I have a small section of roof over the front of my garage, which I would like to insulate and drywall (making my garage fully insulated).
It is 39" high, and 54" by 17', framed with 2×4 trusses, and facing almost directly north (I don't think it ever gets direct sun). I'm located in south eastern Ontario (near Toronto), and the temperature typically ranges from about -30 to +30C. The garage is not directly heated, though is attached to the house by two walls and so some heat loss from the house is going to heat it (even without being 100% insulated, today when it's 13C outside, it's noticeably warmer in the garage).
Click for full size image
There are soffit vents in this section, and since it is open to the garage below it currently gets some amount of air circulation. I was planning on insulating with R14 fiberglass batts.
Can I close it up without issue, or do I need to add vents of some sort? Presumably I need to make sure there is airflow from the soffit vents at least. Since this is the front of the house, if I do need vents, are there any options for discrete vents?
UPDATE: Spray foam installed
I decided to go with the 'hot roof' method, spraying the underside of the roof decking.
At the advice of the spray foam installer, I put some rigid foam boards blocking the soffit space. There is a continuous thermal break from the walls (batt insulation) to the ceiling (spray insulation).
After the spray foam was installed, I immediately noticed how nearly air-tight it feels, which made me a bit concerned about moisture. Since it's easy to do now (prior to drywall) I'm in the process of adding an exhaust vent that vents outside.
It has gotten as low as 0C out, and the garage was still at a comfortable 14C.
Best Answer
You can do a hot roof and not require any ventilation, though I would not recommend it anywhere snow accumulates on shingle roofs. Even if i don't like it, it is allowable.
Whether you need additional ventilation depends on how much you are getting now and what is allowed by the governing building code for your area. According to the International Building Code for One and Two Family Dwellings, R806 Roof Ventilation, you need the equivalent of 1/300 of the area ventilated in free vent area. If the soffit vents do not provide this, you need to install additional ventilation or opt for the hot roof concept.
Also, the attic area needs to accessible by a min 22x30 inch opening. You must also install a class 1 or II vapor retarder on the warm side of the insulation or the ventilation requirements double.
The building code does not address moisture control within the garage space, it's only concerned with moisture in concealed spaces or general ventilation of habitable spaces. You may wish to address moisture in the garage itself with exhaust fans or operable openings. This is unrelated to attic ventillation requirements.