Solid wall insulation

If your home was built before 1930, it is likely to have solid walls rather than cavity walls. Solid walls allow twice as much heat to escape as cavity walls. While they are more expensive to insulate, the savings and comfort improvements are significant.

Internal vs External insulation

You have two main options for insulating solid walls:

1. External Wall Insulation (EWI)

This involves fixing a layer of insulation material to the outside of the house, which is then covered with a special render (plaster) or cladding.

  • Pros: Does not reduce floor space, improves the appearance of the house, protects the brickwork, and minimizes disruption inside.
  • Cons: Expensive, requires scaffolding, may require planning permission (especially in conservation areas).
  • Best for: Detached or semi-detached homes where the brickwork is in poor condition or where a new look is desired.

2. Internal Wall Insulation (IWI)

This involves fixing insulation boards to the inside of the external walls, or building a stud wall filled with insulation.

  • Pros: Can be done room-by-room, usually cheaper than external insulation, maintains the original look of the outside of the house.
  • Cons: Reduces floor space (by about 100mm per wall), requires moving skirting boards, radiators, and light switches, and can be disruptive.
  • Best for: Terraced houses, listed buildings, or homes in conservation areas where the exterior cannot be altered.

Costs and Savings

Solid wall insulation is a major investment:

  • Internal: Typically costs between £7,000 and £10,000.
  • External: Typically costs between £10,000 and £15,000.

However, an average detached home can save over £450 per year on energy bills.

Important: Ventilation and Damp Solid walls need to “breathe” to manage moisture. If you insulate a solid wall incorrectly, you can trap moisture and cause damp and rot. It is essential to use a qualified Retrofit Coordinator (under the PAS 2035 standard) to ensure the system is designed with appropriate ventilation.

Key considerations

  • Planning Permission: External insulation changes the appearance of your house. Always check with your local authority before starting work.
  • Grant Funding: Due to the high cost, solid wall insulation is a priority for grants like the Boiler Upgrade Scheme (if part of a whole-house retrofit) or ECO4.