The formula for heat transfer is:
HT = SF times U-value times Delta T
SF is square footage of surface
U-value is the inverse of R-value
Delta T is the difference in temperature between the outside and the inside.
So the roof is significant because, in this case, it's a big roof. If TNMG's building has 12' walls, he has 3840 sq feet of wall but 6000 sq feet of roof. All else being equal, 56% more energy will pass through the roof just because it's 56% more surface area.
Plus roofs get hot in the summer, especially in the South. I've measured temps of 165 on the shingles on my house roof. So, the Delta T on my walls that day was 20 (95 outside, 75 inside) but 90 (165 - 75) on the roof. That's a bunch more heat transfer IN THE SUMMER. In the winter the sun is helping keep the roof warm and the walls are colder.
Do the radiant barrier foils "make a difference" when put up where there is zero insulation. Of course they will since it does have an R-value. So will a piece of drywall.
My point is that good closed cell foam insulation runs about R-6 per inch. Fiberglass and open cell foams are about R-3.5 per inch. Anyone who thinks a 1/4" piece of foil covered foam or bubble wrap is going to give them R-15 (R-60 per inch) clearly hasn't done much research.
For the truly interested, the Pikes Peak building department requires an energy calc spreadsheet on new construction. Fill in the square footages and R-values of the various surfaces and it gives a total BTU heating load. The T values in columnn G should be adjusted to your local climate but it can be used as is.
Energy Code & Heat Loss Calculation Information
Steve