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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).

front of house

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).

inside, facing right inside, facing left inside, left corner closeup

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.

outside, left corner closeup

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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Are you going to create an access door to the space? –  mike Oct 23 '13 at 6:27
    
For discrete vents, one could DIY a an exhaust fan to blow down and out through a section of existing soffit vent, with an intake duct routed up to a high point. –  mike Oct 23 '13 at 6:29
    
I think we're going to need a roofing specialist to answer this one. I really think the question is "In this small space, can I get away without a vent." –  Chris Cudmore Oct 23 '13 at 13:26
    
@mike Hadn't really thought about that, but I can add one. Had considered exactly the thought on the exhaust fan, but still looking for an authoritative answer on it and some more details (eg, does it run all the time or on a thermostat/timer/something else? how many CFM?) –  gregmac Oct 23 '13 at 14:51
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A key question is what the space will be used for. If you're converting to living space, additional moisture issues come up. You can't just close this off because the dew point will be somewhere in the middle of the space, raining moisture down on your new ceiling. The brick will bring moisture also. I'd include an access door... and maybe solve the venting situation with a humidity controlled fan, vending per @mike. –  Bryce Oct 30 '13 at 3:26
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2 Answers

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.

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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.

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I've had one winter in this house (though the house is 10 years old), and didn't have any issues with ice dams. Keep in mind the garage is unheated. The shingles are in reasonably good shape and there are no signs of leaks, so I don't believe there have been ice dam problems in the past. –  gregmac Nov 7 '13 at 15:28
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