Thanks Bud, I am good on the application of the job, just interested in the validation of the savings. Supposed I could just run a REM/Rate model for my avg housing stock and vary the insulation level of the ducts.
How does Rem handle the ductwork inside the walls? Can you tell the software how many feet of ducts you are insulating?
Here's an LBNL research report by Iain Walker, who tested 24 retrofitted duct systems in the Central Valley of CA. Retrofits increased delivery efficiency by 64-76%, and reduced HVAC system energy by 18%, on average. This is going from uninsulated to insulated and airtightened systems; not exactly what you're looking for, but probably the best you'll find of actual data. Energy reductions varied substantially from house to house, and they suggest targeting homes with higher supply-side leakage to outside.
Heat / cool load calculation software (Manual J) provides ability to run different scenarios for duct insulation levels and duct leakage into unconditioned spaces.
Be careful with two layers of insulation on ducts in humid spaces - could create condensate / mold problems
REM allows you to estimate the length of supply and return duct based on the size of the home and # of registers. I have never done so, but it also allows you to directly enter the length. On two new Energy Star Homes I measured the actual lengths and was within 1% both times. A) 4000 sf home B) 1100 sf home.
Depends on where the ducts are located for the savings. You will get no savings for conditioned space. Still might be a good idea. You get most in a vented attic, some in an enclosed crawl space. More for supply, less for returns. Less still for panned body cavities.
Get the ducts out of the attic. Get them out of the crawl space and the unconditioned basement! Seal them up.