Just visited a single storey ICF house (2700sf) with a 3ton WaterFurnace pump and dump geo system installed.    Theunvented attic space is being used as a Return Air (RA) plenum.   Didn't eyeball the attic space, but it was explained that the RA will enter the attic space thru grilles in the ceilings of various rooms and find its way to the large RA box duct opening in the utility room ceiling and down to the heat exchanger.  The utility room is located smack dab in the middle of the house.

The roof underdeck and gable walls are well sealed with closed cell foam and steel trusses were used.  The attic is used only for utilities and supply air ducts. There are plenty of canned lights, but I didn't notice if they were sealed type. 

Heating is primarily thru radiant in the concrete slab and the forced air will be mainly for A/C.

Will this an acceptable approach or are RA ducts better?   I wouldn't assume the attic dust free, so would the filters be taxed? 

Dave

There may be a code issue with that - fire safety - violates draft stopping rules. Some jurisdictions require foam to be fire rated or drywalled.

Noise may communicate between bedrooms.

Attic may be dusty, but it won't manufacture dust, so I'd expect filters to cope with it.

Two corporate aircraft hangars that we recently did the HVAC in (straight electric with AC) where spec'd like this. It was a metal roof with minimal insulation on the roof deck. Seems to be working after 1 year with no complaints...except for high heating bills. Most codes require spray foam that is used for utility space to be coated with an ignition barrier.

If you have any exposed electric wiring up there, you may run into electrical code compliance issues.

See previous discussion here:

Unvented attic as return air "plenum?"

arkie hit it.
I'm some applications (mainly commercial) all materials in the attic would have to be "plenum rated" which is to say they do not exceed a certain flame or smoke spread.
As an inspector myself, I would consider this attic "within the envelope" making pass through registers and a central return perfectly legal. This can also be done with properly prepared underfloor crawl spaces.
Dust would be essentially the same as any other system (perhaps a little less than a floor level central return.
j

Arkie, Your reference post was very informative as were the additional comments on this post. Next time I'm over there, I'll see if I can peek in the attic space for what's there. The house is in a rural area in which there is no inspection requirement, but the homeowner is interested in doing things right. cheer, Dave