I recently drywalled an attached two-car garage (walls and ceiling).
This left the attic area unvented. I installed two 4"x12" soffitt
vents on both the front and back of the garage (total of 4). These
will provide air input into the attic. I also have two 12" square
vent boxes for the top of the roof. I haven't installed them yet as
the shingles on the garage and house are in need to be replaced, which
I'm planning on doing in the fall. I'm apprehensive about installing
the roof vent due to water leakage until the new roof is installed.
I understand that venting a roof prolongs the life of the shingles,
but since they are at the end of their life, would it really hurt to
wait until the fall. Would the new soffitt vents allow for enough
cross ventilation? BTW, when I installed the drywall and new attic
insulation, I left the cavities clear of insulation where the new
vents are installed to allow for the air circulation.
I could have gone with a ridge vent, but since the house has four box
vents, I decided to match them for the garage.
The garage roof is A-frame with a 12-4 pitch (basically 19'x22')
I'd go with a ridge vent when you do the roof.
If you have electricity in the garage, I would suggest a powered fan in
one of the vents. You could get one with a thermostat that will only
come on when the heat gets to the point that the thermostat is set for,
or a manually powered fan that you turn on and off when you are doing
something in the garage.
Otherwise, you can do what I did on one of my sheds. I built a working
cupola on the roof. I later put ridge vents on either side of the cupola.
We had no vents at all in our garage attic and it lasted fine for the
7 years we were there. The garage ceiling was sheet-rocked except for
a 4x8 hole for access. As with a lot of diets and medical problems,
good advice is often not critical advice.
I doubt you will get much ventilation with just soffit vents.
A roofer could tell you if your soffit vents
and proposed roof vents are adequate. Your argument on shingles sounds
reasonable. You could also ask a roofer if heat will damage underlaiment
(in time until you replace shingles). I don't think it would.
I assume, since you insulated, you heat the garage. A major reason for
ventilating an attic in snow country is to keep the attic and roof at
outside temperatures. If the attic temperature is raised, the roof
temperature is also raised. Snow may melt and run down the roof until it
hits the roof above the soffits where it freezes. That creates ice dams.
You may want to add additional vents. 2% of the ceiling area should
be equal or less than the total vent area. Ridge vents are very good.
You can add a house fan to the ceiling if your garage gets too hot.
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