Insulating garage ceiling with solid rafters

[RSweetland]

Hi
I live in Canada, with very cold winters.

I have an 18.5 x 23 ft garage with 9 foot ceilings. The ceiling rafters are solid wooden 15 inch I-beams spaced 12 inches apart. Above the garage is a flat deck.

At the sides the rafters extend beyond the walls, and have soffit vents all down each side of the garage.

My question is how to best insulate the ceiling with regard to issues like air flow and moisture.

Should I have someone spray closed cell foam directly on the ceiling between each rafter? Will that cause moisture issues to my above-garage deck (the floor of which is covered with Duradek).

Alternately can I place some fiberglass batts between the rafters (btw does it come in 12 inch widths?), leaving a space between the fiberglass and ceiling above it for air flow, and taking care not to block the soffit vents.

Any advice would be appreciated.
Thanks for your help.
Bob

 

[RSweetland]

BTW its an attached garage.

 

[Jackson]

My guess would be that you need rafter ventilation. Like these: http://www.homedepot.ca/product/rigid-extruded-polystyrene-attic-rafter-vents/925601#

I think Lowes has a less expensive version. You would cut them in half or maybe shave a bit more due to the 12" oc spacing.

It's possible with spray foam insulation you may not require these though as there is no air pockets. I'd call somebody that sprays and check, may be the cheapest and fastest route to get insulated.

 

[ezzzzzzz]

I assume that there is a moisture barrier of some sorts between the deck surface and the actual plywood roofing. That should prevent moisture issues inside. The outside decking should not be an issue at all if it is already exposed to the elements and is built for that. On to insulation... I'd used R30 or R-38 in between those BCI's. If they are on 12" centers then the actual opening is 9" to 10". You can buy 24" wide 4' batts and split them lengthwise with a razor knife. I place a length of board on the glass to compress it and use as a guide to cut the batts. These will rest on the 'I's' when stuffed between those BCI's. I'd also consider adding stiff wire, wood slats, chicken wire or drywall (preferred) to keep the insulation up there. I'm also assuming there are different codes where you live. I had to use 5/8" fire-rated drywall on the ceiling and 1/2 drywall on the walls along with taping and mudding on my attached garage down here in Va.

 

[Jackson]

Ya, disregard my advice. Thought you had a sloped roof and venting. Even though you clearly do not... Just seal it off at the soffits with some rigid foam sheets.

 

[RSweetland]

thanks for the advice!