Internal wall insulation for listed and period buildings
How to insulate a solid-wall, listed or conservation-area property without trapping moisture in the fabric — consent, build-ups, performance and evidence.
Roughly 5.9 million buildings in the UK were built before 1919. Almost all of them have solid walls, almost all of them are cold, and almost all of the insulation sold in this country is designed for a completely different kind of building.
This page covers what internal wall insulation involves in a listed or period property, what consent you need, what performance is realistically achievable, and how a vapour-open system differs from the boards most contractors will offer you.
Why internal rather than external
External wall insulation is usually cheaper per square metre and avoids disruption inside the house. For a listed building or a property in a conservation area it is almost always the wrong answer, because it changes the external appearance. Brick bond, stone coursing, string courses, window reveals and eaves detail are the things that give a period building its character, and they are exactly the things external insulation buries.
Internal wall insulation keeps the outside of the building untouched. That single fact removes most of the planning objections, and it is why almost every heritage retrofit is done internally.
The trade-off is that internal insulation makes the wall itself colder. The masonry now sits outside the insulation layer rather than inside it, so it stays wetter for longer after rain and it loses the small drying benefit it used to get from indoor heat. A vapour-closed system in that situation traps moisture at the interface between insulation and wall. A vapour-open system lets it keep moving. That difference is the whole argument.
What makes a system suitable for a historic wall
Vapour permeability
Moisture must be able to pass through the assembly rather than accumulate at a boundary. In practice this means low Sd values through every layer, with no polythene, foil facing or impermeable membrane anywhere in the build-up.
Hygroscopic buffering
Lime and perlite absorb moisture at humidity peaks and release it again as conditions dry. That buffering smooths out the spikes that cause surface condensation and mould.
Material compatibility
Lime-based, mineral, and softer than the masonry it is applied to. Cement renders and gypsum plasters are harder than historic brick and stone, and they fail at the interface.
Thin build-up
Every millimetre taken off a window reveal changes the proportions of the room. A system that performs at 25–50 mm preserves detail that a 100 mm board destroys.
Continuity at junctions
Most failures happen where the insulation stops: reveals, floor junctions, joist ends and party walls. The system has to have an answer for each of them.
Documented evidence
A conservation officer needs something to point at. WTA Merkblatt 6-4 is the internationally recognised standard for internal insulation of historic buildings.
Do you need planning permission or consent?
Three different regimes get confused with each other constantly. They are not the same thing.
| Situation | What is required |
|---|---|
| Unlisted house, not in a conservation area | No planning permission for internal work. Building Regulations still apply where the work is a controlled thermal element. |
| Unlisted house in a conservation area | Conservation area controls govern external appearance. Internal insulation does not usually engage them. |
| Listed building, any grade | Listed Building Consent is required for works affecting character, and internal insulation almost always does. Applies to the interior as well as the exterior. |
Carrying out works to a listed building without consent is a criminal offence, not a planning technicality. It is worth saying plainly because homeowners are frequently told otherwise.
What helps a consent application succeed is evidence that the proposal will not harm the fabric. BreatheTherm supplies hygrothermal modelling reports, material data and a record of comparable approvals to support applications. Your conservation officer will usually want to see the proposed build-up, the vapour resistance of each layer, and how reveals and joist ends are detailed.
The build-up
A typical internal wall specification uses four or five layers applied directly to the prepared masonry. There is no frame, no battens and no cavity behind the insulation, which is deliberate: a void behind an insulation layer is where moisture collects.
| Layer | Function | Thickness | λ (W/m·K) | Sd (m) | Guide price /m² |
|---|---|---|---|---|---|
| PerliScratch | Lime-perlite scratch / key coat | 3–5 mm | 0.080 | 0.02–0.04 | £4.60 |
| PerliTherm | Expanded perlite insulating render, hydraulic lime bound | 50 mm | 0.064 | 0.2–0.4 | £45.00 |
| AeroBond | Fibre-reinforced lime-polymer adhesive and basecoat | 3–6 mm | 0.12 | 0.02–0.04 | £20.00 |
| Aerogel layer | Silica aerogel blanket for constrained build-ups | 10 mm | 0.017 | 0.04–0.08 | £23.70 |
| PerliFinish | Breathable lime finish coat | 20 mm | — | 0.10–0.20 | £6.60 |
Guide prices are supply only, excluding VAT, and are indicative. Every build-up is specified per project. See the full cost guide.
Floors are treated on the same principle, using a membrane-free build-up so the slab can continue to dry upwards rather than pushing moisture into the wall base.
| Layer | Function | Thickness | λ (W/m·K) | Sd (m) | Guide price /m² |
|---|---|---|---|---|---|
| PerliMat | Vapour-permeable geotextile separator | 3.5 mm | n/a | Vapour open | £3.00 |
| PerliBase | Mineral insulating floor underbase | 75–200 mm | 0.045 | 0.3–0.6 | £59.25 |
| PerliStruct | Lime-perlite structural screed / underbase | 30–150 mm | 0.080–0.090 | 0.20–0.40 | from £46.00 |
| AeroBond | Fibre-reinforced lime-polymer adhesive and basecoat | 5 mm | 0.12 | 0.02–0.04 | £20.00 |
| Aerogel layer | Silica aerogel blanket | 5 mm | 0.017 | 0.04–0.08 | £23.25 |
| PerliScreed | Vapour-open, membrane-free floor screed | 75–200 mm | 0.045 | 0.3–0.6 | £59.25 |
Guide prices are supply only, excluding VAT, and are indicative. Every build-up is specified per project. See the full cost guide.
Explore the interactive layer guide to see where each component sits in the assembly.
What performance can you expect?
Achievable U-values depend on the existing wall and the build-up depth available. Across the range of specifications the system delivers between 0.17 and 0.45 W/m²K.
A site-specific hygrothermal calculation is produced for every project rather than a generic figure, because the same build-up on a sheltered south wall and an exposed west gable behaves differently. Full technical specification and modelling detail.
Which buildings can it be specified for?
| Construction type | Suitable? | Notes |
|---|---|---|
| Solid brick walls | Yes | The core use case. Typically 215 mm or 330 mm solid brick with lime mortar. |
| Solid stone walls | Yes | Includes rubble-filled and Cotswold limestone construction. |
| Cob | Yes | Vapour-open throughout is essential; cob must be able to dry inwards and outwards. |
| Timber frame with infill | Yes | Wattle-and-daub and brick-nogged panels. Junction detailing is specified case by case. |
| Flint | Yes | Irregular substrate; a thicker levelling coat is usually required. |
| Listed buildings (all grades) | Yes | Subject to Listed Building Consent. We supply supporting documentation. |
| Conservation areas | Yes | Internal work does not alter external appearance, so area restrictions rarely apply. |
| Walls with active damp | Diagnose first | Rising damp, failed rainwater goods or defective pointing must be resolved before insulating. |
| Modern cavity walls | Possible | Not the intended application. Conventional measures are usually more cost-effective. |
| Basements below ground | Case by case | Depends on water table, existing tanking and drainage. Speak to the technical team. |
Evidence from completed projects
Three examples, each a different construction type and a different constraint.
17th century Cotswold farmhouse
Solid stone walls, cob partitions and original lime plaster, under Historic England's strictest requirements.
- U-value achieved: 0.22 W/m²K
- Energy bills: down 52%
Mansion block, 12 units
Internal wall insulation across twelve flats with hygrothermal monitoring installed throughout the building.
- U-value achieved: 0.19 W/m²K
- Zero moisture failures at 36-month review
Bedroom renovation, Upper Park Street
Bare brick walls and a retained lath ceiling, insulated in only 25 mm of PerliTherm.
- Wall heat loss: down 46%
- Ceiling heat loss: down 38%
What happens next
- Free property assessment. A specialist visits, inspects the wall construction, checks for existing moisture problems and measures what depth is available.
- Written recommendation. A proposed build-up, the U-value it achieves, an indicative cost, and a hygrothermal risk note.
- Consent support, if needed. Documentation for a Listed Building Consent application.
- Installation. Typically 2–5 days per room, or 1–3 weeks for a whole house. Lime plaster then needs 3–7 days to cure before decoration.
Book a free property assessment
A specialist visits your property, assesses the wall construction, and produces a written build-up recommendation with indicative costs. No obligation, no hard sell.