"tom" wrote in message

You're right about keeping the roof as close to the outside temp as
possible. But if you insulate right up against the roof decking, you've
lost the ventilated air space that really does the job of keeping down
the ice dams.



so Edwin Pawlowski wrote:You are correct about a normal heated house,
but does this still apply to a
detached garage or shed? My garage has no venting at all. The roof is
not
insulated, but the walls are. I run the heat out there maybe four
hours on
a weekend day. I may add some insulation on the roof also in the
future.
but since it is a rather minimal use, I don't see any particular
advantage
to venting unless I separate the area at the ceiling joists. The other
160
hours a week it is not heated at all.


Depending on where you live, yes, it can still apply. The OP's in
Ontario, Canada. I used to live just north of the southernmost part of
Ontario, namely Michigan. If he's gonna heat it in the winter, it'll
melt any snow up there, and that'll flow down to the eaves, and
refreeze. It's a bad thing, even with all the Ice and Water Shield in
the world. You're losing most, if not all of your heat straight up,
Ron. Tom