Venting Open Ceilings – A Special Case

If you have the opportunity to visit one of those grand old cathedrals back in England, take time to glance up at the vaulted stone ceiling. Setting the detailing aside, it’s a vast array without a rafter in sight. But there’s always a steeple somewhere, venting condensation and moisture into the fresh air.

They had ample time to build those stone buildings way back then. In fact, cathedrals could take a century to complete. Canadians don’t have the luxury of time, and so we use a network of timbers to support our roofs. Most times we conceal these behind ceilings, and seldom give them a second thought.

That’s All Well and Good Unless We Have an Open Ceiling

If we have a closed ceiling, we may be able to get away with soffit ventilation feeding vents on the end gables. Or alternatively, a couple of extractors high up on the roof surface. The odd drop of condensation from the rafters is no big deal. It drips harmlessly onto the ceiling insulation, and soon evaporates.

However, and that’s the point of this article, if we have an open cathedral ceiling with exposed timbers, any humidity between them will tend to drip into the living space below. That’s because it is trapped in a three-sided box. Gravity dictates it has nowhere else to go, except down. Unless, of course we can vent it out.

venting open cathedral ceilings in calgary
Continuous Roof Vent (Image U.S. Oak Ridge National Laboratory)

Venting the Spaces Between Rafters in Open Ceilings

At first thought, we might be tempted to vent each open ceiling bay individually. But that would be expensive and cumbersome, things we wish to avoid. Therefore, we install a continuous roof vent along the ridges of our asphalt shingle roofs. The image is a cross section of a single bay in this elegant solution.

However, in practice the continuous roof vent presents as little more than a shadow line beneath the shingle capping. The illustration show how the air flows, per rafter bay, from vents in the soffits all the way to the fresh air above. The benefits include reduced humidity against the open ceiling, and far less chance of ice damming on the roof overhangs.

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