What U-Value should I target in my roof? – Can I Just Ask? | Ep.30

Roof U-value targets sit in a slightly different context to wall targets, for a couple of reasons. The regulatory starting point for an existing roof being improved without changing the rafters is 0.35 W/m²K — a threshold that’s achievable with a relatively slim build-up, such as filling between 100mm rafters and adding a 20–40mm wood fibre board on the inner face. That’s a significant improvement over the uninsulated state typical of many older buildings, and in the context of historic buildings with small rafters and a reluctance to alter the visual profile of the roof, 0.35 is often the practical working target.

The stronger recommendation, however, is to push below 0.2 wherever possible — and where a new roof is being installed, building control will generally require a target of 0.15. The reason isn’t just energy efficiency, though that benefit is real. It’s comfort, and specifically summer comfort. Roofs are exposed to far more solar radiation than walls, meaning the risk of overheating through the roof plane is considerably higher — and a well-insulated roof with a dense, high-thermal-mass material like wood fibre dramatically shifts the time at which that heat reaches the interior, often beyond the point where external temperatures are already falling again. The comfort difference between a 0.35 and a sub-0.2 roof on a warm summer night is immediately and tangibly noticeable.

The build-up required to reach 0.15 is more substantial — typically 150mm rafters combined with around 120mm of wood fibre board — but it remains a manageable construction and one that pays dividends across both seasons. The moisture risk in a well-designed roof build-up is also relatively straightforward to manage compared to walls: roofs aren’t subject to wind-driven rain ingress in the same way, and good detailing of the airtightness and vapour control layer takes care of the primary condensation risk. The conclusion here is clear: if you’re going to insulate a roof, it’s worth going further than the regulatory minimum.