New Shop construction question insulating the roof

[Scooter Read]

Hi, Im Scott and new to the site,
I have been building a new addition to my current detached garage for a larger shop.
This will be a 24X48 story and a half stick built building. Currently its all framed up and dried in.
My question is with the insulation, I am thinking I would like to use the spray in foam for the roof rafter which are: 2x6" instead of fiberglass batts.
My contractor says if w do this I do not need any vents, no gable vents, no ridge vent, no soffit vents.

This seems a little too air tight to me. I'm in North Carolina, I will heat the shop from an outdoor water stove when needed. Fiberglass batts will only get me to R-19 with the 2x6's . I never heat this much over 55 degrees so the R value may not all that important . My biggest concern is with the spray in foam being so air tight and no vents upstairs. I do have a window at each end though if that makes any difference.

Is the spray in foam the way to go this project?

[Cameron Wood]

Some discussion here:

https://sawmillcreek.org/showthread.php?301619-How-to-go-about-insulating-2x4-vaulted-ceiling-in-detached-garage

[roger wiegand]

The old "buildings need to breathe" theory is pretty thoroughly debunked these days. Creating as tight a building as you can and then controlling ventilation with devices like energy or heat recovery ventilators is pretty much the way to go for energy efficiency and comfort. That's the route we took in building our house 10 years ago and it continues to be the most comfortable I've ever lived in. I spray-foamed the barn which I converted into my workshop and it seems to maintain remarkably consistent humidity year-round without mechanical ventilation. It's a big building with only one occupant so I've never noticed an absence of ventilation. When spraying finishes and such I do open windows for a cross breeze, and set up a big fan blowing out if I'm doing a lot. The building costs under $100/month in the dead of winter to heat in MA.

[Jim Becker]

It's a standard practice with closed cell spray foam under the roof deck to not be ventilated. You're creating an envelope that provides insulation value, air and moisture sealing and additional structural rigidity. If your roofing material is "dying" to be ventilated as your preference, put on the deck and mount the roofing on purlins/strapping above the deck with air flow between the hard deck and the roofing material, such as metal sheeting. That ventilation is purely for the roof material; not for the conditioned space inside the building, but not really required. One other thing, since this is not residential space, you don't have to go "full monty" on the R value up top if you don't want the huge expense that would involve. An R21+ (3-4") of closed cell spray foam under the roof deck provides really good results...because there is no air infiltration. The sealing factor is "yuge" in that respect.

FYI, my shop has closed cell foam for insulation. 2-3" on the walls and 3-4" on the underside of the roof deck. There is pretty much zero air leaking anywhere the insulation is so the biggest source of any loss is at windows and doors because of their nature. But even there, there is minimal air infiltration. It's more about them being less efficient than the walls.

[Tom Bender]

You're in NC so heating is not so much of a concern, so roof R factor is not so important as Jim says. Controlling humidity might be worth some effort. Seal it up and dehumidify in winter. A/C in summer. Your wood will be more stable and rust should be nil. Plant a couple of sugar maples for shade.

[Eugene Dixon]

You're in NC so heating is not so much of a concern, so roof R factor is not so important as Jim says. Controlling humidity might be worth some effort. Seal it up and dehumidify in winter. A/C in summer. Your wood will be more stable and rust should be nil. Plant a couple of sugar maples for shade.

Not sure where the OP is located within NC, but when I lived in ENC, heating days exceeded cooling days, albeit by a slim margin. Go figure. Certainly not MI weather, to be sure.

[Mitch schiffer]

SprayJones on YouTube has some good videos on this as well as anything else related to spray foam.

[Scooter Read]

thanks
for the feed back so far, I am waiting on the quote from the contractor for the insulation part.
I finished all the wiring this past weekend, the roof is next the I can do the insulation part
I am thinking spray the roof rafters and then fiberglass the rest.