Attic Venting for Post-Framed Garage
I’m planning to build a post-framed two-car detached garage in North Idaho, Climate Zone 5. The building will be insulated and minimally heated with radiant heat in the slab. The roof will be dual-pitch with an offset ridge. Rather than attach the roof metal directly to the roof framing as is typical in post-frame construction, I plan to sheathe the roof with 5/8” cdx as a base for a standing seam metal roof. The soffits will be enclosed with T&G cedar. My question is: Given that the sheathing will be attached to the purlins which run perpendicular to the slope, would I be better off installing gable vents and an unvented soffits/ridge or vented soffits and vented ridge? My thought is that the purlins running perpendicular to the slope would effectively disrupt the airflow along the the underside of the sheathing in the soffit/ridge vent scenario. Even though the garage will only be minimally heated, I still plan to do a thorough job air-sealing on the interior, which will get a drywall finish. Code requires me to vent the attic; would gable vents be sufficient? Or is soffit/ridge venting still preferable?
Thanks in advance for your wisdom!
Adam
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Replies
If the garage has a flat ceiling with insulation on it, then vented ridge and soffits will still work fine. The perpendicular purlins will reduce the velocity of airflow along the bottom of the sheathing slightly, but probably not the total volume of airflow at all. That said, unless there is going to be a lot of moisture generation inside the garage, gable vents would probably work just fine.
Ceiling will be flat with insulation blown in above. Probably not all that much moisture generated in the garage, except during winter/spring with snowmelt/rain being tracked in by vehicles and dissipated by the heated slab. Not sure how much to be concerned about that, but it’s definitely the season for condensation around here. Unvented ridge makes me a little nervous, so I’ll prob go soffit and ridge venting if the purlins aren’t much of an obstacle. Thanks for the advice!
Adam,
Does the ceiling follow the roofline, or is there an attic?
The ceiling will be flat with blown in cellulose and a conventional attic space above.
Adam,
As Peter said, it doesn't matter much. As long as you provide adequate vent area either will work well.
5/8" CDX is a good substrate for metal roofing. Just check the specs on spanning in the 4 ft direction. Sheathing is designed to span across its longer dimension. With some, like 7/16" OSB, that means running it vertically over Purlins.
Hi Malcom,
Thanks for the advice. Just to clarify, the purlins are going to be 2x6 @ 12” o.c. set flush with the trusses in joist hangers, which is the spacing that’s required for snow load in my area. Are you recommending I install the sheathing perpendicular to the purlins? Also, i realize I didn’t mention the purlin size/spacing. Does this have any bearing on venting issue?
Adam,
No you are more than fine. It's common to see roofs here strapped with 2"x4"s at 24" oc. That's the kind of spacing that can cause problems.
I don't think the venting will suffer much either. Attics are a lot more forgiving than cathedral ceilings if they don't have excessive air leaks, and adequate volume of air exchange with the outside.
Ah, yeah that makes sense. Thanks again!