What U-Value Should I Target For My Retrofit Project? – Can I Just Ask? | Ep.14

U-value targets for retrofit are often treated as a straightforward regulatory question, but the reality is considerably more nuanced. Building control generally steers projects towards 0.3 W/m²K, with a threshold value of 0.7 — and Part L does allow for more conservative targets where a strategy has a payback period exceeding 15 years or reduces room area by more than 5%. In practice, that often means 0.7 becomes the realistic working target for internal wall insulation. For external insulation, hitting 0.3 is relatively uncomplicated — 120mm of wood fibre will get you there — but internal insulation brings a different set of considerations that make chasing the same number both harder and potentially counterproductive.

The starting point matters enormously. An uninsulated solid wall typically sits somewhere between 1.5 and 3.0 W/m²K, and even 20mm of natural fibre insulation can roughly halve that conductive heat loss. That’s a significant performance improvement even though it falls well short of regulatory targets — and it comes with very little moisture risk. As insulation thickness increases internally, so does the stress on the wall fabric, the risk to embedded timbers, and the complexity of the moisture analysis required to understand what’s really happening within the build-up.

The practical upshot is that thinner layers of internal insulation, installed well, often deliver the most meaningful improvement relative to risk. Even a modest layer isolates the occupant from cold masonry, raises internal surface temperatures noticeably, and transforms the comfort of a space — without pushing the wall into territory where moisture becomes a serious concern. Chasing a 0.3 U-value internally is sometimes possible, but it should be the result of a thorough specification process, not a starting assumption.