When having uneven floor temps, it is very important not to be using oversized cooling & heating equipment. Longer runtimes with the dampers set for the different heating & cooling seasons will help even out floor temp differentials.. In northern SC, the 2.5% summer design is near 91ðF dry bulb with a 17ðF wet bulb drop for about 50% Relative Humidity. Therefore, you need a smaller A/C with longer runtimes to get the humidity level down to 50% or lower within your home for comfort. I would estimate that a in a properly weatherized home in your climate, a 3-Ton unit would cool & properly dehumidify your home. I'll put a link to a FREE Whole House load-calc. Use the calculator with the software to figure the infiltration CFM. I'd use 0.4 ACH X's the volume of your home in cu.ft. of air. Okay; 2000-sf X's 10' ceilings is 20,000-cu.ft. * .4-ACH is 8000-cu.ft., it will give the amount; but using * .0167 it's 133.6-CFM; enter on the correct line so it figures the latent & sensible loads correctly. Here is a link that might be useful: FREE Whole House Load-Calc

Divide 1600-CFM by an initial filter velocity of 300-fpm = 5.33-sq.ft. of free-open air area. Hart & Cooley A 25X20 only delivers 721-CFM; not HALF what is needed for 300-fpm velocity. . Therefore, 'two' 25X20's would only deliver 1442-CFM; you need to flow 1600-CFM at 300-fpm with a cheap 1" deep throw-away filter. You actually need a second 30X24 filter area 'to get above' what is needed for 300-fpm initial velocity. Best to use; but if can't...> Another 24X24 filter area; using both filters would then flow 1553 CFM at 300-fpm initial velocity through a new clean filter.; that is close. You could reduce the 4-Ton's CFM to 1550-CFM with no problem. Even 350-CFM per Ton might be a wise move; that is 1400-CFM. Unit is probably oversized & that CFM would provide better humidity control & improved COMFORT. This post was edited by udarrell on Thu, Jun 27, 13 at 15:57

A properly sized unit should be able to maintain reasonable humidity levels (though maybe not at 78 degrees without a dehumidification mode). The dual-stage unit will help control humidity. The only way of extra dehumidification with the XL16i that I know of is with the TCONT803/Honeywell TH8321 thermostat, which will overcool to control humidity. Ask your contractor why he proposed the VisionPRO IAQ when the BK terminal is already in use by the XL16i, thus not allowing the dehumidification feature of the VisionPRO IAQ (which is great as it actually slows down the blower). If you want an alternative that can make use of the VisionPRO IAQ, I'd go with the XL15i. It's a single-stage unit, but sized properly it should control humidity fine with the added advantage of the VisionPRO IAQ. Here are possible setups (ask your contractor what size he would recommend--I would NOT go with a 3 ton for your size home). Now, there are different air handlers that can go with these units, but the 4TEE3F39 air handler is tall due to the Vortica blower. The 4TWX5024A1000A (2-ton XL15i) with the 4TEE3F31B1000A yields 13.00 EER, 15.25 SEER and 8.60 HSPF. The 4TWX5024A1000A (2-ton XL15i) with the 4TEE3F39A1000A yields 13.00 EER, 16.00 SEER, and 9.00 HSPF. The 4TWX5018A1000A (1.5-ton XL15i) with the 4TEE3F31B1000A (only rated match with the 1.5-ton outdoor unit) yields 13.00 EER, 15.00 HSPF, and 8.50 HSPF. Feel free to print out my responses if it will be easier for you to go over these possible issues with your contractor. Good luck.

"There was no excuse not to do that. " yes, i agree. especially since it was in the contract. i got a bit bullied at the end because he got behind and stressed out...another thing would be to call him and try to make him fulfill the contract, but i'm afraid he would say it isn't possible (what he said last year, after he agreed to do it.) removing the entire ceiling, installing all new drywall and finishing it seems really overwhelming and expensive and horrible since this ceiling is less than a year old (and i did all the finishing myself which was a lot of work) thus my attempt to find another solution - like blown in insulation, venting, or roof coating. everyone agrees that these aren't real options and that removing the whole ceiling is the only way to insulate it? one last gasp: would a vent in the pitch like my drawing help at all?