we need more info...
what kind of foam...I assume open cell? And when you say .55/ft, what exactly are you meaning? What r value/thickness? Foaming the attic is really only something to consider if you are puting hvac equipment up there. Otherwise if you want air tight ceiling plane, you can use air tight drywall on the ceiling and then blow in cheap cellulose.
Expect around $3-6 per square foot to shoot enough foam in the attic, depending on the required r value.

---not sure about "open cell" ---
- we had talked to a cellulose guy and he said the whole house would be approx $1.20 sq/ft (think it was broken up by $0.60-.65 for the walls and $0.50-.55 sq/ft for blowing cellulose up in the attic...)

yes ductwork will be up there (not sure on R value) - live in the south..which the way i see - no need for tech shield, and of course air vents (not sure how much that would save us - but maybe it would offset some of the cost of the foam..)

but then again - if its $5-6 sq/ft - that is a pretty hard pill to swallow-- we were thinking along the lines of $2... durn

I had an estimate of about $10k for open cell at r-30 for 2000 sqft of attic floor space. It is hard to compare because the roof is steep and a little complicated. A flatter roof should be cheaper and I'm sure a better way to price is by rafter area but I have no idea what that is.

Foaming the rafters is a great concept but hard to justify even in your case with duct work up there. I think our radiant barrier was like $200 and air vents are pretty cheap.

Our energy audit suggested $200 a year savings (and all of their savings estimates were high because they are taken in isolation) for foaming the rafters. So 50 year payback which was probably optimistic.

Can you get the ductwork into conditioned space? In retrospect, I probably could have spent

putting ducts in the conditioned space takes considerable
foresight and planning.

designer/hvac company has to be on board in the planning stages.

this is why the foam business has grown, the ducts and equipment goes in the attic in most cases.

foam is often a last minute add on.
it works, when installed to meet code r-value requirements
and tested to assure air tightness of install.

now your attic doesn't need ventilation, as you are
making it into an unvented attic.

I've been running roi on efficiency upgrades for years.
while the foam in the walls isn't cost effective,
in many cases open cell foam in the roofline is.
roi or payback isn't 50 years. the 5000 sq ft
home from two years ago has already recouped 1/4
of the upgrade costs and as their utility rates will
go up 30% in the next two years..less than an 8 year
payback for this particular home.

understand that to create an air tight air barrier
between living space and attic is beyond what most builders
trades people actually deliver (hvac, sheetrock, stove & bath vent installers and electricians who install IC recessed lights instead of ICAT)

by adding the foam these holes made by tradespeople no longer provide entry into the living space below.

easier to add an added cost to the homeowner than
to properly train trades to seal holes they make.
and design homes that will be affordable to live in
as utility costs do what they always do..rise.

having a consultation or energy rating done
would give you the unbiased information based on
testing, inspections, verifications and efficiency
for your home would be of benefit to you.
check with Resnet for a rater in your area.
experienced, unbiased information by people who don't have a dog in the fight.

sizing of hvac systems are changed when equipment and ducts are in semi conditioned unvented attic. load calcs to
ensure equipment isn't oversized is always a good investment. here in the south oversizing = short cycling and doesn't allow for dehumidification.

at the very least make sure that the ductwork and
all connections including supply boxes and return airs
are mastic sealed. an average of 30% duct leakage in new construction is not uncommon.

best of luck.

air sealing the ceiling plane is not hard at all, just takes a little bit of time. A blower door test can help insure the sealing was done properly. Obviously this does not address duct work in the attic space either like spray foam would.
As mentioned, I would recommend seeing if the duct work could be placed below the ceiling. In our area its easy because most homes have basements. South not the case.

Have your insulator give you a price for open cell foam to hit your required r value in your area. This is the prescriptive method. Many foamers will try and do the performance method claiming they do not need to spray the required r value by code because foam stops the air infiltration unlike cellulose and especially batts. (a near worthless product IMO). However new energy code is going to require air sealing anyway, so that will start to take away some of the advantages of foam. (or push more people towards it...who knows).

...talked to a reputable company--- they are in the $1.90 sq/ft range for open cell foam (think it was R 19)...said they are doing more of the following though...on top of the sheetrock spraying a 1/2 inch or so layer of closed cell foam and them putting cellulose on top of it - said it was half the cost....that may be an idea-- has anyone had any experience with that approach?

Foamed our entire home when built in 2002. 3000' 2-story was $7000 more than fiberglass at that time. Believe we've probably saved that $7000 in lower energy bills. Also so much quieter! But know, your success will be dependent on the installation crew. Make sure they are experienced and go to the trouble of checking other homeowner references.

You can go the route of flash and fill, or like I suggested, use air tight drywall techniques and save yourself the cost of the spray foam. Spray foam is an easy way to seal up the house that your subcontractors build leaky and subpar. Hold your subs to a quality job and follow air sealing techniques, and spray foam is completely uneeded. I design houses that test out tighter and more efficient then most homes that use spray foam everywhere...and I never use spray foam. Cheaper house to build, cheaper house to operate.

its a lateral move. comparism would be actually
sealing all penetrations and insulating conventionally.
time & labor vs cost of foam install..

the whole point of foam for the roofline is to
put ductwork and ducts in conditioned (semi conditioned)
space. the side benefit is that the holes in the ceiling
are not sucking hot attic air into the house as pressure
boundry is @ roofline.

keep in mind that you are building in the south
which is a cooling climate, not a heating climate.
houses dry to the interior here, and to exterior
up north.

best of luck.

You are forgetting spray foam does nothing to address some of the most leak prone places in a wood framed home such as where top and bottom plates meet, box sills, etc. Gaskets and caulk are a fraction of the cost of foam even after adding in the small amount of extra labor. I have tried to make foam work, I really have cause it is a good product used with the system, but it just never pencils out. (for new construction I might add).
Also in a heating climate homes can dry in either direction. Its where you place your moisture barrier (if using one) that helps dictate that direction.

But regardless, with the topic at hand, it does make some sense to seal the attic above the ductwork in the OP situation.

I didn't address wall or sole plate leakage as the
questions were for unvented attic.
you handled the air tight drywall approach nicely, btw.
for my clients..I always recommend sill seal or a double bead of caulk between slab & sole plate..even on second floors we caulk sole plate to subfloor.

I've tried to make it work where if someone made a hole
for example a hole cut in the ceiling for a supply box
that they were responsible to properly seal the hole.
too hard to ride heard over a bunch of tradespeople.

this is where our situations differ..you run the job,
I'm just there for efficiency recommendations, installs
and performance.

I never ever recommend foam in walls.
roi is too high..and we seal walls ok..its the
ceilings & floors (on piers)!
instead I recommend a 1" hi density foam sheathing board
to exterior..conventional insulation and ada.
air sealing such as sole plates, around windows doors
etc.
caulk is a wonderful product..my clients usually start with
a case. most quickly understand that it is in their best interest to take an active part in the air sealing.
but for our climate..and the many mistakes in air barriers
ducts & returns...foam for the roofline is a no brainer.

in a perfect world...houses would be designed in the south
with ducts in cond space. every tradesperson would seal each hole they make.

I'm going to have to think about this for a bit!
"Also in a heating climate homes can dry in either direction. Its where you place your moisture barrier (if using one) that helps dictate that direction."

Um, I dont think humid air cares which side of the assembly you put the VB on. If some rain drenched brick get heated by a summer sun, moisture will move inward. During the heating season, it generally does move outward.

getting off topic here...
heating climate, yes the vapour drive is to the exterior. Because of that, code required a poly vapour barrier on the interior (warm side) of the walls. Now that new code requires exterior foam sheathings or some form of continuous insulation, this now reverses the drive inward since the wall can not dry outward due to the exterior vapour barrier. If poly were to remain on the interior it would trap moisture in the wall. The interior barrier is now eliminated and instead a vapour retarder (latex paint for example) can be used in its place.

we put open cell foam in the exterior walls and the attic when we built our home in 2009. 3100 sq feet, 3 hvac systems with ductwork in attic. Utility bills (elec and gas) run less than $200/month. It is quieter, and there seems to be less dust. When we built, our contractor calculated a 5 year payback on the foam. It was $6000 more than cellulose, and I'd say were saving at least $100/month on the utilities.