closed cell won't absorb water and adds structural rigidity...it is the only insulation approved for use in flood zones. Closed cell is also more expensive.

we looked at insulating the roof or the attic floor and chose to do the latter.

Yes, if foaming deck there's no sense in a vented attic. Not sure what OD means but there is certainly ventilation going on even if you dont sense it.

Generally, unvented attics only make sense with vaulted ceilings, complicated rooflines and when you have HVAC ducts in the attic space.

CC is a superior product not only from the water and strength issues Annie mentioned but it also forms an air and vapor barrier at only 1-2" compared to the 5-6" needed for open cell. That said, most choose OC because of costs. You can get more R for less money. Just be sure to insist on the proper thickness to meet code minimums which will be much more than what sales people and installers recommend.

it depends on where you live.
In La. & Fla. for example open cell is used
in attics.

if you have to rely on closed cell for structural
strength...your builder needs to learn to beef
up framing members. the whole structrual strenght
thing is from closed cell salesmen. passed on
to builders and homeowners.
its bs if you build right.

best of luck.

energy-rater, it is not bs...it is true. CC does add structural rigidity. Granted you want to build so your house is sound without any insulation, but that doesn't change the fact that it does add structural rigidity above and beyond what you get from 2x4s.

We also noted how much quieter this house is since the CC went in.

The additional "structural rigidity" that the closed cell foam manufacturers tout increases the resistance of the building frame to lateral forces from wind and earthquake which is required by building codes and/or acceptable engineering practice to be resisted by the building's floor, wall and roof sheathing. Adding the stiff foam would increase this resistance by some unknown amount which might be desirable in a high wind or earthquake area as additional protection. But structural designers do not like unknown/unmeasurable force resistance so the claim remains an interesting sales pitch at best.

Here is a link that might be useful: foam insulation

depending on your location and what you have in the attic, I rarely spec or recommend spray foams. Mainly due to cost. TO get the IECC amounts, you need to put on a lot of foam. In our area, it would be 7.5" of cc or 11" of open. Most insulating contractors I talk to claim you need half that because its "such a good insulator and air tight". Anything can be made air tight, spray foams are just the fast and easiest (also the most expensive). Spray foams do no insulate any better then blown cellulose or fiberglass. r value is r value. If code states an r49, you put that in. Most projects I do I detail the ceiling plane as the air barrier allowing r60+ of cheap blown product ontop. I never put ductwork in a house attic, but that is more location specific. We all have basements in the midwest, unlike the south. (with that being said I am bidding out a large institutional project with 145,000 sqft of sray foam in the attic........)

back to the question however. If I am specing spray foams in attics, typically I will do open cell due to cost, but also since it does not hold water like closed cell, I feel more comfortable with the sheathing layer on top of it drying with a leak/water intrusion. closed cell holds the water at the sheathing layer possibly causing rot to the structural sheathing and you would never know it.

Why are you insulating your roof deck at all? Are you creating a closed attic because of HVAC ducting in the attic? You would need to eliminate all of your attic vents if you do so and put all of your R30+ at the attic roof line. If you are keeping an open ventilated attic, the insulation goes on the attic floor, and you improve your attic ventilation so that it remains close to ambient temperature.