Technical considerations in retrofit begin with understanding the building — its construction type, condition, energy performance and the constraints that will affect what measures are feasible. Getting this right is essential: the wrong measures, specified without proper regard for the building's characteristics, can cause harm as well as wasting investment.
This guide covers the key technical factors that affect retrofit planning and measure selection across the UK's diverse housing stock.
UK housing stock spans several centuries of construction methods, and the technical characteristics of a property are largely determined by when and how it was built. Understanding the construction type is the starting point for any retrofit assessment.
Solid brick or stone construction with no cavity. Wall insulation requires either external solid wall insulation (EWI) or internal wall insulation (IWI). Both have significant implications — EWI changes the external appearance, IWI reduces floor area and both require careful detailing at junctions. Moisture management is particularly critical: solid walls regulate moisture through vapour permeability, and insulation systems that trap moisture within the fabric can cause serious harm.
The most common construction type in UK social housing. Where the cavity is unfilled and in good condition, cavity wall insulation is typically straightforward and cost-effective. Where cavities are already filled, narrow or show signs of failure, different approaches are required. The condition of the existing cavity fill must be assessed before any further insulation is specified.
Flats present particular complexity — individual flats may share walls, floors and ceilings with adjacent properties, making whole-house assessment more complicated. Party wall heat loss, communal heating systems and the need to coordinate works across multiple leaseholders add layers of complexity that require careful planning.
The Energy Performance Certificate provides a standardised measure of a property's energy performance on a scale from A (most efficient) to G (least efficient). For funded retrofit programmes, EPC rating is typically used to identify priority properties — most schemes target homes below Band D or Band C.
However, the EPC has significant limitations as a tool for retrofit planning. It is based on standardised assumptions about occupancy and behaviour, and the RdSAP methodology that underpins it does not capture all the factors relevant to retrofit design. A pre-retrofit assessment under PAS2035 gathers substantially more data than an EPC survey and should always be the basis for retrofit measure selection.
The fabric first principle holds that improvements to the building fabric — insulation, draught proofing, glazing — should be prioritised before low-carbon heating systems are installed. The logic is straightforward: a heat pump in a poorly insulated home will work less efficiently than the same heat pump in a well-insulated home, and the energy and carbon savings from the heat pump will be smaller.
PAS2035 does not mandate a strict fabric-first approach, but the Medium Term Plan must sequence measures logically so that earlier measures do not compromise later ones. Installing a heating system before the fabric is improved, or installing solar PV before reducing the demand it needs to meet, are examples of poor sequencing that the RC must identify and address.
Ventilation is the technical consideration most frequently overlooked in retrofit, and the one with the most significant consequences when it goes wrong. As a home is made more airtight through fabric improvements, its natural ventilation — the uncontrolled air movement through gaps in the fabric — is reduced. Without compensating ventilation measures, indoor air quality deteriorates and moisture levels rise.
The ventilation strategy must be designed alongside the fabric measures, not added afterwards. The options range from simple background ventilators (trickle vents) and intermittent extract fans for modest improvements, through to Mechanical Extract Ventilation (MEV) or Mechanical Ventilation with Heat Recovery (MVHR) for more comprehensive upgrades.
Key principle: Never specify insulation or draught proofing measures without simultaneously specifying the ventilation strategy. The two are inseparable.
Planning constraints affect what external measures are feasible for a given property. Properties in Conservation Areas, Listed Buildings and Article 4 Direction areas may face restrictions on external wall insulation, replacement windows and solar panel installation. Permitted development rights — which allow many retrofit measures without planning permission — do not apply in these areas.
Checking planning status at the assessment stage, before a measure package is specified, prevents the costly discovery of constraints after design is complete.
The condition assessment at Stage 1 must identify any structural defects that could be exacerbated by retrofit measures or that must be addressed before measures are installed. Damp penetration, failed lintels, poor pointing and roof defects all fall into this category. Insulating over an existing moisture problem does not solve it — it typically makes it worse.
Where significant structural work is required before retrofit can proceed, the programme timeline must accommodate this. Identifying structural issues at assessment stage rather than during installation avoids the most disruptive delays.
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