Retrofit Insulation in Hertfordshire: A Practical Guide to Insulating Your Existing Home

Retrofit insulation means adding insulation to an existing building — as opposed to insulating a new extension or conversion during construction. It covers cavity wall insulation, solid wall insulation (internal or external), loft insulation, floor insulation, and pipe and tank insulation.

The Hertfordshire housing stock is dominated by 1930s–1970s semi-detached and terraced properties. Most of these have cavity walls — a gap between the inner and outer leaf of brickwork — that were left unfilled at construction. Filling that cavity is the cheapest and most impactful insulation upgrade available for these properties. Older Victorian and Edwardian properties (common in St Albans, Hertford, and Hitchin) have solid walls with no cavity — these require a different approach.

The Energy Saving Trust estimates that a well-insulated home can save £300–£600 per year on energy bills compared to an uninsulated equivalent. With energy prices remaining elevated, the payback period for most insulation measures is now 3–8 years — making retrofit insulation one of the best-returning home improvements available.

Retrofit insulation also improves your property's EPC (Energy Performance Certificate) rating, which is increasingly important for mortgage lenders, tenants, and buyers. From 2025, landlords in England must achieve a minimum EPC rating of C for new tenancies — making insulation a compliance issue as well as a comfort one.

If your home was built between 1930 and 1990 and has an unfilled cavity wall, cavity wall insulation (CWI) is almost certainly the most cost-effective insulation upgrade available. The process is straightforward: small holes are drilled in the mortar joints of the outer leaf, insulation material is injected under pressure to fill the cavity, and the holes are filled. The whole process takes 2–4 hours for a typical semi.

Three materials are commonly used: mineral wool (glass wool or rock wool) beads, which are the most common and cheapest option; EPS (expanded polystyrene) beads, which have a slightly higher thermal resistance and are better in wet conditions; and polyurethane foam, which provides the highest thermal performance but is more expensive and harder to remove if problems arise.

The key risk with CWI is moisture. If the cavity is already damp, or if the property is in an exposed location with driving rain, CWI can cause moisture to bridge from the outer to the inner leaf — leading to damp and mould inside the property. A pre-installation survey is essential. TCM always checks for existing damp, wall tie condition, and cavity width before recommending CWI. Properties in exposed locations (east-facing walls, coastal-influenced areas) may be better suited to internal or external wall insulation.

For more on damp diagnosis and treatment, see our guide on how to tell if you have rising damp and our Damp Proofing & Waterproofing service page.

Victorian and Edwardian properties — which make up a significant proportion of the housing stock in St Albans, Hertford, Hitchin, and Harpenden — have solid brick walls with no cavity. These walls have a U-value of approximately 2.0 W/m²K — far above the 0.18 W/m²K required for new extensions under the 2026 Building Regulations. Insulating them requires either adding insulation to the inside face (internal wall insulation, IWI) or the outside face (external wall insulation, EWI).

Internal wall insulation (IWI) involves fixing insulated plasterboard (a composite of PIR insulation and plasterboard) or a timber frame with mineral wool insulation to the inside face of the external wall. It reduces the internal floor area by 80–100mm per wall and requires all skirting boards, architraves, window reveals, and electrical sockets to be repositioned. It is typically done as part of a broader refurbishment — see our Property Refurbishment service page for details of how TCM integrates IWI into a full refurb programme.

External wall insulation (EWI) involves fixing insulation boards to the outside face of the wall and finishing with a render or cladding system. It maintains the internal floor area, eliminates more thermal bridges than IWI, and can transform the external appearance of a property. However, it changes the external appearance — which may require planning permission in conservation areas — and is more expensive. EWI costs £8,000–£15,000 for a three-bedroom semi, compared to £4,000–£8,000 for IWI.

Both IWI and EWI can achieve a wall U-value of 0.18–0.30 W/m²K — significantly better than the uninsulated solid wall, but not as good as a well-insulated cavity wall. For properties where CWI is not possible, solid wall insulation is the only option for meaningful wall heat loss reduction.

If your loft is uninsulated or has less than 100mm of insulation, topping up to 270mm of mineral wool is the cheapest and quickest insulation upgrade available. The Energy Saving Trust estimates savings of £150–£300 per year for a typical semi-detached property. The cost is £300–£600 for a standard loft, and the installation takes half a day.

The standard approach is to lay 100mm of mineral wool between the ceiling joists (the first layer) and 170mm across the joists (the second layer, at 90° to the first), achieving a total depth of 270mm and a U-value of approximately 0.13 W/m²K. This exceeds the 2026 Part L requirement of 0.15 W/m²K for new roofs.

If your loft is used for storage or access, the insulation can be installed at rafter level (a warm loft) rather than joist level (a cold loft). A warm loft uses rigid PIR boards between and below the rafters — more expensive than mineral wool but necessary if the loft space is to be kept accessible. For full loft conversions, see our Loft Conversion service page and our guide on energy-efficient loft conversions.

Heat loss through the ground floor accounts for approximately 15% of total heat loss in a typical Hertfordshire home — less than walls or roof, but still significant. The approach depends on the floor construction.

Suspended timber floors (common in pre-1960s properties) can be insulated by lifting the floorboards and fitting mineral wool between the joists — a relatively straightforward operation that costs £800–£2,000 for a ground floor. The insulation must be supported on netting or battens to prevent it from falling into the void below. A vapour control layer is not required for suspended timber floors (the void below provides ventilation), but draught-proofing the perimeter is important.

Solid concrete floors require either lifting the floor and installing PIR insulation below a new screed (major works, £3,000–£8,000) or installing insulated flooring panels on top of the existing floor (which raises the floor level by 20–50mm, requiring all doors and skirting boards to be adjusted). Solid floor insulation is usually only done as part of a broader refurbishment — see our Property Refurbishment service page for how TCM sequences floor insulation within a full refurb programme.