How can I increase the effectiveness of the A/C in a hot 2nd floor converted attic master suite?

Question

Just bought a 1940's masonry construction house. House has a finished basement (cold as a meat locker), first floor (comfortable) and 2nd floor (too hot). The 2nd floor is an attic that has been converted to a master suite (with vaulted ceilings), and it's way too warm. The furnace is really old, so I'm installing a new one (with credit from seller) that has a 4-ton blower (old one is 3 ton).

The master bed (24 feet x 12 feet) has a single supply and two returns (all on the same knee wall) all at same height (wall is only 43" high and then it follows the pitch of the roof). Each return is about 6 feet either side of the supply.

Running an additional supply duct is going to be very destructive. The HVAC guy says using a 4-ton blower, putting a damper in the main trunk in basement, and splitting the supply duct in the attic will probably fix it.

Before I commit, I'd like to get some opinions on how probable a solution this is. I would have thought splitting the ducts wouldn't increase the air flow, as it's constrained by the diameter through the walls (isn't it?).

Would an inline duct fan help?

Answer 1

The HVAC guy has a hammer, and your problem looks like a nail. More ducts! Bigger equipment! Wrong. Your problem is easy to diagnose: this room's ceiling is terribly under-insulated. When people convert attics into rooms, they almost always (like 99% of the time) insulate the ceiling incorrectly and insufficiently. You need a lot more insulation; the lack of enough is the problem, and adding more is the solution. There are two ways to do this: by adding rigid foam boards under the rafters or above the roof sheathing. If you're not planning to redo your roof anytime soon, the latter option is probably right out. So you need to remove the ceiling drywall, put, say, 5" of polyiso foam (XPS, EPS or mineral wool would work too, but you'd need more) under the rafters, put up new drywall, and live with a 5" reduction in ceiling height.

A secondary problem is that the duct is in the kneewall area, which is unconditioned space. So the nice cold air is passing through a duct in a space that's maybe 130 degrees or more. You can wrap the duct in a ton of insulation, which will help. Adding a radiant barrier under the exposed roof decking in the kneewall area will help too. There are lots of other approaches that will work here, such as insulating under the roof decking in the kneewall area to bring it into the conditioned space.