How to Insulate Internal Walls in the UK – Options, Costs and DIY Guide

Internal wall insulation is the method of choice for homes that cannot be insulated from the outside – typically Victorian and Edwardian solid brick properties, listed buildings, and homes where the external appearance is protected by planning restrictions. Unlike cavity wall insulation, which fills an existing void between two skins of brickwork, internal wall insulation adds a new insulating layer to the inside face of an external wall. The trade-off is a small reduction in floor area in each treated room, but the thermal improvement can be substantial – a solid brick wall has roughly a tenth of the thermal resistance of a well-insulated modern cavity wall, and bringing it closer to contemporary standards dramatically reduces heat loss and improves both comfort and energy bills.

It is worth being clear upfront about what internal wall insulation involves: it is a significant home improvement, not a weekend project. Fitting it properly requires moving sockets and switches, rerouting pipes if present in the wall, taking skirting boards and architraves off and refitting them, and replastering or finishing the new surface. Some homeowners tackle it room by room over time; others do it as part of a wider renovation. Either approach works, but going in with realistic expectations of the disruption involved is important.

Do you need internal wall insulation?

The first question is whether your walls actually need insulating from the inside – and the answer depends entirely on your wall construction type. Homes built after around 1920 typically have cavity walls – two leaves of brickwork with a gap between them. If that cavity has not already been filled, cavity wall insulation is a far less disruptive and cheaper option than internal insulation and should always be considered first. Internal wall insulation is specifically for solid wall construction – a single thick leaf of brick or stone with no cavity – which is common in pre-1920 properties.

A simple way to identify solid wall construction is to measure the wall thickness at a window or door reveal. A solid brick wall is typically 220-230mm thick. A cavity wall is typically 270-330mm including the cavity. If you are unsure, your home’s EPC certificate will list the wall construction type, or a local builder can tell you within minutes by looking at the brickwork pattern from outside – solid walls show a different bond pattern to cavity walls.

Methods compared

Dry lining with insulated plasterboard

Dry lining is the most commonly used method of internal wall insulation in the UK and the most practical option for a confident DIYer. It uses composite insulated plasterboard – a laminate of rigid PIR or EPS foam insulation bonded to a plasterboard facing – which is fixed directly to the existing wall using adhesive dabs and mechanical fixings. The combined board insulates and provides a ready-to-decorate finish in a single product, without the need to build a separate timber frame first.

The most common products are 60mm boards (38mm PIR + 12.5mm plasterboard) and 72mm boards (60mm PIR + 12.5mm plasterboard). The thicker the insulation layer, the better the thermal performance, but every millimetre of thickness is room depth lost at the perimeter of the room. In a typical Victorian bedroom, a 72mm board on two external walls reduces the floor area by around 0.3-0.5m2 – noticeable but not a major issue in most rooms. For rooms where space is already tight, the 60mm option is a reasonable compromise.

Installation involves marking out the wall, applying thermal bridging tape around the perimeter, applying adhesive dab spots to the back of the board, pressing it into position against the wall and securing with mechanical fixings through the board and into the masonry. Sockets and switches need to be extended forward on proprietary socket extension boxes to bring the face flush with the new board surface. Skirting boards are removed before installation and refitted afterwards against the new board face. The finish is then taped, jointed and decorated in the same way as standard plasterboard.

Stud wall with mineral wool or PIR

The stud wall method builds a new timber or metal frame against the existing wall and fills the cavity between the studs with insulation before closing it with plasterboard. It requires more skill and time than dry lining but delivers better thermal performance for the same total wall thickness, because the insulation fills the full depth between studs rather than being limited to the composite board thickness. It also provides more flexibility for concealing services – cables, pipes and conduits can be run within the stud cavity rather than chased into the original masonry.

Mineral wool – glass or rock wool – is the traditional fill material and remains a cost-effective and effective choice. It is non-combustible, easy to cut and handle, and provides good acoustic insulation as well as thermal performance. PIR rigid board cut to fit between studs delivers higher thermal resistance at a given thickness, which is valuable where wall depth is limited. The studs themselves are a source of thermal bridging – metal studs conduct heat more than timber – so timber studwork is generally preferred for this application. A 100mm stud cavity filled with 100mm mineral wool delivers a U-value improvement that is broadly equivalent to 60mm of PIR insulated plasterboard at dry lining thickness.

Costs and expected savings

The savings from internal wall insulation are real but the payback period is longer than simpler measures like draught proofing or loft insulation. For a whole-house professionally installed project, payback periods of 20-30 years are not unusual at current energy prices – which is why the DIY route makes a significant difference to the financial case. A competent DIYer tackling dry lining room by room over several years dramatically reduces the cost and brings the payback period within a more reasonable range. The savings also compound with other improvements – internal wall insulation works best as part of a whole-house approach that includes loft insulation, draught proofing and heating controls.

Government grants under the Great British Insulation Scheme and the Energy Company Obligation (ECO4) may cover solid wall insulation costs for qualifying households. Eligibility is based on income, benefit status and EPC rating. If your property has solid walls and you are on a qualifying benefit or have a low household income, checking eligibility before funding the work yourself is always worth doing – the grants can cover the full cost in some cases.

Common mistakes to avoid