Supply ducts, as opposed to return ducts, cannot use wood as any part of the duct. As an additional comment, wood that is exposed to temperatures in excess of about 175 deg is permanently changed. It is recommended that wood never be heated above about 150 deg. The ignition temperature is lowered and the strength diminishes. This damage is cumulative and increases with each such heating. Fuel-fired hot air furnaces might approach such temperatures in the plenum under certain conditions.

The question of "do you need to" would better be "will it pay off" as I see you understand. The answer has to be that it depends. If both the basement and main floor to be used a lot and heated at the same time, insulation between them will have little benefit. The added insulation will have the most benefit if you heat the basement a lot when the upstairs cooler. It will have some benefit if you are heating the upstairs when not heating the basement. You are really going to have to look at your projected use and what temp you will keep "unused" areas set back to. It could be the case that the heat that leaks by even with no insulation will pretty much serve to keep the area at the level that you want so no heat calls are necessary in that zone. If you add insulation, you might be running the circulator in the cooler area when you might not be otherwise.

We live in AK and have had pex-type hydronic infloor heat in our home for 16+ years. The tubing has gypcrete poured over it to a depth of 1.5". We have tile, vinyl, marble composite, and floating genuine wood plank flooring. We are now in the process of designing our new and last home. I'll tell you my experience from our current home and then what we're planning for the next one. Vinyl: I know you aren't considering it, but for those who might think about it, we did have bad shrinkage problems in one area where the water in one loop went way to high due to a temporary equipment failure. I won't do vinyl again, anywhere. You should think about the possibility of equipment malfunctions as part of your process; you can't guarantee the ideal temp will be maintained in such an event. Ceramic tile and marble composite tile: Tile is one of the best types of flooring for heat transfer. It's the one type of flooring where you can feel the heat (in a good way). Our kids tended to use the back entry floor as a drying rack for their snowsuits and gloves; just lay them out on the floor and they're dry in an hour. It is a hard surface and people tend to see it as "cold" even though it's the warmest floor in our home. The downside: All our grout joints have cracked because of floor movement. I don't know whether this is worse in infloor heat, but our back entry is only eight feet across with adjoining hall and bathroom being narrower, 1/4" joints with sanded grout, cracked badly in both areas. The master bath has the composite marble; that room is about 17 feet across with 1/8" joints, unsanded grout, and there is a single grout line that cracked all the way across the room. Apparently that grout line fell in the lowest "bounce" point. Incidentally, these two rooms were installed by different tile guys, so I don't think it was a case of inferior work. I'll do tile again, but I will make sure the floors in those areas are heavily reinforced to prevent movement, and I'll store extra grout for later fixes. I'll almost certainly put a tile product in my kitchen and entry areas. Hardwood: We have floating 7/8" hardwood planks, clipped together (Junckers brand). We acclimated the flooring as directed before installing it. The manufacturer recommended a felt-type layer under the flooring. Because of the gypcrete having a slight cupping from wall to wall, the floor squeaked at first. My husband took it back up and put down more of the felt lining in some spots that were lower, and that cured the squeaks (although a few spots are still creaky, especially in winter). The biggest problem is the shrinkage between planks. They're tongue-and-groove, but they aren't snap-together and the metal clips aren't enough to hold them taut together. We have a friend who put down wood strips between the tubing runs and nailed down oak tongue-and-groove flooring, having gone through the preliminary step of lengthy acclimation period beforehand. He is also disappointed that there are gaps in his floor also. I will plan on using engineered hardwood flooring but never use plank strips again, even snap-together type. I'd go with a thinner product next time too. We do have rugs over areas of the wood floor, but rugs are insulating and therefore not to be overdone without sacrificing efficiency. A friend of ours with infloor heat used a glue-together plastic laminate flooring, and that was very successful. Because of that, I would imagine something with a plywood type backing would perform better than just hardwood strips. Yet another friend of ours installed a bamboo snap-together flooring from Costco over radiant infloor heat. It failed badly, each plank bowing and cupping. The manufacturer had approved it for floor heat, so they stood behind it and refunded the money. They replaced it with another type of engineered hardwood and it is working well. I didn't mention that we have carpet in a few areas, and that is the most insulating of all the materials (a bad thing for floor heat) even though we used the type of pad that was recommended. I won't be using carpet next time. As to the question of the floor feeling warm, we don't notice it being particularly warm to our feet except in the tile areas (delightful but hard). When we visit friends without infloor heat, though, the difference in comfort is VERY noticeable. We feel generally colder in their homes and especially our feet feel colder. After having infloor heat, I'd never have anything else. As an aside, I did talk to a flooring specialist the other day and she said cork floors are usually approved for infloor heat and perform about the same as wood flooring. I'm sorry this is so long, but maybe some of the info will be helpful. A number of friends have gone with infloor after being in our home, so no matter what type of floor covering you use you will probably enjoy the effect of the heat.

Thank you Kevin. I have been "doing my own research" (LOL), and there seems to be another option to handle the difference in temperature which involves using a separate water heater, connected to the oil burner, to feed the floor heat. Seems like the mixing valves would be less costly, but IDK. In any case, this is not a project which is high on the priority list, so I think I'm good for now!