Ventilated (Cold) Pitched Roof Assembly

In a ventilated pitched roof the insulation lies across the ceiling, following the horizontal plane rather than the slope. The roof space above is left unheated and open to outside air, and the covering, underlay and roof structure all sit within it at close to outdoor temperature. The envelope of the building turns the corner at the ceiling, not at the rafter.

The test against its nearest sibling, the insulated rafter-line roof, can be made from inside the building. Put your head through the hatch: if the insulation is under your feet and the space around you is cold and shares air with outside, the roof is this assembly. If the insulation follows the slope above you and the space is part of the heated building, it is a rafter-line roof and belongs in that entry.

The roof space works as a buffer that tolerates small amounts of moisture because outside air moves through it continuously and the covering above is not a sealed skin. Nothing in the void has to be perfect for the arrangement to succeed, so long as that exchange is not interrupted. What it does not tolerate is having its air path closed or being asked to remove a moisture supply it was never expected to receive.

That vulnerability is what makes this a system rather than a detail. Boarding the void for storage, fitting recessed lights in the ceiling, leaving the hatch unsealed, drawing new cables through the ceiling line or extending an extract duct into the space are each ordinary domestic acts, and each removes part of the arrangement. Read individually they look harmless; read as a system they are the failure.

The insulation and the ventilation inlet compete for the same space over the wall head, and the competition is decided in a place nobody sees. Carry the insulation right out to the wall to meet the wall insulation and it can block the inlet; hold it back to keep the inlet open and there is a cold wedge over the top of the wall. Resolving both at once is the point of the eaves detail rather than a refinement of it.

The ceiling decides how hard everything above it has to work. With a continuous barrier, what reaches the void is limited to what passes through the construction itself. Open that plane with recessed fittings, an ill-fitting hatch or an extract terminating in the space, and warm moist air is delivered into an unheated volume through the opening rather than through the material, which is a different order of supply. The coldest thing it can find there is the underlay.

Boarding out the void changes the assembly in ways that read as storage rather than construction. Boards laid on the ceiling joists compress the insulation beneath them, and they cap the joist bays so air can no longer move freely across the space. The result is a roof that still has its openings, still has its insulation and no longer has a working void.

The underlay ties the covering to the void. A vapour-open underlay allows some drying upward and reduces what the ventilation alone must achieve; a closed one puts the whole duty back on the openings. Underlay that sags between rafters can also drop into the path at the eaves, so a choice about a sheet becomes a change to the airflow it sits above.