Solid Wall Insulation

As much as 45% of the heat in your home could be escaping through un-insulated solid, stone or timber walls. Insulating the walls internally or externally will significantly reduce heat loss in the winter and also reduce overheating in summer.

We recommend that you consider insulating your solid walls, either internally or externally.

Whilst this is more expensive than many other insulation types it can increase comfort, reduce fuel bills and may add value to the property.  Funding is available through Green Deal and ECO , NEST and PZIL (external only).  Part funding through ECO is available to all, full funding might be available to those on certain benefits, see section on funding schemes.

Investment From £80 per square metre (external)* From £40 per square metre (internal)
Financial Savings Up to £400 per year Up to £380 per year
Carbon Dioxide savings Up to 2,100kg per year Up to 2,000kg per year

*This cost assumes you are already doing repairs to the wall so excludes scaffolding etc.

This work is likely to come under the Building Regulations unless you are only working on a small area. We recommend that you check with your local Building Control.

Recommendations from English Heritage are that traditional wall constructions should be insulated using breathable materials, vapour barriers should be avoided and non waterproof paints used.

External insulation

Options for external insulation include:-

Render applied over insulating batts (the most common method).

Timber or other cladding over insulation board.

Planning permission may be required for external insulation. Window sills and roof overhangs may require modification to accommodate the required depth of insulation.

Internal Insulation

Internal solid wall insulation has several advantages. It does not affect the external character of the building, and can be done one room at a time as convenience and finance allow, although loss of space and internal features can be an issue. Internal insulation options include:

Using timber studs with a porous insulation material, and cladding with plasterboard.

Use a rigid porous insulation material such as Calsitherm or Pavadentro and a lime based plaster.

Insulated plaster board laminates with either polystyrene or phenolic foam are available from 18mm up to 90mm. Phenolic foam versions are about 50% more effective than polystyrene types per inch.  Installing them involves the boards being fitted directly to the inside of the wall or on timber battens. The 18mm boards are useful for door and window surrounds.

Aerogel Insulation Boards can achieve the same insulation value as phenolic type board with around half the thickness, but cost 3-4 times more. Spacetherm, for example, is available from The 10mm thickness costs less than £20 per square metre when prices were last checked. Another company which may supply aerogel is Hillserve (

Flexible linings such as Sempatap, being only 10mm thick, do not take up as much room but have much lower insulation value. They are of most use on walls prone to condensation where there is not room to use thicker insulation boards. Cost is from around £18.00 per square metre plus labour.  See for Sempatap.

Insulated Plaster is less effective than dry lining but is worth considering if loss of space is an issue. See Or Lime Green’s Eco Render Ultra

Thermilate insulating plaster could be applied as an alternative to Sempatap as it has similar properties for a given thickness but would be more easily applied to undulating walls. Thermilate also supply an insulating paint which can be added to either Sempatap or the Thermilate plaster, costing around £1 per square metre more than ordinary paints.

Oak framed walls

Oak framed walls are more difficult to insulate. If the building is not listed, the infill panels can be replaced with a natural insulation material and sealed with lime render. However, if the building is listed, the infill panels may well be part of the buildings listed characteristic and replacement would not be permitted. In this case, the most that can be done is to make sure that the panels are sealed as well as possible and any cracks are filled with a breathable product that is compatible with the infill material.

If in doubt, seek advice from an installer that is experienced in treating traditional walls.

Permitted developments on listed and conservation buildings

Permission should always be sought for working on a property that’s listed or in a conservation area or AONB.

Insulating plasters may be of use where thicker insulations are not permitted.

Extra information:-

  • A vapour barrier may be required on the warm side of internal insulation when fitted between battens unless breathable internal insulation is used.
  • If present sloping ceilings can be insulated at the same time. •We do not recommend multi-foil insulation, as it is controversial.
  • More useful information is available from the following sites:-

Local suppliers and installers include:-  (insulating lime render for external option)

Interest-free loans in Powys. See the funding section for details of grants and loans that may be helpful for solid wall insulation.