Adding Insulation Above the Rafters Moves Every Edge

The rafter-line record is careful to describe this as a choice about position rather than quantity. The thermal layer can sit above the rafters, between them, below them, or in any combination, 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.

Each position carries a different set of consequences. Between the rafters the layer is interrupted by structure. Below them it is continuous but lowers the ceiling and moves the control layer with it. Above them it is continuous but raises the covering, which is the case this guide follows.

Raising the covering plane raises where the covering arrives at the wall head, so the eaves projects from a higher line and the whole edge is reconstructed rather than adjusted. The junction record describes the eaves as already congested, with the underlay discharge, the insulation overlap, any ventilation inlet and the fascia and gutter all landing in the same shallow space, and adds that the depth available narrows exactly where the two insulation lines need to overlap.

The rafter-line record names this junction as usually determining whether the roof has a continuous thermal line or a break at its lowest point, because the insulation on the slope has to meet the insulation in the wall around a wall plate, a gutter and often a restricted rafter depth. Adding a layer above the rafters changes the geometry of that meeting.

The verge is the gable edge, and the published junction record describes it as the position most exposed to wind uplift, where coverings are lost first and the fixing requirement is correspondingly different from the field. It also has to shed water sideways without delivering it onto the gable wall, and the last course of covering has support on one side only.

Because three requirements meet on that one line and all of them are fixed relative to the covering plane, a verge does not extend upward when the plane rises. It is rebuilt in a new position, with its own weathering and its own restraint, which is why the record lists it alongside the eaves and the abutment as something that moves.

Where the slope runs into a wall, the rafter-line record states that the covering, the underlay and the insulated line all stop at different heights, and that a raised covering plane moves the flashing line, so the abutment is redrawn whenever insulation is added above the rafters.

The abutment record explains why that is more than a roofing operation. Weathering there is layered: the roof turns its own covering up the face, the vertical element oversails that upstand with a flashing retained in a chase or a bed joint, and in a cavity construction a tray inside the wall discharges above the cover flashing. Moving the line moves where the chase has to be, and the concealed tray sits inside masonry that the roof detailer cannot see and cannot add to later without opening the wall.

The rafter-line record notes that where insulation sits above the rafters, a rooflight kerb grows with it. The kerb record explains why that is not a cosmetic change: the kerb takes over the structural and enclosing duties the opening removed from the deck, gives the waterproofing a face to turn up and a level at which to be held, and carries the insulation and the vapour control around an aperture that is colder than the roof around it.

It also sets the terms for the rest of the unit's life. The record observes that how high and where a kerb stands governs whether the glass can be reached for cleaning, whether an opening light has room to swing, and whether the upstand can be worked on without disturbing the roof covering. A kerb that grows is therefore a decision about maintenance as well as about weathering.

There is also a load path question. The record describes counter-battens and the batten zone as the means by which the covering load is brought back through the thermal layer to the structure below, where insulation sits over the rafters, and notes under movement that the covering plane can now be carried on layers rather than directly on the rafters, so fixings pass through compressible material.

That is a question for the roof designer rather than something settled on site, and the record asks it directly: how is the covering supported and fixed where insulation sits above the rafters. It sits alongside the structural question of whether the roof is being asked to carry a covering of different weight at the same time.

The last consequence is the least visible. The record warns that adding insulation to an existing pitched roof without assessing what is already in the build-up can remove a drying route the roof is currently relying on, and that filling the rafter depth completely removes the vented space above the insulation so the drying duty transfers to the vapour behaviour of the underlay and any counter-batten cavity.

It treats these as a combination rather than as independent selections, and states that a professional assessment governs the combination. In an existing roof it adds a prior question: what has actually been established about the layers already there, before anything is added to them.