Most South London homes have solid walls, not cavity walls. A PAS 2035 retrofit assessment identifies the right insulation approach and unlocks grant funding.
The majority of homes in Lambeth, Wandsworth, and Southwark were built before 1919. Victorian and Edwardian terraces, purpose-built flats in converted houses, and Edwardian maisonettes dominate the housing stock. These properties have one thing in common: solid walls. Unlike post-war cavity wall construction, solid walls cannot simply have insulation injected into a gap. Insulating them properly requires a different approach, and a PAS 2035 Retrofit Assessment is the starting point for getting it right.
A solid brick wall typically has a U-value (a measure of heat loss) of around 2.0 W/m2K. A well-insulated modern wall might achieve 0.18 W/m2K. The difference is significant: solid walls account for a large proportion of heat loss in older South London properties, and addressing them correctly can transform both comfort and energy bills.
The challenge is that solid wall insulation, whether internal or external, is more disruptive and costly than cavity fill. It also interacts with other building elements: moisture, ventilation, thermal bridging, and condensation risk all need to be assessed before work begins. Doing it in the wrong order or without proper assessment can lead to damp, mould, and structural damage.
There are two main approaches to insulating a solid wall.
Internal wall insulation (IWI) involves fitting insulated boards or a studwork frame filled with insulation to the inside face of the wall. It is less visible from the street and does not require planning permission in most cases. The trade-off is a small loss of floor space and some disruption during installation: skirting boards, electrical sockets, and radiators all need to be temporarily removed and refitted.
External wall insulation (EWI) involves applying an insulated render or cladding system to the outside of the building. It preserves internal floor space, can improve the appearance of the property, and is generally more thermally effective. However, it changes the external appearance of the building and may require planning permission, particularly in conservation areas. Many streets in Lambeth, Wandsworth, and Southwark have conservation area designations, so this needs checking before proceeding.
A Retrofit Assessment carried out to the PAS 2035 standard surveys your property in detail before any recommendation is made. The assessor will examine wall construction, existing moisture levels, ventilation, heating systems, and the fabric of windows, roofs, and floors. The resulting Improvement Measures Report specifies which insulation approach is most suitable for your property, the order in which measures should be installed, and any preparatory work needed first.
This matters because introducing insulation to a property with existing damp or inadequate ventilation can make things worse. PAS 2035 exists specifically to prevent poor-quality retrofit work on older properties like those common across South London.
Solid wall insulation is one of the more expensive retrofit measures, typically costing between £8,000 and £25,000 depending on property size and approach. However, it is one of the measures that ECO4 and the Great British Insulation Scheme can fund for eligible households.
ECO4 eligibility is based on benefit entitlement or local authority referral (LA Flex). The Great British Insulation Scheme covers single insulation measures for properties in Council Tax bands A to D, or for households on means-tested benefits. A PAS 2035 Retrofit Assessment is required to access both schemes.
If you own or rent out a Victorian or Edwardian property in Lambeth, Wandsworth, or Southwark and want to understand your insulation options, a Retrofit Assessment is the right starting point. It gives you an independent report on what your property needs, in the right order, with grant eligibility confirmed.
Carn Energy provides ABBE-accredited PAS 2035 Retrofit Assessments across South London, with a particular understanding of the older housing stock common in these boroughs. View our Retrofit Assessment service or find your area to get started.
ABBE-accredited, TrustMark-registered PAS 2035 Retrofit Assessments across Lambeth, Wandsworth, and Southwark.
Book a Retrofit AssessmentWondering what an EPC costs in Lambeth? We break down current 2025 prices for domestic, landlord and retrofit assessments, and explain what affects the cost.
ECO4 has ended. The Warm Homes Plan is the government's successor scheme, with billions set aside to fund insulation and heating upgrades for eligible households. Here is what you need to know.
Before retrofit work or grant funding can proceed, you need an independent PAS 2035 retrofit assessor. Here is what to look for and why independence matters.
ABBE-accredited EPC assessments across Lambeth, Wandsworth, and Southwark.
