I don’t want to open another debate on the unvented — vented attic debate but…
Building single story gabled (8:12) roofed home in CZ 4A. Built on a concrete slab with ductwork planned for the attic and trusses in the roof.
What would be the best way to protect the airhandler and ductwork other than a unvented attic?
If you were to insulate a gable end wall in the attic with dense pack cellulose how would you install the insulation when the wall is built with 2X4 on flat (truss)? How wide could the bays be? Construct bays out of osb and 2X4?
Could one install an attic floor above the ductwork? Spray that with a minimum of SPF for air-sealing and place loose blown cellulose on top of that?
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How high are your ceilings? A smart solution would be to run the ductwork in a dropped ceiling chase, put the air-handler in a closet or mechanical room. Partial enclosure of the attic is possible but you're going to have to resolve some fussy details. Good luck.
If the only available space is the attic, you could conceivably build an insulated closet for the air handler up there, and a chase along the tops of the bottom chords that's insulated and air-sealed on the top and sides. If the floor plan is compact and you can keep the duct runs short it might not be too bad.
A good alternative might be a couple of mini-splits... skip the ducts and air handler entirely.
Many people have been down this road before, and the choices remain the same.
It's a little late in the design process to try to correct a fundamental design error.
Ductwork and HVAC equipment belong inside the home's conditioned envelope. Since your designer didn't know that, you are now scratching your head.
The most straightforward solution is to create an unvented conditioned attic. If you don't want to do that, you can experiment with half-measures, as David said. Good luck.
Not really a design problem but just "cold feet" really. The original decision was to go unvented conditioned attic, therefore no need to move the ducts. I have always had my doubts. Seeing the framed space, I don't feel any better about the decision. I was trying to lessen the amount of unvented space...