Built me a Barndo and I'm ready to run my HVAC ducting. The barndo is 40'x60' with 30'x40' living area. Rectangular Rigid ducting is very expensive. About $50 for 4 feet of 8"x12". My second floor rests on top of 8" C purlins. I would like to use the gap between the C purlins to create my own interstitial ducting. The C purlins run parallel to the gable ends on 16" centers. As I see it, the metal c purlins form two of the vertical walls of my "duct". The bottom side of the 2nd level floor, 1" plywood, will be my upper side of the "duct". I can cover that wood with "[" shaped HVAC panning. I only need to create the bottom side of the "duct". Again a "]" shaped HVAC panning. The panning can be attached to the c purlins by self tapping metal screws. I am going to run a main trunk perpendicular to the c purlins and drop down 3 branches to the proposed interstitial ducts.

I think it can be cost effective and a sound alternative to typical HVAC ducting. ???

*** The Barndo has Closed Cell spray foam on the interior of the exterior walls at 2 inches, and 4 inches on the interior of the roof.

THANKS!

Return air maybe, but for supply air you may get sweating on the steel (which could turn into a nightmare), unless you insulate it.

Yes. Closed Cell Spray Foam, 2", on the opposing sides. * And the entire living area will be completely "sealed". No soffit vents, no attic. + pressure.

Plywood cannot used fot supply air. Cool air will cause water to to migrate through unconditioned side of wood. Mold will grow. Even though the "duct work" is in conditioned space. One degree of temperature difference cause condensation. Wood is an insulator. During cooling cycles, the inside of the plywood will be cooler than the outside of the plywood.

Next, a 12X8 square duct is only good for 400CFM. Which is only good for only good for 12,000 BTU of cooling. That is only 1 ton or about a room AC.

I didn't consider metalsheet and screw and duct sealant.

Also, square duct must have turning vanes at direction changes to prevent eddie currents and turbulance.

Even if you solve condensation and mechanical installtion procedures, I dont think you will move enough air to do the work you need.

I will post a field duct sizing chart. You need 400 to 500 CFM per ton of cooling.