, not the technology placed within it. This sequence offers a route for improving performance across millions of homes, supporting both the occupants who live in them and the wider move towards a lower carbon future.