Insulated Rafter-Line Pitched Roof Assembly

A rafter-line insulated roof follows the slope rather than the ceiling. The insulated envelope turns the corner at the wall head and runs up the pitch, so the space beneath the roof is part of the heated building and can be occupied, lined and lived in. Everything that a loft would have contained now sits inside the envelope with the people.

Against its nearest sibling, the ventilated pitched roof, the test is where the boundary of the heated building runs. Look at the underside of the slope from inside: if it is lined and the room extends up into the roof shape, the insulation is on the rafter line. If instead there is a flat ceiling with a cold, vented space above it, the roof belongs in the ventilated pitched entry regardless of how much insulation is present.

The thermal layer here can sit above the rafters, between them, below them, or in any combination of those positions, and that is the decision the whole assembly follows from. It is not a choice about how much insulation there is; it is a choice about where the rafter sits in the temperature gradient, which decides what the underlay has to do, whether a ventilated space above the insulation exists at all, and where the air and vapour control layer has to run.

The assembly is also unusually sensitive to the perimeter. A layer added above the rafters lifts the covering, which moves the eaves, the verge, the abutment and every kerb; a layer added below the rafters lowers the ceiling and changes head height at the point where the roof meets the wall. Neither is a local addition, which is why insulation position is settled before the roof edges are drawn.

Insulating only between the rafters leaves the rafter running through the thermal line, so the coldest internal surfaces sit in stripes following the structure. Adding a layer below the rafters covers that pattern but moves the control layer inboard; adding a layer above keeps the rafter warm but changes how the covering is carried. The pattern of temperature across the ceiling is therefore a direct read-out of where the insulation was placed.

The control layer and the service zone are a pair. Where the control layer sits directly behind the lining, every downlight, socket and fixing punctures it, and the assembly relies on repairs made by trades who are not thinking about it. Where a battened zone is provided inboard of it, the same work happens in a void, and the layer that has to be continuous is left alone.

Filling the rafter depth completely removes the vented space above the insulation, and the drying duty transfers to whatever is left: the vapour behaviour of the underlay and any counter-batten cavity above it. A closed underlay with no space beneath it and full-depth insulation removes every drying route at once, which is why these are not independent selections and why a professional assessment governs the combination.

Insulation above the rafters lifts the whole covering plane and pushes every edge of the roof outwards with it. The eaves has to come further out, the verge has to be rebuilt in a new position, the abutment flashing sits higher and any rooflight kerb grows. A decision taken about a layer in the middle of the roof therefore arrives as a redesign of the perimeter.