I am a building and solar contractor in Central Florida. Our desire to to build zero energy homes and we've experimented with various ways to improve efficiency.
We are getting ready to build a duplex for ourselves that will be very tightly sealed between living space and attic. We are even actually considering drywalling the ceiling before building the interior walls so that the drywall envelope is not broken where every wall is located. (The electrical contractor is the main guy affected if we do this but we would leave a sheet or two of drywall off in the middle of the rooms to make it easier to wire. The only thing the plumber has to do in the ceiling is run his stacks through to the roof.) We want to avoid putting ductwork into the attic for the obvious reasons. On an 8' ceiling it gets challenging to put ductwork in conditioned space as you can't drop your ceiling below 7' in those areas where your duct work would go. The floor plan is designed so that we would be able to hit the entire apartment with air by lowering the ceiling in a hallway and one bathroom.
So the thought is this. . . Since we will have a (virtually) completely thermally sealed separation between attic and living area at the 8' ceiling level, why don't we just hook the A/C supply to this cavity (between the 7' and 8' sheetrock. We would then penetrate the cavity on the adjoining rooms that are 8' tall with an appropriately sized register so that the air is adequately and uniformly circulated (at least that is the goal).
The reasoning is that it is simpler and cheaper than creating a duct box in the same space and we don't see what benefit the ductboard box would have since the cavity is 100% in conditioned space. Considering that a 7' ceiling is going need to be supported by 2x4s at a minimum, you really only have 8" to work with in the cavity. That's not enough height to do anything with a ductboard system.
The units are 3 bedroom/ 2 bath, 1,200 Square feet in size and they will have vinyl, double pane, Low-E windows, a galvalume natural metal color metal roof, 2' soffet overhangs, and will be VERY well sealed between living area and R-50 attic. I haven't run the numbers but I suspect that one ton of air would suffice.
Please advise on the following:
1. Is there any reason why we should not create an air distribution chase this way? Would there be moisture or condensation issues of some kind? Would be be able to adequately balance rooms by register sizes is this situation?
2. What is the best high SEER way to deal with such small BTU needs where a ton of air might suffice, considering that most A/C units are much larger than that? We want to avoid short cycling the units and we want to control humidity? Is this where an inverter based variable type of system makes sense? I've thought about using a mini-split and avoiding an air chase all together but it seems like you need a unit in each bedroom and the living area and that would be overkill for a small place such as this.
Thanks so much for your thoughts and input.