Around a third of UK homes have solid (uninsulated) walls, and insulating them is one of the highest-impact retrofit measures — but also one of the riskiest if done badly. The two options, internal wall insulation (IWI) and external wall insulation (EWI), behave very differently, especially with respect to moisture. Understanding the trade-offs is essential before committing.
The head-to-head
| Factor | Internal (IWI) | External (EWI) |
|---|---|---|
| Moisture risk | Higher — wall left cold & wet; dewpoint at interface | Lower — wall kept warm & dry |
| Thermal bridging | Hard — interrupted at every floor & party wall | Better — continuous wrap |
| Embedded timber (joist ends) | At risk of decay | Kept warm and dry |
| Internal space | Reduced (lost off each room) | Preserved |
| Appearance / planning | No external change | Changes façade; may be restricted |
| Disruption | High (rooms emptied, skirtings, sockets) | External; rooms stay usable |
| Cost | Often lower upfront | Often higher (scaffold, render) |
| Thermal mass benefit | Lost (insulation inboard of mass) | Retained (mass stays warm-side) |
Why internal is the higher-risk option
Internal wall insulation moves the warm side of the wall inboard, leaving the original masonry colder and wetter than before. This creates two specific risks: interstitial condensation at the insulation-to-wall interface (which can hit the dew point), and decay of timber joist ends embedded in the now-cold, damp wall. It's also interrupted at every internal wall, floor and party wall, leaving thermal bridges where mould can form. IWI can be done safely — but only with a continuous air barrier, the right vapour-control strategy, insulated returns at junctions, and verification by hygrothermal analysis. (See the dedicated IWI article.)
Why external is usually preferred
Where it's permitted and practical, EWI keeps the whole wall warm and dry, wraps the building in a continuous insulating blanket that controls thermal bridges, protects embedded timbers, retains the wall's thermal mass on the warm side and preserves internal space. Its drawbacks are appearance/planning, cost and the need for good junction detailing — but on moisture and thermal-bridge grounds it is the lower-risk choice. (See the EWI article.)
How to choose
- Can you do EWI? If appearance, planning, boundaries and budget allow, EWI is usually the safer, higher-performing option — choose it.
- If EWI is ruled out (listed building, conservation area, attractive façade, no boundary room), IWI is the fallback — but commit to doing it as a designed, moisture-safe system, modelled (often with WUFI) before installation.
- Mix where sensible — some homes use EWI on rear elevations and IWI only where the front must be preserved.
- Always design the ventilation alongside — both options make the home tighter, so ventilation must be upgraded to match.
- Model the moisture risk first — for any solid-wall insulation, a condensation-risk/hygrothermal assessment is the difference between a safe build-up and a hidden failure.