How Much Internal Insulation Can You Add? – Can I Just Ask? | Ep.15

How much internal wall insulation you can safely add is one of the most common questions in retrofit — and one of the most poorly understood. The starting point is wall thickness. A 100mm single-skin brick wall offers very little scope: 20mm is realistic, 40mm an absolute maximum. The standard 225mm nine-inch brick wall is more workable, with 60–80mm typically achievable, and possibly up to 100mm depending on location. Geography matters significantly here: in the relatively dry south-east of England there’s considerably more latitude than in wetter, colder climates like Manchester or the west coast of Scotland, where wind-driven rain and lower temperatures reduce what can safely be added. Stone walls are a different challenge again — so variable in type, porosity, and exposure across the country that every project sits somewhere on a spectrum of risk, with conservative build-ups almost always the right approach unless rain screen cladding or render is being added to the outside.

But thickness is only part of the picture, and arguably not the most important part. Post-occupancy evaluation consistently shows that a thinner, well-installed system with good airtightness and continuity outperforms a thicker one that’s been poorly executed. The question of how much insulation to add should always be considered alongside how the building is heated, ventilated, and maintained — not in isolation.

The real limiting factor in many projects is embedded timber. Physical condensation doesn’t need to form at the insulation-to-wall interface for damage to occur: at around 87% relative humidity in the surrounding masonry, joist ends can accumulate enough moisture to facilitate decay — well below the threshold a Glaser analysis would flag. It’s this sub-condensation moisture risk that shapes specifications more than any theoretical U-value target, and it’s why the answer to how much insulation you can add is almost never as straightforward as a single number.