To VENT or NOT VENT, that is the question…
Hello!
Looking for help on how to properly vent a roof assembly for a pole barn garage we plan to have build with the pictured post and rafter design. What makes me wonder how to more easily vent is due to the rafters being perpendicular to the ridge which would be the normal method for venting. Picture 2, shows our assembly, with planned plywood roof decking, vapordry sa, cedar breather vent mat and standing seam metal roof. OR, do I skip the venting, ensure good airsealing and insulate with spray foam with an unvented roof assembly? Wasn’t sure if this was an option in my zone/climate.
Our home is in climate zone 5B near the central cascade mountains in Eastern Washington State. Cold and snowy winters, warmer summers.
Thanks for the help!
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Replies
mountaincabin,
There is no way to vent the assembly as shown. If you want to use an vented assembly you will need to add another layer under the sheathing that allows airflow from the eaves to the ridge. Otherwise your alternatives are either foam board above the sheathing, or closed cell spray foam between the rafters.
If I were doing it I would:
- Run a membrane (Tyvek Commercial, taped as an air-barrier) over the rafters and then strap with 2"x4's @ 24"oc, before sheathing the roof.
Have you considered running beams along the tops of the walls, and using scissor trusses instead of the pole roof? That would simplify the insulation and also get rid of the posts in the garage.
You could drop a ceiling. So long as there is an air gap between the bottom of the purlins* and the top of the insulation vents at the eave and ridge would work.
*(Framing members in a roof that run horizontally are purlins.)
> "So long as there is an air gap between the bottom of the purlins* and the top of the insulation vents at the eave and ridge would work."
I don't think so, in this case. The space between each set of purlins would be sort of dead zone with no air movement, only the top two spaces immediately opposite the ridge vent would see any actual air movement. The rest of the roof would have open air UNDER the assembly, but no air movement between the insulating material and the underside of the roof sheathing. Moisture would still get to the sheathing and condense there without a vent channel to carry it away. I do think a drop ceiling would be better than nothing, but I still consider it a risky assembly.
I agree with Malcolm here: closed cell spray foam under the sheathing, or a new vent space above the sheathing, or rigid foam above the sheathing, are your only safe options here.
Bill
Malcolm, I thought of this same idea, taping a vapor barrier then adding 2x4s, then sheathing. My only hesitation is that this will add quite a bit of cost for me vs. adding insulation later with no vents. Just wasn't sure I could do unvented assembly in my zone as I understand vented is standard.
I don't like spray foam but in this case it would allow me to move forward as planned with a future opportunity to insulate vs. change building design now and add costs. Plus eliminating vents would be nice as we are in wildfire zone which in this case would add further protection by not having the vents at all (we have vulcan vents on our house). Thoughts? Spray foam unvented assembly ok in zone 5 cold climate?
Unvented assemblies done with closed cell spray foam are generally safe in all climate zones due to the insulating material and that it's fully adhered. It also has the advantage of going up quick. The big downside is cost. Do make sure you get a competant installer too -- nearly all issues with spray foam are due to inexperienced crews.
Bill
mountaincabin,
Given that, spray foam may well be the best option for you.
The question is, was the structure engineered to carrier the weight of the insulation and a drywall plus the local record snow load. In most pole building they have value engineered the structure to the lowest price point that will not collapse.
Note close cell spray foam is much more expensive than open cell foam. If you get talked into open cell moisture can get thru it and condense on the structure and rot it away.
If this house is still in the planning stages and the goal is a tight well insulated space. I think you should sharpen a pencil and do the math and a fill budget down to completion. What you will find is every dime you saved by starting with a pole building is going to cost you 15 cents by the time you make it tight and well insulated.
Most of the time people don’t ask how to make a silk purse out of the sows ear of a pole building until they have had it built. If that is the case one of the spray foam options in this article will work.
https://www.finehomebuilding.com/project-guides/insulation/five-cathedral-ceilings-that-work
Walta
Walta,
I agree I'm not generally a big fan of pole buildings, but as I read the OPs description, this is an insulated garage, not a house.
I don't disagree, problem here is our builder had to cancel our build due to problems he found on a small reno project he was doing now will take most of year and has next year already booked. We had a window and lost it with our stick frame design. This pole barn building is a plan B and I'm designing it will stick built features. Thanks for the comment but now just looking for ways to insulate at a future time without going in debt. I think I have a solution with those who responded here so thank you.
Are the roof and siding already on?