Can I skip ridge ventilation on an existing cathedral ceiling shed roof?
Homeowner Long Island New York and I’m re-siding the house. The contractor suggests I add ridge ventilation to the cathedral ceiling shed roof. He suggests a cor-a-vent type ventilation at the ridge.
Right now there is only soffit ventialtion for the cathedral ceiling. The house was built in 1983 so I doubt there are any baffles over the insulation creating an air path under the roof deck. Any opinion on how necessary the ridge ventilation is for this shed roof cathedral ceiling? I’m worried about adding the ridge ventilation and not being able to air seal it properly, allowing cold air to come into the house through the ridge. Thanks for the help!
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Replies
Mike,
The roof structure may already have moisture issues. Codes don't allow you to fill rafter bays with permeable insulation unless you vent them, because warm interior air can make its way though it to the sheathing and condense on the underside. I would choose one of these roof assemblies and emulate it.
https://www.greenbuildingadvisor.com/article/five-cathedral-ceilings-that-work
Hi Malcolm. Thanks for the suggestion. So I’m assuming you don’t think the soffit vents are enough? If I was building a new cathedral ceiling from the start, it would be easy to have continuous soffit to ridge ventilation but it’s much more tricky now with the house already built. You wouldn’t be worried about air getting in the house through the ridge making it colder in the winter?
Specifically, Malcolm, since the house is already built it’s very hard to have an air tight ceiling at the top plate since I can’t caulk where the sheetrock meets the framing.
So it seems pretty easy for the air to get into the ridge vent and just come down the wall into the house.
Mike,
Your main concern is the one I described - moisture accumulation in the rafter cavities. Not all un-vented roofs with permeable insulation fail - yours maybe fine - but they are very risky. That's why codes don't allow them and designers don't specify them.
There is no point in adding venting if there is no vent space above the insulation, and no point in only having venting at the soffits with no way of exhausting moist air. Unfortunately there is no middle ground. What you have with all roofs is a binary choice: Vent, with all than entails (that is a vent cavity with both an air intake at the soffits and an outlet at the peak), or choose an un-vented assembly which doesn't have permeable insulation against the roof sheathing.
Malcolm thanks. One final question. I can more easily add the vent at the cathedral ceiling shed roof ridge bc it is a separate roof. However I have a bunch of other shed roof areas that terminate at a wall. They all have soffit ventilation. Do I also have to cut new holes and put ridge vents there where it butts up against the wall. That’s a riskier detail but I know it’s done. It would entail cutting new holes in multiple areas.
Mike,
The roofs don't know they terminate in a wall, all they know is they have no vent path. If they are cathedral ceilings then yes a continuous vent at the end-wall. If they are trussed roofs you may be able to vent high on a gable end, or install spot vents near the peak.
All this is premised on there being a cavity above the batts in these roofs. If there isn't, there is no point installing ridge vents.
Thanks so much Malcolm!