Heat can travel through solid walls more easily than through cavity walls. In the UK, houses built before the 1920s generally have solid walls, so improving the insulation of solid exterior walls is the most effective way of keeping the heat in and cutting energy bills. It also cuts condensation and helps to reduce carbon dioxide emissions.
Insulation can be applied to either the external or internal face of the outer wall, and each method has its advantages and drawbacks. Often the best strategy is to combine solid wall insulation (SWI) with other renovation work, whether external or internal, or undertake SWI in phases, for example when redecorating a room..
Internal wall insulation
Several methods can be used to insulate the internal walls. These all have the advantage of maintaining the external appearance of the building, but they reduce the interior space. Broadly, the thicker the insulation, the less heat is lost. However, check the manufacturer's specifications to get the most efficient material.
Internal insulation: careful fitting of insulation around corners helps reduce thermal bridging
The most compact approach involves applying a drylining, typically a laminate of insulation and plasterboard, directly to the wall using adhesive. This should contain an integral vapour control membrane. This is most suitable for straight, even walls with no serious damp problems.
Alternatively, wooden battens can be fixed to the wall and the space between them filled with insulation material. An air gap (at least 25 mm) should be left between the insulation and the wall. Finally, insulation-backed plasterboard with integral vapour barrier is fastened to the battens.
Points to note
- Treat any dampness before installation
- Take care around windows, doors, and at junctions with internal partition walls to avoid creating thermal bridges.
- Vapour barrier should be on the warm side of the insulation
- Seal joints between plasterboard sections and around pipes, etc.
- Seal joints at junctions with floor and other walls
- Skirting boards and furnishings, such as kitchen units or bookshelves, will need to be fixed to battens, not to the insulation material!
- Electrical cabling should be enclosed in metal conduits
External wall insulation
Insulating the wall by adding an extra layer to the outside obviously changes the appearance of a building. But, it does provide protection to the existing walls, and causes less inconvenience to the occupants. If insulation is done as part of a more general renovation, the labour and material costs will be reduced.
External wall insulation
Various systems of exterior cladding or rendering are used. Essentially all involve fastening a layer of insulation to the masonry, then covering it with an external render or cladding. Alternatively, a thick layer of of insulating render is applied to an expanded metal support fastened to the wall.
Points to note
- Work should be carried out by a specialist installer
- The system should have been tested and proven
- Certificated systems with a 25-30 year life are available from members of the Insulated Render and Cladding Association
- Windows may need to be replaced to accommodate the thicker walls
- Planning permission may be required
- Costs can be reduced if insulation is done as part of renovation or redecoration.
Note that recommendations for fire safety with regard to external cladding are under review following the tragic Grenfell Tower fire in 2017.
Costs and benefits
The cost of installing solid wall insulation will vary substantially depending on the technique employed, size of house, and nature of the project. However, the Energy Saving Trust has estimated approximate costings and savings, based on a three-bedroomed semi-detached house.
For internal wall insulation, the benchmark cost of installation is £42 per square metre. For external wall insulation, the cost is reckoned at £8000-23,000, depending on the type of property, with potential fuel bill savings of up to £245 per year, and carbon savings from 500 to nearly 2000 kg/annum.
So payback is very much a long term prospect. However, solid wall insulation, whether internal or external, has the biggest potential for emission savings from the building stock, according to a review by the Energy Saving Trust published in December 2013.
It is vital that any solid wall insulation is carefully designed and installed in sympathy with the style and fabric of the building, its location, and even the habits of its occupants. Beware of 'one size fits all' solutions, which can have potentially serious unintended consequences.
Rendering being applied to external wall insulation.
The Building Research Establishment (BRE) has identified various problems that can arise due to faulty installation of external wall insulation. These are highlighted in the published study of Post Installation Performance of Cavity Wall and External Wall Insulation.
- Overheating, especially with internal wall insulation.
- Increased relative humidity leading to damp and mould, due to increased airtightness not properly addressed (e.g. through extractor fans). Can be associated particularly with untreated thermal bridges.
- Humidity raises the risk of dry or wet rot of timbers, and also insect attack.
- Internal wall insulation may mean that the external wall no longer dries out, so that moisture is retained.
- Short-term reduction in air quality due to the volatile organic compounds (VOCs) used during the installation, such as formaldehyde from adhesives.
- Increased airtightness can cause long-term decline in air quality inside the house.
- Enhanced risk of condensation in adjoining property.
External wall insulation is eligible for funding under the government's ECO (Energy Companies Obligation) programme (see Insulation and heating grants).
Call the Energy Saving Advice Service on 0300 123 1234 to check, or phone Warmer Homes Stafford on 0800 677 1785 for up-to-date local advice on grant funding. See also Insulation and heating grants.