I live in an 1800 sq. ft. 20 year old single story ranch on a slab. I’m located just outside of Kansas City, KS in climate zone 4A. I want to increase the attic insulation, but before doing so, I wanted to improve the duct work in the attic since the builder did a really poor job with both the design and the install.

The house has 2 x 4 walls with 2 x 4 ceiling and roof trusses. I have 320 sq ft of raised floor (9″ off the attic floor) storage down the middle of the attic. The return air is currently 100% flex while the supply has some rigid trunk but is mostly flex as well. I plan to replace the existing duct work with a 16 x 8 return trunk with short runs of 8″ flex to the individual returns (6 existing, plus one new return). One the supply side I plan to use rectangular trunk with 6″ flex (R-6 or R-8) branches. None of the flex branches will be more than 20′ long, and most will be less than 15′ long.

I would like to take the attic insulation up to at least R-50. The existing insulation is loose fill fiberglass, but only about 8″ – 9″ deep, some areas where repairs were done in the past has even less.

All of the branch ducts will be running parallel with the ceiling trusses, while the trunks will lay across the ceiling trusses. I had been planning to just run the flex between the truss bays in contact with the sheetrock attic floor/ceiling, and then cover it all with enough fiberglass insulation to get to R-50.

However, the more I read on this site and others, the more confused I have become. As I understand it, if I lay the flex on the sheetrock and cover it up even to an R-50 level, then I risk having the flex sweat in the summer time creating potential mold and moisture problems.

Is that actually a risk in my part of the country? Can I just bury flex with fiberglass and be OK? Alternatively, can I enclose the flex (and trunk lines) in some kind of airtight chase built out of sheetrock or rigid foam and then cover that with loose fill insulation? Or is it more economical to cover the flex with spray foam, rather than fabricate some type of chase for the supply ducts. Since I haven’t done this type of work before, maybe there is an obvious reason building the airtight chase on the attic floor won’t work. I don’t see why I couldn’t just build a chase out of 1″ or 2″ xps, lay the flex duct inside, close the “box” with another piece of xps, tape and spray foam the seams, and cover that with all the insulation I want. However, I haven’t been able to find any discussion about that type of approach on this or any other site.

I’ve read on this site about sealing the attic with spray foam at the roof, instead of insulating the attic floor. The roof has an 8/12 pitch with two dormers so I would have about 2500 sq. ft. or more of roof to cover, versus upgrading the existing attic insulation.

Any help I can get understanding how to best accomplish this project would be greatly appreciated. I plan to live in this house another 20 to 30 years, but really need to improve the energy efficiency.

Roger