I have a few questions about spray foaming my attic. If I decide to move forward, the work will be done by a professional. I’m hoping the folks on this forum who are a lot more knowledgeable than I can help me with my decision.

First, some details:
– House is in Northern NJ
– Built in the mid-1940s
– Stick construction
– Relatively simple gable roof currently covered with asphalt shingles that will probably need to be replaced in the next five years
– The roof stops at the edge of the house – there are no eaves/soffits, and thus no eave/soffit vents
– The attic is vented by two gable vents and three roof deck vents (there is no ridge vent)
– Attic floor area is about 700 square feet, and it’s currently covered with really old (paper crumbles when you touch it) fiberglass batt
– Home energy auditor estimated the current ceiling R-value to be about R-4, and there is no air sealing
– Attic contains central air equipment and ducts and is also used for storage
– Ceilings are plaster over what appears to be a precursor to drywall (no lath)
– The ceilings curve down along the front/rear walls of the house where the pitch of the roof encroaches on the rooms

I have a spray foam contractor coming out on Sunday to provide a quote. Over the phone, we discussed lightly spray foaming the floor of the attic for air sealing, and then spray foaming the rafters and gables for insulation.

Essentially, this would create an unvented attic.

Here’s how I’m thinking about this:

Pros:
– R-value is R-value, so spray foam will improve comfort just as much as the equivalent R-value of cellulose
– Better air sealing than cellulose
– Not as messy as cellulose both in general, and especially if the roof were to ever leak (fortunately, haven’t had this problem)
– Creates an unvented/semi-conditioned space, which will improve the performance of the HVAC equipment
– Potential to stiffen up the structure, especially with closed-cell (not that it needs it, just a side benefit)

Cons:
– Price, based on what I’ve read
– Permancy (won’t be able to get to wires/electrical boxes without some serious foam removal)
– Flammability
– Potential to hide problems (like a leaky roof deck)

Overall, money aside, my feeling is that an equivalent R-value of spray foam will improve comfort and efficiency more than cellulose. And I like the idea of an unvented/semi-conditioned space for the HVAC equipment and the stuff I store up there.

Am I missing any major downsides? Are there horror stories out there about people converting vented attics to unvented in old houses and causing tons of domino-effect problems? Any reasons not to do this?

Any info you can provide will be appreciated. Thanks!