Do I need to add roof vents if I close off a small attic space in the garage?

Question

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 2x4 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).

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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?

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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.

Answer 1

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.

Answer 2

In the current form, this roof appears to be at risk for an ice dam. Heat will build up at the top of the roof without any vent, melt the snow, and it will refreeze at the bottom of the roof, backing up under the shingles. The result is a leak inside the garage. On this size roof, and in your location, they've likely installed a barrier under the singles to prevent most leaks, but these ice dams can still do a number to your gutters.

The cheap solution is to install some vents at the top of the roof, some soffit vents at the bottom, and insulate between the garage and roof. Since the concern is melting snow, I would exclude any low profile vents, as good as they may look, since that would be quickly blocked by the snow itself.

Given the desire to have a clean appearance, I'd personally call in the professionals for closed cell spray foam directly to the underside of the roof. They should be familiar with any code requirements (drywall) and you eliminate the need for a vapor barrier and vents. The only downside is if the roof ever needs to be repaired/replaced, they may damage the insulation in the process. Since the underside of the roof cannot breath with the insulation applied directly, I do worry that moisture can build up in the roof deck itself, but this risk seem fairly low as long as you don't have a leak.

On the side note of moisture in the garage, I'd recommend something that's both fire and mildew proof. Rather than standard drywall or even green board, I'd prefer something like the purple board. Leave an air gap between the bottom of the drywall and the concrete floor. If you're really concerned about moisture, install an exhaust vent in the ceiling and run it out a soffit. A timer switch for the exhaust vent would be useful so you can leave it running for an hour.