Ceiling insulation question

[Left Coast Chris]

I have a shop roof that has 2x12 joists spanning 16' and are 2' on center.   The sheathing is 1/2" OSB.   There is no ceiling.  The joists and underside of the OSB is exposed.   I would like to insulate the ceiling and have some 3" fiberglass foam sheets that could be cut to 22-1/2" wide and placed or glued directly against the OSB without an air space.   The joists run across the roof not down slope so it would be hard to vent out a air space since there is no exterior overhang of the joists.

I know it is a caution to go with no air space due to condensation but we are a comparatively dry climate and foam is often shot on surfaces on metal buildings.   I have not heard of any problems in the area but was wondering if any of you have had problems.

Thanks for sharing any experiences.

[Brucer]

Venting above the insulation does two things. It allows any trapped moisture to vent before it condenses onto the insulation. And if you have snow in the winter, it keeps the roof deck cool so the snow won't melt.

If you're insulating it to keep heat in during the winter, you will want to put a vapour barrier on the inside of the insulation. 6 mil polyethylene is best. Without the barrier, moisture from inside will migrate into the insulation and condense.

Best talk to a local heating/ventilating contractor to see what the practice is in your area.

[Left Coast Chris]

We only get snow once in 5 or 6 years and when we do it is very brief and light.   We get maybe a dozen or two frost days though.   Generally condensation is not much of a problem and they do use a bit of spray foam directly on roof panel underside but mostly with steel.  We also use vapor barriers directly over the outside side of studs then outside sheathing over that and only sheet rock on the inside.   I will discuss with a HVAC contractor.   Good thought.  Thanks.

[Saga Dan]

You would be fine in glueing the insulation to the underside of the osb sheathing.  You do not need to vent  as long as you don't have an air space.  You would have to be careful if you ever do install a ceiling to make sure things are vented or packed full of insulation.  I have seen many apartment buildings roof decks rot away due to adding insulation at the roof level and in the roof joist bays.  (Talking flat roof installations) 
Go ahead and add the insulation it will be a big help.  You will still have some thermal bridging at the joist locations but not too bad.  If you are concerned about it keep and eye on it and see what happens throughout the seasons. 

[Randy88]

Every area is different but around here, buildings that insulate like your wanting to do have a tendency to be sweat inside during the more humid months.   Most around here that do as your planning end up taking it all back out, and insulate on the bottom of the 2x's and putting up a false ceiling and having a roof vent of some sort.   

I'm no expert by any means, I'd also recommend talking to several heating experts in your area and even some state university people if you have them, they are more unbiased because they are not really selling anything to you.   

I've seen and head some real horror stories about insulating buildings, anywhere from rotting them out to having a shower inside at certain times during the years depending on conditions, to having insulation fall off in chunks due to moisture and even mold growing inside buildings and having to destroy the whole building to solve it.    Best of luck on whatever you do.   

[ely]

I would have the closed cell foam insulation sprayed directly on the ceiling and joists. that way there is no way it can leak and it will be air tight seal. use the product you have on a wall project in the future.

[Left Coast Chris]

Thanks guys for the options.  Lots of different experiences out there.   If I can find some vents that can penetrate through the end walls without letting rain blow in Im thinking of going in that direction.   I will need a vent to the outside at each joist space.   I agree that an air space is most often a good thing if you can get it.   Spraying foam directly on is certainly good too but since it is an out building with an aging roof I could see a leak at some time in the future saturating into what ever is there.    The air space at least allows things to dry out if there should be a leak.

[zandoval]

Do they still do this kind of spray??? It's used allot in central Texas and offers fire resistance and insulation...

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[maple flats]

I was in the insulation business for 20 yrs. Don't do as you suggested, it will spell disaster even in a dry climate. Either make a ceiling and vent the space, even using roof vents if necessary, or spray a vapor barrier insulation directly on the underside of the OSB and on the joists. When I was in it, Urethane would have been a good choice, there may be others now, I got out of that about 20 yrs. ago.

[Left Coast Chris]

The roof is only a 2.5:1 pitch with asphalt shingle roofing.  The roofing has added adhesive tar under the flaps.    I am very reluctant to spray the underside of the OSB and joists in case there was ever a leak it would be hard to find and hard to dry out.   I'm going to cut the ridged sheets of dense fiberglass for each joist space then nail cleats on the bottom inside to hold up the 22-1/2" wide strip of ridged insulation in place.   This creates an air space that can be vented out one end on the exterior end wall.  Other end of the joist is an interior wall.    I think that is about the best I can do.