Edmonton has a great deal of low-slope and flat roof construction - mixed-use buildings, walk-up apartments, commercial units and a stock of mid-century houses with shallow roofs. Ceiling spray foam insulation in Edmonton is very often about a deck that cannot be ventilated at all.
A flat roof cannot be vented the way a pitched roof can
Ventilation of a roof assembly relies on air moving from a low intake to a high exhaust, driven by slope and buoyancy. Take the slope away and there is nothing to drive it. What gets called a vented flat roof is usually a cavity with a couple of vents in it and no reliable air movement, which is worse than an unvented assembly done on purpose, because it is designed as though drying will happen and it does not.
The long winter is the reason the detail has to be right
A vapour drive that runs outward for four or five months puts moisture at the underside of the sheathing and keeps it there. In a vented pitched roof that moisture has a route out. In a flat deck it does not, and it will condense on the coldest surface it can find. Closed cell foam applied to the underside of the deck removes that surface from the equation by keeping the sheathing warm and by being its own vapour control in the same layer.
Rooftop units and the penetrations nobody insulates
Commercial and mixed-use decks are full of penetrations - drains, curbs, mechanical units, conduit. Each one is a hole through the insulation plane, and on a flat roof they collect. The flat area between them is usually the easy part of the job; the detailing around them is where the performance is won or lost.
Older houses with shallow roofs
A shallow-pitch house from the fifties or sixties often has just enough roof space to be called an attic and nowhere near enough to work in or to insulate to a modern number. Treating that space as an unvented assembly and insulating at the deck is frequently the only route that gets the house to a sensible level.
Ceiling Spray Foam Insulation Edmonton: Common Questions
Can a flat roof be vented properly? Not in any reliable way. Ventilation needs slope to drive it, so a flat assembly with a few vents is designed around drying that will not actually happen.
Where does the moisture end up in a long winter? At the underside of the sheathing, which is the coldest surface in the assembly. Keeping that surface warm is the point of insulating at the deck rather than at the ceiling.
Do the rooftop penetrations matter that much? On a flat roof they are usually where the losses are. The open field of the deck is straightforward; the curbs, drains and mechanical bases are the parts that decide how it performs.
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