Solid Wall Insulation: Is External or Internal Insulation Better? – Can I Just Ask? – Episode 2

For solid-walled properties, the choice between external and internal wall insulation is one of the most common questions in retrofit — and the honest answer is that both have genuine merits and neither is universally better. External wall insulation keeps the work outside the building, avoids disrupting occupants, allows thicker insulation layers with lower moisture risk, and can improve the weather resistance of porous or absorbent masonry. It also has the effect of warming the wall fabric itself, which is thermally advantageous and reduces condensation risk within the build-up. The practical downsides are significant though: roof overhangs, boundary constraints, adjacent buildings in terraces or semis, existing services, and the sheer visual impact of adding a substantial layer to the outside of a building all need to be resolved at the design stage — and the work is entirely weather-dependent.

Internal wall insulation brings its own set of advantages that are often underestimated. Even a relatively thin layer has a dramatic effect on perceived comfort, because it raises internal surface temperatures quickly — the wall stops feeling cold to the touch and radiant heat loss from occupants drops accordingly. It can also address airtightness in the same operation, making it an efficient combined measure. The trade-off is that it reduces floor area, requires the building to be occupied or managed during works, and demands a carefully considered specification to manage moisture risk — particularly where there are embedded timbers, absorbent masonry, or limited capacity for inward drying.

In practice, the two approaches aren’t mutually exclusive. Some projects benefit from a combination — external insulation on elevations where it’s feasible, internal where constraints make external impractical. The key in both cases is that the specification needs to be appropriate for the specific building, not applied as a generic solution. That matching of approach to building type and condition is where the real work lies.