How Roof Insulation Affects Your SAP Calculation

Roof insulation plays a bigger role in SAP than you’d assume. It’s one of the simplest elements of a build, yet small choices in the build-up can shift a design from a comfortable pass to an unexpected fail. Thickness is part of it, but the way the roof is constructed matters just as much.

SAP 10 sets a strong notional standard for roof performance, which means the model expects modern homes to lose very little heat through the top of the building. When a design falls short of that expectation, the numbers tighten quickly. Roof insulation becomes one of the first places to look when a design is close to failing.

Insulation Depth. Is 270mm Enough?

The obvious factor is insulation thickness. Many drawings still show 270 mm of mineral wool in a cold roof. It meets common guidance and has been the standard for years. But in SAP terms it often isn’t strong enough, especially once you include the effect of timber. Increasing to 400 mm is one of the simplest ways to bring the U-value down to a level that aligns with the notional dwelling. It’s cheap, requires no redesign and has no impact on the appearance of the home.

Timber Roof Joists & Thermal Bridging

The part that most people miss is the influence of timber. Insulation isn’t the only thing in the roof. Joists and noggins all bridge heat from the warm side of the ceiling into the loft. Timber isn’t a good insulator, so every extra piece slightly increases heat loss. In a U-value calculation this is treated as a percentage of the roof area. If that percentage is higher than expected, the overall performance drops.

This is why the way the joists are arranged matters. Over-tight spacing, unnecessary noggins or heavy trimming for services can increase the bridging factor. Even if the insulation depth looks good on paper, the combined effect of extra timber can weaken the U-value more than expected. Keeping the structure clean and avoiding over-engineering helps preserve the performance the architect intended.

Why ceiling-level insulation usually performs better

Rafter-level insulation is another area where performance is often overestimated. Cutting PIR boards between rafters looks neat, but rafter insulation carries much more thermal bridging than horizontal wool. The rafters continue from the warm side to the cold side, so they act as a direct heat path. Depth is also limited by the rafter height, which often means compromising on thickness or adding extra layers to reach the right U-value. It can work, but it rarely matches the cost or performance of simple horizontal insulation unless the construction is planned very carefully.

This is why ceiling-level insulation remains the most reliable approach for SAP. It avoids most of the bridging, allows generous thickness and gives predictable performance. When a design is close to the limit, improving the roof is often the easiest fix. A little more mineral wool, a cleaner joist layout or a small change in material can restore the margin without touching anything else in the build.

If you want the roof, walls and glazing balanced properly from the start, our SAP calculation service can help shape the design before it reaches Building Control.