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 a cost-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.
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 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, their benchmark cost of installation is £42 per square metre, giving annual savings on heating bills of some £470 and a carbon saving of 2.4 tonnes.
For external wall insulation, the cost is reckoned at £5000-6000, giving savings of about £500 per year, and carbon savings of around 2.5 tonnes.
Grants
External wall insulation is eligible for funding under the government's Carbon Emissions Reduction Target (CERT) programme. This obliges electricity and gas suppliers to promote energy efficiency measures, in part by providing grants and special offers. The availability and level of these do vary, and it is worth shopping around to get the best deals.
Rendering being applied to external wall insulation