Yes, you can spray it on and it can be ventless assuming you meet the required r value for your climate zone for an unvented roof assembly. Cost will not be cheap. Around here it would cost roughly $3-5 a sqft to shoot enough open cell on there to hit the r value required. Talk to an insulator and get some real numbers from them. That is the only way you will know the cost, not from anyone on here.

So much cheaper just to get the ductwork and airhandler out of the attic. I just built on the NC coast which is primarily a summer house (very little heating - mostly a/c like you). Never considered foaming the rafters. Just keep ductwork in conditioned space and caulk and foam the ceiling where needed. Foam will never payoff but ductwork in conditioned space always will.

Cost was about $1500 for 2100 sqft 2 story home. 10 sqft or so for the air handler at $500. $1000 extra for open web floor trusses to put the ductwork in. No extra charge from HVAC installer. The downsizing of the a/c unit probably saved $600.

Sealing the attic does have other benefits particularly regarding water intrusion in driving rain. But the foam cost would be about $4000 vs $1500 for similar savings (IMO ductwork relocation saves more money). Your cost will be very dependent on roof pitch and complexity.

I should have mentioned that I am building on the water and want a ventless system to keep out rodants and bugs.
Can you use blown in fiberglass insulation or does it have to be foam without any air vents? Thank you

without getting overly techinical, you can not use blown fiberglass or cellulose in a ventless design like you are requesting.

I don't pretend to understand S FL bugs but is this really a known entry point? Could you not seal the ceiling with a little foam as a barrier?

But - no question, the unvented attic is better at water penetration.

I agree with David. A good builder should be able to build a traditional vented roof without access points for critters.

OP in my area open cell will cost about $1.20 per
sq ft. this will be for doesn't meet code install
of 3". to meet code the open cell foam should be
6" and cover the faces of all rafters.
this will meet code for cathedralized ceilings
of R-25. the cost of foam for the 6" install
is approx $1.40 per sq ft.
this is based on two of the foam companies who
do installs for my clients.
costs will vary greatly as posted abovve..$3-5.00
apparently costs are much higher in that area.

getting an experienced educated installer..not
the salesman..is the key to a good install.
if the attic is not air tight then the
value of the install is lessened.

best of luck.