Yet another roof venting question
MALCOLM TAYLOR
| Posted in General Questions on
By far the most common method of venting trussed-roofs is to couple the soffit air intakes with a series of roof vents situated near the peak. Drive through any subdivision and the houses sprout them like mushrooms – and if they are sized appropriately they seem to work quite well.
One of the injunctions repeated here is that having a combination of gable and ridge vents can be problematic because they can short-circuit the movement of air from the eave to peak. Why would having those two vents near the peak cause problems when a series of roof top vents doesn’t seem to?
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Replies
Malcolm,
Gable vents won't necessarily cause problems, especially if the house has an airtight ceiling. But on windy days, gable vents are less effective -- you tend to get air entering the gable vent on the windward side of the house, and air leaving through the gable vent on the leeward side. That provides less air flow than would occur if the air entered at the soffits and exited at the ridge vent.
The soffit vent and ridge vent combination is less affected by wind -- although obviously, wind is a strong factor in all attic venting scenarios.
Hey Malcolm.
I always refer to Bruce Harley's response to this question in FHB: Should I Close Off My Gable Vents?.
And I have heard the same sentiment over and over again from others: don't overthink the roof venting, just make sure that there is an effective air barrier at the attic floor.
As Joe Lstiburek says "Where the air leaves isn’t as important — whether it’s a ridge vent, or mushroom caps, or gables." IMO, this includes any combination thereof. But do pay attention to high/low vent ratio. And ceiling air sealing.
Most concerns about airflow short-circuiting are unfounded. Lots of mixing occurs in any open space.
The biggest function of roof venting is to keep the air under the roof similar dew point as the air outside. Holes of reasonable size anywhere will do this, diffusion through these openings is a powerful driving force even without airflow.
The air flow with ideal vent opening arrangements is just a nice bonus.
Thank you all for the replies. I am in the midst of replacing my roof and needed a bit of reassurance my new venting strategy made sense.
Nothing like having to work on a steep cut-up roof you designed yourself to make you rethink the virtues of simple shapes.