That's 8' not 8"s

Perhaps I mis-wrote - you miss my point. I'm not advocating actively cooling the attic, but rather applying insulation to the underside of the roof sheathing and doing away with all attic venting. The attic volume should then vary only +/- 10 or so degrees different from the conditioned space and heat gain / loss to / from ductwork therein will be greatly (up to x8 or so) reduced.

The load reduction is typically so much that my partner and I tell homeowners NOT to pay for sprayfoam UNLESS they are also prepared to replace their heat pump with a much smaller (- 1/3) unit at the same time, else they wind up being oversized, over humid and uncomfortable despite the sprayfoam investment.

engineer, I think I did misunderstand the concept that you were describing. On the "closing off attic" subject, here are a few questions: Is spray foam applied by a commercial company or can it be done DIY? What thickness/R-value is recommended? I there a good way to calcuate cost/sq/ft? (roof sq/ft vs. house sq/ft, right?) The top roof vents would be fairly easy to close up, but what is the recommed way of closing off the over hangs? To seal off the overhang vents, do I/they just blow in insulation over the perforated vinyl or is there another method? Is there any downside to sealing off the space ie. moisture, mold etc. If this method is used, is it also recommended that the attic insulation be improved? I currently have R-30 fiberglass & was thinking of blowing in some of the natural fiber insulation to bring the R-value up to approx. R-49. Would this be redundant? If this improvement actually can reduce heat load by almost 1/3, I would be looking at a combined 3.5 ton system vs. the 5 ton system I currently have. Is this realistic?

Best you make a call to a foam installer and get an on-site inspection followed by a quote. It will address your questions in the best way.

Dick

Commercial co with $50-100k spray rig. Figure about R6 per inch, but the cost-reducing effectiveness of that R value is about triple what the fiberglass insulation folks can claim. My sprayfoam guy closed off the overhangs by stuffing chunks of fiberglass batts into them to serve as backstop for sprayfoam.

No downside - attic stays clean, cool and and dry. no other attic insulation should be required if 3-4" of foam is applied and it completely encapsulates the upper truss chords. That should cost ~ $3 per SF of roof underside area. Way way up north you might want to bump that to 4-6".

I can't accurately comment on load / tonnage reduction w/o running a load calculation specific to your home, but the example decrease you cite is not atypical for a southern house.

Engineer,
What type of spray foam is recommended for this purpose?
Do load calculations take this method of sealing the underside of the roof sheathing any differently than for conventional insulation, or do adjustments need to be made somehow?
Does the foam stick well over time?
What happens in the event of a roof leak? Does the water come through the foam?
Should the conventional attic insulation (on the floor of the attic) be removed or kept in place?
Should the trusses (that run along the roof sheathing) be encapsulated to the same 3-4 inches?
Thank you!

Closed cell polyurethane, about 2 pounds per cubic foot, commonly abbreviated pcf

Structural load calculations do not yet, to my knowledge, take into account rigid foam. Thermodynamic load calculations do, at least mine (Elitesoft) does. I tell it the construction type, r-value, and that all the ductwork is in the conditioned envelope.

So far my experience suggests that if there are adhesion or shrinkage problems they'll manifest themselves early, well within a typical 1 year warranty. Not all foam, foamers, and rigs are created equal. Ensure that you get references.

2 pcf rigid sprayfoam is a vapor barrier - water will not pass through it

Floor insulation may be removed or left in place - the stuff is a bloody nuisance, though.

In future I plan to ask that the truss chords in contact with the sheathing be encapsulated, but I'd be happy with an inch. Asking that they be much deeper would greatly increase the total foam in the job and drive the cost up to a point of greatly diminished return. We also plan to try to have a blower door test done before the foamer leaves the site - air leaks are sometimes elusive.