How do I insulate a solid wall?
Why solid walls are different, internal against external, and the moisture problem that makes this a specialist job.
- Difficulty
- advanced
- Time
- 1 hr
- Read
- 2 min
- Safety
- warning
Short answer
Internal insulation is cheaper but loses room space and risks condensation inside the wall; external is more effective and more expensive and changes the appearance. Either way, the vapour behaviour of the materials matters more than their insulation value.
Houses built before about 1920 usually have solid walls, and they were designed to absorb and release moisture. Insulating them is worthwhile, and it is also the one insulation job where the wrong specification causes serious damage rather than merely underperforming.
Safety
Step by step
- Confirm the wall really is solid.Solid walls are typically thicker than 260mm, often with a header-and-stretcher brick pattern. Cavity walls have a very different and much cheaper solution.
- Understand the moisture risk before anything else.Adding insulation makes the original wall colder, so moisture can condense inside the build-up. This is what damages timbers and causes hidden mould.
- For older buildings, favour breathable systems.Wood fibre, lime plaster and similar materials let moisture move through. Sealed plastic-based systems on a breathable wall are how the serious failures happen.
- Weigh internal against external.Internal: cheaper, room by room, loses 50-100mm of space, and disrupts skirting, sockets and window reveals. External: better performance, no lost space, no internal disruption, considerably more expensive and changes the appearance.
- Check the planning position.External insulation changes the exterior and often needs permission, particularly in a conservation area or on a listed building.
- Deal with the cold bridges.Window reveals, floor and ceiling junctions. Untreated, these become the coldest spots and attract condensation and mould exactly where you have just decorated.
- Get a proper assessment.This genuinely needs someone who understands moisture in your specific construction. The cost of the assessment is small against the cost of getting it wrong.
- Do the cheaper measures first.Loft insulation, draught-proofing and floor sealing are cheaper per unit of heat saved and carry no comparable risk.
Tips
- Cavity wall insulation is a much cheaper and simpler job. Confirm which you have before assuming this applies.
- Insulating one room internally creates a boundary at that room's edges. Get advice on the junctions rather than stopping at the wall.
- Grant schemes often cover solid wall insulation because it is expensive. Check what is available before paying in full.
Questions people ask
Is internal or external wall insulation better?
External performs better and does not lose room space, but costs considerably more and changes the appearance. Internal is cheaper and can be done room by room.
Why is solid wall insulation risky?
It makes the original wall colder, so moisture can condense within the construction. In older breathable buildings the wrong materials trap that moisture and rot the structure.