Is there a web site that talks about summer time heat losses with different R valve, say 4. 6, 8 insulation on the ducts, in a southern attic?
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Not that I know of. Duct insulation here in my part of Central Texas only comes in R6 and R8. Whether either value is actully achieved having mush more to do with the quality of the installers than the rating of the insulation.
Oh, and neither really does that much when your attic goes to typical summer temperatures of 130-140º--recall that R value is pretty straight forward, install to spec, an R8 insulated duct will heat up 1/8 as quickly as an uninsulated one.
But, we are back to having a ceiling are to attic deltaT of around 50, and the indoor-outdoor walls only 15-20.
I know of none, but that's
I know of none, but that's not a topic I'd search for. Personally, I'd insulate to the highest R-value possible. Much of the time attic ducts are covered (at least in part) by attic insulation. If you are going to insulate ... make sure you seal the ducts to eliminate/reduce leakage ... which could be a bigger energy issue by far than duct conductive heat loss/gain. NO GREY DUCT TAPE, though.
I'd get the biggest R-value you can, though to insulate. The attic, like the other poster said will be say 120+, your supply air may be 55 deg in contrast w/ the house at say 75 and the outside say 95.