Ventilated Voids That Have Quietly Stopped Ventilating

The record puts the first rule bluntly: an inlet without an outlet is not a partial ventilation system; it is not one at all. Air entering a closed space displaces nothing and stops. It then adds the case that is worse, where a high-level outlet exists but the inlets have been blocked: the outlet still draws, and the air it draws is taken from inside the building through whatever gaps exist in the ceiling, so the roof ends up pulling moisture into the void rather than removing it.

The openings work through the difference between them rather than through their own size. Placed on opposite faces or at different heights they are exposed to different pressures and air crosses the void; placed together on one side they sit at almost the same pressure and the void breathes barely at all. That is why the record says the location of openings is agreed before their provision is counted.

The cold deck record describes joists running across the flow direction, trimmers around openings, upstands for rooflights and anything laid into the space as each dividing the void into bays, and concludes that a perimeter arrangement which ventilates the roof as a single space is not ventilating a roof that has quietly become a set of separate compartments.

Geometry does the same thing at a larger scale. The ventilation record names hips cutting off the run to a ridge, valleys interrupting the eaves line, dormers and chimneys dividing the space, and small enclosed areas above bay windows or over porches ending up connected to nothing. A nominal arrangement of eaves and ridge openings describes the simple part of a roof and leaves the awkward parts unserved.

The ventilated pitched roof record treats this as a change to the roof rather than a use of a space. Boards laid on the ceiling joists compress the insulation beneath them and cap the joist bays so air can no longer move freely across the space, and the result is a roof that still has its openings, still has its insulation and no longer has a working void.

The same record notes that the void is the only component people routinely fill with belongings without regarding that as a change to the roof, and that anyone entering it is standing on the thermal layer while anything left there is sitting in the ventilation path. It also warns that entering a roof space involves standing on ceiling construction that may not be intended to carry a person.

This is the case the records return to most often, because the work is correct in itself. The ventilation record lists the thermal layer among its components precisely because, where it is carried up to the underside of the deck or pressed into the eaves, it becomes an obstruction in the circuit while still looking like correctly installed insulation. It names the misunderstanding directly: insulation installed to a high standard can be the reason a roof no longer ventilates, because the path and the thermal layer share the same space.

The cold deck record describes the competition as not neutral. Push the insulation up to the deck and the ventilation path disappears while the roof looks fully insulated; hold it down to preserve the path and the thermal layer is the thinner of the two claims on the depth. The bypass record supplies the component that resolves it, a baffle that holds the path open while keeping moving air off the insulation face.

Voids behind coverings and claddings fail the same way. The published junction record describes tile battens lying directly on the underlay as acting as small dams, holding water against the batten along its whole length, and counter-battens running up the slope beneath them as what lifts the tile battens clear so water passes underneath and continues to the eaves. The crossing point is where that path is either created or absent, and it is concealed for the roof's whole life.

On a wall the same logic applies to air as well as water. The timber frame record says the battens run so that they never dam the void they were installed to create, and that they hold the cladding off, keep the void open for drainage and ventilation, and carry wind load through the membrane into the frame. A batten run the wrong way does all three jobs except the second.

The eaves record describes the mechanism that catches the most careful owners. Fascia, soffit and trim details that close the edge neatly can close the void with it, which turns the ventilated concept into an unventilated one without any visible change. It adds that renewing fascia and soffit boards is often treated as decoration although it disturbs the underlay edge, the inlet and the gutter line at the same time.

The ventilation record explains why that happens so easily: the inlets sit in the zone where insulation, fascia, soffit and gutter are all being fitted at once, and the path is completed by several trades who are each doing something else, none of whom owns the circuit. It also notes that openings close over time with nests, cobwebs, wind-blown debris and paint, so a circuit that worked at completion may not be working now.

Retrofit cavity fill is the case where the void disappears entirely. The record says nothing else about the wall changes: the same outer leaf, the same ties, the same trays and the same drainage exits remain in place, but the space they were all detailed around is no longer empty. It names the assumption to avoid, that an unfilled void offers nothing, and says it was doing a job which the fill has to take over rather than simply add to.

It also names what has to be found first. Vents, flue ways, ducts, boxes and tray ends each become either a place fill escapes into or a designed air path that is blocked, and because none of these is drawn on an ordinary existing house, they are located by inspection of both faces and of the roof space rather than assumed from the age or the style of the building.

The cold deck record asks the question that has no comfortable answer: how will anyone confirm, after the ceiling is closed, that the air space above the insulation was left open. It describes holding insulation clear of a deck across a whole roof, and keeping every bay connected to the openings, as difficult to achieve and almost impossible to verify once the ceiling is up, and says that verification problem is a real part of the assessment.

The ventilation record generalises it: a circuit cannot be inferred from a finished roof, because openings can exist without a path between them. Recording the intended arrangement is what lets a later alteration be judged against a design rather than against appearances. That record, and an inspection before closing, are the only things that survive the moment the void disappears from view.