I just received my estimate to apply spray foam to the attic, gables and knee walls in my existing home.  We live in eastern central Florida.  The estimate was for Demilec Sealection 500 (1/2 pound).  I have the option of applying sufficient thickness to make R-19 which I from my calculations is about 5" or R-30 which is about 7.9 ".  The prices for each look reasonable as they are much lower than another estimate I received for the same product.  I am concerned that while the current code says R-30 for ceiling insulation (the old requirement was R-19), R-19 will provide me with about 93.4% efficiency, the additional 2.9" of foam to reach R-30 will only increase the efficiency by 2-2.5%.  For an increase of 33% in installation costs, it may take a long time to recoup that 33%. The existing fiberglass batts are loose, fallen fallen down and are generally not functional from years of additions and modifications so anything should be a significant improvement.  Any thoughts are appreciated. 

I'm curious, what was your quote for how much sqft?

It's pretty common to split the R between a foamed roof deck and blown cellulose/fiberglass (or even batts) on the attic floor. If there are f.g. batts on the attic floor and that interface is reasonably air-sealed, leave them in place, pulling them back only as far as necessary to install the foam. If the attic floor/ceiling interface has lots of air leaks, it's worth foam-sealing what you can when the foam guys are there, bearing in mind you can't foam over things like flues or recessed lighting. (You can box over recessed lights with 3" clearance, and air-seal the box though.) If there are still gaps in the batts where they meet joists or go around vent stacks, etc, a 3" over-blow of cellulose to fill in the whole thing is a cheap fix, and will improve the performance of the batts by reducing convection losses in the fiberglass by ~90%. (This is an easy DIY project with a box-store rental blower. Be sure that it's all raked smooth & even.)

If the ducts & air handlers are in the attic, sealing & insulating the ducts with at least R6 (2" of open cell foam works) is worthwhile when splitting the R between roof deck & attic floor.

Don't confuse fraction of heat gain with "efficiency". It's all about how many BTUs you reject, and how much it costs to pump out the rest. R30 is usually cost-effective over a longer term present-value financial analysis with either half-pound foam or other. On roofs you're talking about much bigger delta-Ts than indoor/outoor air temps too, due to the solar aborption factors of the roofing. It may be only 95F out, but your roof deck could be 140F, a 65F delta-T against 75F room air, instead of the 25F delta between room & outdoor temps. This adds up to a ceiling-radiated heat comfort issue as well as a cooling load compressor duty-cycle issue.

R19 at the knee walls & gables is fine, since the exterior walls convection-cool much better than roofs (especially roofs with pitches of 2:12 or lower, which would have a strong indicator for the cost effectiveness of "cool roof" materials.)