Originally Posted by EE playing with HVAC
I think that it is worth going to R8 insulation if you were going to change out the duct anyway. The R8 insulation cost about $10 more at HD than R6. The savings from R8 according to the graph would be about 3%. There are 9 ducts in the house so the savings on the electric bill would be 3%/9 per duct. The electric bill is around $150. So $150*3%/9 is $0.50. So it would be a 20 month pay back.
It may still be worth it to change out even if you didn't need to. Going from R4 to R6 is a 5% savings. Going from R6 is an additional 3% savings. So you will get around a 8% savings going from R4 to R8. 6" R8 is $35 at HD. The savings would be $150*8%/9 = $1.33 per month per duct. Which makes it about a 26 month pay back.
What should be considered a good pay back? 1 year, 2 year?
Sure, if you're going to replace a duct anyway, use the highest R-value available. When my house was built 2 years ago, code called for R4.2, and that's what they used. It went up to R8 briefly sometime after Katrina, but then for whatever reason, it got knocked down to R6, where it stands today. I just happened to see a bunch of R6 duct recently at Home Depot and bought one as an experiment. After replacing one R4.2, I didn't observe any difference in the temperature of the air coming out of it; it appeared to gain as much heat as the original. I guess there might have been maybe a couple of tenths of a degree difference, but I don't know if that's going to significantly improve the comfort level in my home. For the time being, at least, I'm focusing on sealing the ductwork, caulking all the gaps around ceiling penetrations, touching up underinsulated spots, etc. I think fixing conditioned air going into the attic and attic air coming into my house has a better bang/buck ratio as a first step.