Looking for some advice on floor insulation on my house project. The first floor is over a semi-conditioned basement. Currently, there is no insulation. the existing floor is 100 year old t&g pine on top of 2x8 floor joists. With the rehab, the existing floor must be built up 3/4" to match the floor of the addition. The finished floor will be the 3/4" existing pine, 3/4" (TBD), 3/4" radiant heat panel (pex), 1/2" to 3/4" engineered or hardwood floor. I'd like to provide a thermal break over the basement, so my question is; would it be better to use 3/4" foam board under the pex panel or 1/4" foam with 1/2" plywood for a stronger floor. The house is in central Maryland.

In general from a whole-house heat loss and basement humidity/temperature point of view it's more effective to air-seal & insulate the foundation walls and foundation sill/band-joist than insulating the floor.

Foam board under the PEX panel would be fine, but is woefully inadequate for thermally isolating the basement. With a radiant floor you'd need R11-13 batts (faced or unfaced) snugged up to the subfloor to achieve reasonable isolation from an air-sealed & insulated basement. If the basement is air-leaky and is going to run cold, R19s would be better.

Using 3/4" t & g OSB/plywood subflooring would be the least problematic approach to raising the floor since it would stiffen the floor leading to less squeak & creak, and provide a flatter reference surface for the layers above it. It would add about R1 to the stackup. From a thermal point of view, going full-depth with fiber (even crummy low-density R25s) on the 2x8s would reduce the downward heat flow about as much as R4 foam above the subfloor + R19s between the joists:

Most species used for joists run R1.2/inch, and at 7.25" you're looking at ~R8.7 of thermal bridge at full depth, vs. R6.6 with R19s. Add R0.9 for the 3/4" OSB (instead of foam) and you're at R7.5 of thermal bridging. That's R1.2 lower-R at the joists but a much bigger difference at center cavity. The net heat flux is about the same.

But it's still worth air sealing & insulating the foundation, which will reduce infiltration as well as heat loss (which is still significant, even at 50F basment temps.) The infiltration itself is a heat loss in winter but also a big humidity gain to the basement in summer in an MD climate, leading to the musty-stinky basement smell and mold growth on things stored there. IRC 2012 code min foundation insulation for climate zone 4A (MD) would be R10 continuous insulation, or R13 if it's fluff-between-studs. Studwall basment insulation on it's own is a bit risky in MD, since interior vapor retarders would trap ground moisture in the studs, with mold potential, and without an interior vapor retarder the is risk of wintertime moisture drives building up the framing. A cheap solution is to put at least R2 of rigid foam between the studwall & foundation, and insulating the studwall with unfaced batts. Foil faced polyiso, or EPS with a plastic or foil facer would be preferable at that thickness. If it's facer is on one side only, the facer should be next to the foundation, not the studs. An inch of EPS under the bottom plate of the studwall is useful as a thermal & capillary break too, to keep ground moisture from wicking up from the slab.

Thanks Dana1, your posts are always an education.

The basement is 100 year old poured concrete walls originally with a dirt floor. At some point about 4" of concrete was dumped and smeared over the dirt floor and it varies from 6' to 6.5' from the floor to the joists. It's covered with epoxy paint. Exterior grade runs from 6" to 3' exposed. Insulating from the outside is not an option unless I want to rip 75' of existing porch off first. Humidity is an issue but controlled by a water heater heat pump and a dehumidifier (as needed). The area will be storage and a small work shop, so 55 to 60 is the max temp required. There is some water seepage on rare occasions, but that will be fixed with landscaping changes.

The plan is to spray foam the sill/band board and air seal as much as possible. Radon is a concern here so we'll be testing for that. I can tap into the required vent stack on the addition.

That leaves the basement ceiling and basement walls. The ceiling/upper floor joists are filled with wires, plumbing and bracing, so adding batts or trying to fill the space would be a pain plus I'm trying to avoid fiberglass. It previously had fiberglass batts that provided a wonderful habitat for mice. After ripping it out and repairing half the wiring in the house, I'm looking for something a bit more "solid" that would allow access to the wiring and plumbing. I was thinking a couple of inches of foam board (difficult) or a couple of inches of spray foam (expensive) against the bottom of the floor with the thermal break on the top of the floor to offset the thermal bridge through the rafters. Would that be viable or should I just forget anything up top and not worry about the rafters?

Would spraying the basement walls with 2" of closed cell foam be a mistake?

The easiest course I can take right now is installing 2" faced R-max ridged against the bottom of the floor. It's R-13.1 plus I figured ~R-3 for the exposed joists plus about R-1.8 for the materials above the floor. The joists are about 14% so I figured my average R value to be about R-13.5 for the whole floor. I can install this right now.

I need 50 or more degrees to foam the rim board and I'm still working on the best way to do the basement walls. Am I on the right track?

The ideal solution in the basement is 2" Thermal poly-iso fastened to the concrete walls, if they are flat enough. If not, closed cell spray foam will work just as well. Both will stop moisture migrating through the concrete and help condition the basement. For the floor, strongly suggest you consider removing the concrete, digging it out more and installing EPS and thick layer of taped plastic to stop the moisture drive through the floor. This obviously will be a longer project, but well worth it in the long run.

Thanks Bob, Digging up the floor is a possibility, but it will be a "down the road" one. The basement isn't talk enough to be finished living space, but it will be great storage space. Right now I just need to get the upstairs temperature under control. I'm looking at a 57 degree first floor surface temperature. Comfort level is priority one.

When using poliso on the walls in an antique retrofit it's important that the cut edge of the polyiso not be resting on concrete (or dirt), or it can wick ground moisture into the foam. If the basement has a history of minor flooding, stop the polyiso above the high tide mark.

Closed cell spray foam works great, and is self-sealing against air infiltration, but is blown with HFC245fa, with a global warming potential about 1000x that of CO2, which makes polyiso (blown with pentane at only 7x CO2) a greener option. It's arguably "worth it" for sealing & insulating the band joist and foundation sill, which is often tough to do with cut'n'cobbled foam board, but for the major surfaces of the walls it's a tougher case to make.

In a central MD climate an inch of EPS (also blown with pentane) trapped o the concrete wall with a 2x4 studwall insulated with batts (unfaced rock wool preferred, R13 kraft faced fiberglass would be OK, but not foil-faced) delivers higher thermal performance than 2" of closed cell foam or 2" polyiso. Put an inch of EPS under the bottom plate of the studwall as a thermal & capillary break. Since the studs are not structural, 24" o.c. spacing and single top plates are fine. The R-value of the wall foam offers plenty of wintertime dew-point margin at the foam/fiber boundary, as long as it has a reasonable semi-vapor-permeable air barrier on the interior side, such as half-inch wallboard with a coat of standard latex primer.

With a basement temperature target of 57F there is no economic case on energy savings for insulating under the slab, since the subsoil temps are about 57F in central MD:

http://www.earthrivergeo.com/img/geothermal-article/geothermal-subterrainean-temperature-contour-map.jpg

But insulating the slab reduces the mold potential from summertime air infiltration, since the average dew point of the outdoor air in that climate is north of 60F.

But in fact, the basement temp will be above 57F in winter if you insulate the walls, assuming you keep the first floor room temps at 65F or higher. If you have a boiler or furnace, and a hot water heater in the basement, the standby and distribution losses should keep the basement above 60F after insulating & air sealing the walls & band joist.