the ROI is rarely there on spray foams in walls, if ever. Marketing BS has people thinking spray foam is the next step up from batts to ultimate energy savings. The truth is, unless you are going with closed cell (most do not) your wall r value increases 1 or 2 points, however you do get a boost in air sealing within the stud bays. However you still have a thermal bridge in either system, and spray foams can not address that, as well as air leaks at your top and bottom plates of your walls. I would be curious to see how you are getting r21 in your exterior walls. if they are using fiberglass batts, you arent hitting r21. For zone 6, it would be money in your pocket to increase that value (especially after thermal bridge and batt insulation short falls your whole wall r value is closer to r16) by adding exterior foam sheathing. The extra upfront cost of adding a couple inches of foam on the exterior will be cheaper then the spray foam upgrade, as well as decreases the thermal bridge shortfalls (reduced "cold drafting" on your walls) and provide a better air seal assuming seams are caulked/foamed/taped. Get your HVAC guys to do a load simulation and adding foam sheathing should reduce your tonnage for the equipment, thus reducing the upfront cost of it and offsetting some of the cost of the sheathing. After that, depending on your energy usage, rates, size of home, etc your payback on your sheathing will typically be less then 5 years. I would also recommend switching from batts (again, assume you are using them) and going with a blown product such as fiberglass or cellulose. It will do a better job of fully insulating the wall cavity. Finally not everything needs to be looked at as ROI. Some things will provide added comfort but not always come with a payback. Full foam below slab could be one of those items. I have had clients that do it and are happy they have, and I have clients that save the couple k and dont, and wish they have. One even took up their entire wood floor (floating) and added foam on the slab after the fact since it was so cold. These are people in your same climate zone 6. People never like to spend money on things they dont see, and insulation is typically the first area people do not always care about. Most builders do not understand air infiltration, thermal bridging, and how to do energy simulations and payback so most do not understand the advantages to increasing the shell insulation and tightness of the home. Most stick by code minimums (like in your case). Energy rates may be low at the moment. But do keep in mind...energy rates will always keep going up.

Basements inherently have more humidity. To combat this, have your entire concrete walls and floors sealed with oil based watertite. Otherwise you will need the dehumidifier to run all the time. It is possible your airflow is not sufficient, causing the ductwork to get very cold which will increase condensate. So possibly increase ductwork and insulate the ductwork with a vapor barrier are more solutions to try. I would not try to use the AC to dehumidify the basement, basements typically are cold enough and if you run the AC in the basement they get uncomfortably cold.

Basements which are mostly underground have a small heat load. In the summer the basement can actually become too cool if you leave the vents open. When I installed my new system I didn't get a separate zone for the finished part of the basement. I didn't think it was needed but now I know better. I would be concerned creating a zone for the 1500 sq ft basement with a single stage furnace. I am willing to bet the first floor system is over sized which further aggravates the problem. I suppose a contractor who is very good a zoning could do it correctly. But then again someone who understands the physics behind good zoning design would probably recommend upgrading to a variable speed 2-stage furnace.

Sizing can be less of an issue when you have multiple units. Because each unit is controlled independently, provided of course the unit itself is not over sized for the area in which it serves resulting in excessive short cycling. In your case 3 ton plus 2 ton isn't necessarily equal to 5 tons of cooling... unless you drive those thermostats really low. By having two systems with total capacity of 5 tons (which, most of the time this would be oversized for a 1700 - 1800 sq ft structure) has a few advantages. 1. You have the ability to use one system only during low heat days or set the opposing unit higher. The unit set higher only runs if load conditions warrant.... meaning if it's not hot, the thermostat controlling the unit does not call... the unit can not run without a call... 2. Utility bills can be much less... because you rarely use both systems at the same time. You run the bedroom system when you sleep, the other when you're awake. If you have a home office in one the bedrooms you may need to rethink things a bit. 3. Extra capacity for when it's hot out. When it's hot you have the capacity to cool the home. (even while entertaining guests --- additional people add heat) Remember heat waves come, heat waves go... a so call 'properly sized' AC system is essentially properly sized roughly 10% of the time... especially if that system is a single speed AC system. Alot of people I come across like to sleep cool. So for that reason I doubt I would change the configuration of the space. Once you realize what you have, the choices you make are mostly budget type concerns. Essentially a set up like this runs like a 2 stage system. This kind of subject is above the pay grade of many... choose your contractor wisely. I service the Katy, Texas area.