The vents would be there to reduce the amount of heat build-up, therefore they

would vent the space above the insulation and under the roofing material. Some would argue that ventilation is not required if insulation is sprayed under the roof sheathing that the roofing (asphalt shingles) is laying on.

Why do you ask?

If the roof is insulated with spray foam, the attic becomes part of the conditioned space and will be closer in temperature to the space below. If no conditioned air is fed to the attic, it will generally end up warmer in summer than the space below.

we did exactly what you are talking about and installed sprayfoam in our house building a wall with the foam between house and garage. We then put Sprayfoam in our garage ceiling rafters and gables and sealed off our vents.

when you spray foam an attic you are enclosing your attic space as part of the building envelope so leaving a vent is like leaving a window open.

since you are insulating an unconditioned space you are just going to be slowing how fast it heats and cools but since attached garages share walls with houses anything helps. In the summer we leave the garage door open for a few hours in the morning to cool it off and in the winter we keep it closed and it stays surprisingly temperate.

we are in a dry climate and so closed or open cell doesn’t matter but if you are somewhere humid you might check out if you need the more expensive but watertight closed cell.

I'm a fan of conditioned attics (no pun intended.) I've built a number of them and I retrofit one in a former home which worked fine for the nearly 18 years I lived there. I don't see the benefit of an unvented attic above unconditioned space, indeed it could lead to moisture issues in the attic space depending on the climate zone. Proceed with caution.

If you use spray foam against the roof sheathing you would not need the passive venting. But talk to some roofers about how often they find rotted sheathing and frustrations in replacing it, whether closed or open cell foam.

You would need a thermal barrier covering for the foam if the garage ceiling is open to the foam, or if there is a garage ceiling with an accessible attic above and you use it for any storage.

In our cold climate, I've only insulated above a garage when either there is living space above the garage or the garage itself will be heated. If the house is in a hot climate, I suppose insulation in the rafters or roof would keep it cooler, useful for all those people who seem to use the garage as party space.

The problem is the need to deal with moisture-laden air which is likely to accumulate in the unvented, unconditioned attic space except, perhaps, in an arid climate. The main mechanism for heat gain in an attic is radiation. The use of radiant barrier sheathing is the best investment you can make to deal with that. Vented attics work well in all climate zones. Skip the spray foam insulation and vent the attic space above the garage.

The roof decking OSB is the kind that has radiant barrier (looks like foil glued to sheet) on the back side.

I think what I'll do is this in the garage attic which is separated from the foamed in living space attic:

• Batting in walls
• Blown in or thicker batting in ceiling
• 4 sheets of 4x8 plywood or OSB decking over top of joists for storage space (yes that may compress / reduce effectiveness of insulation)
• Soffit vents along garage walls incl garage doors
• Ridge vents (if not too thick/tall) along upper ridges
• Optional: Thru-wall grate on one wall to allow in-flow of air and thru ceiling vent, both with some sort of doors if I wish to close off in colder months - this would allow some outside air to flow into the otherwise hot/stuff garage and up into the attic and out...

I just can't determine if putting foam in the roof rafters (with openings at top/bottom for venting) would be worth considering over blown in over the ceiling when over an unconditioned space like a hot garage. And I don't want that space 'sealed' over an unconditioned garage.

If someone has a better solution or can find fault in this - please let me know.

You have the airconditioned system in the garage right then it will not make it warmer you can opt for foam in the attic.

@CoolAir Inc.

No - actually the attached garage is not air conditioned, just the living space.

The attic over the air conditioned living space will be partitioned from the attic over the un-conditioned garage and and have foot in the roof.

The partitioned attic space over the un-conditioned garage is what I'm questioning as to approach.

HU,

You indicated that you have radiant barrier sheathing on the garage roof. It doesn't work with spray foam insulation applied to the underside of the roof sheathing.

If you insulate under the roof deck and vent the attic volume you will have accomplished nothing but running up the cost of your project. Insulate the garage ceiling with whatever your favorite material is and vent the attic space above the garage.