HVAC ducting on the second floor – GreenBuildingAdvisor

Hello All

Two story floor plan in the making with an additional basement. Zone 5A. First floor ~ 2500 sq feet living space with two storey great room. Second floor would be around 2900 sq feet space (extra space above garage). The space above garage will be conditioned and part of the second floor. There will be a vented attic at the top, without any intention of making it a usable space in future. Plan is for R80 blown in cellulose in the attic. Exterior walls would be R40 (Zip + Rockwool comfortboard). Will have an ERV.

Two forced air HVACs – one in basement for Basement + first floor, and second on the second floor, primarily for the second floor.

Question is how to run the ducts in the conditioned space in the second floor. I do not want any perforations in the ceiling air barrier. 

Current plan is to have a conditioned mechanical room in the space over garage (at one end of the second floor). We cannot find a good place for the mechanical room in a central location on the second floor. The second floor has a straight hallway in the middle, so the man trunk will straight run over the hallway (bulkhead/drop soffit) and give laterals to a master suite (with two closets and bathroom) on one side, and two other bedrooms (with closets and bathrooms) + office, on the second side.

Initial plan was 9″6′ first floor ceilings, and 8″6′ second floor ceilings. No ducts in Attic.

HVAC contractor has suggested to increase the ceiling on the second floor to 9″6′ to have a 12″ bulkhead, and use 2’x 12′ ceiling joists (upgrade from 2′ x 6′) for top lateral take-offs. This is adding to costs. Plus then the ducts will flow in the ceiling, and we will have to air seal ceiling supply vents. I just want to have no penetration in the air sealing of the ceiling (planning to use SIGA Majrex).

I’m unable to clearly find how bad are “Side” lateral take offs compared to “top” lateral take offs. My thoughts are that we can increase the second-floor height by 6′ (from 8″6′ to 9″) and use “side” take offs, and those take-offs can run parallel to ceiling, but below ceiling, and run through bulkheads in closets/other spaces, and the supply vents would be on the lateral walls of the bedrooms, close to the ceiling. Making a 12″ bulkhead will then only drop ceiling height from 9″ to 8″  in the hallway. 

Does this sound okay?

Any other ideas? 

I googled, and some ideas are to run an inverted soffit in the attic, with that space being conditioned, but that sounds complex to me to pull it off. I just want no penetration in the second floor ceiling/attic.

Thanks in advance!!