I am installing a staple up retrofit and am using plates to cover 100% of the subfloor.  My question is whether the radiant barrier is worth doing.  If its purpose is to reflect heat back up to the floor then is its effect minimized if the plates then just reflect the heat right back?  It makes sense to me that if there is any wood exposed then that would be the recipient of the reflective heat from the barrier but what if the entire floor is reflective as well?

The emissivity of the aluminum plates is extremely low in the first place, so the radiated heat transfer downward is very small indeed, so RB would be nearly worthless.

But even in plateless staple ups the value of RB is rarely worth the labor involved, and you'll get as-good or better performance by upgrading to higher density batts on the fiber insulation (use R15 instead of R11 between conditioned space floors, use R22-25 instead of R19 over unconditioned space.) Even under 150F roof decks in FL the reduction in heat gain to the attic ceiling you get with RB is only ~20-25%. If you're running 120F water, and the plates hit 110F, the radiated heat flux would be very small even if you painted 'em with high-E paint.

Plated or unplated, MOST of the heat transfer from the PEX is conducted & convected, only a small fraction is radiated. The "radiant" in "radiant floor" has more to do with the emissivity of the floor into the conditioned space, but even there the bulk of the heat transfer is conduction & convection, not radiated. (The air adjacent to the floor gets warmer than the ceiling. Were it predominantly radiated the ceiling would be warmer than the surrounding air.)

Thanks, this is what my intuition was telling me.  I think I will put my efforts into just adding more r value.  A follow-up question.  Since I am part way through my project and have installed the RB to this point would it be a problem to just push the RB up against the plates to make room for more batts?  I have heard that contact between the RB and the plates is bad.  I don't feel like taking the insulation down and taking the RB off since I stapled it onto the paper barrier on the batt.

What R value have you already installed w-RB attached, and is it between 2 living-space floors, above a basement, a crawlspace or... ??? (And how much air gap is there between the tubing and the RB?)

R value is 20 and it is between an unheated basement and the first floor.  I have about a 3 inch gap between the RB and the plates.

if the RB touches plates, it is no longer a barrier of any kind. radiant barriers require airspaces to function. but if you don't care about the barrier, the space is not needed.

At R20 with RB above with a bit of gap, just leave it.  If the basement temps average 60F or above, going more than R20 would be overkill anyway. 

If it's below 50F down in winter there it means the sills & band joists, doors, windows etc, are leaky as hell, and you should spray-foam seal/insulate them all the way around, and fix all of the weatherstripping, etc.  Deep well temps and average annual soil temps in the Newburgh area are ~ 50F.  Odds are that even uninsulated it'll coast through the winter well above 50F, over 60F if there's any sort of wall & band-joist insulation.




Even when the sub-floor gets up to 100F, the heat loss to the basement through the R20 to a 60F room below is less than from a 70F room through an R20 wall to a 30F outdoors, (and less than the loss from a 70F room through an UNinsulated non-radiant floor to a 60F basement.)  Most homes built before 1990 have nowhere near R20 walls- R20 is "good enough" over semi-conditioned space in your location.  Your seasonal average sub-floor temps are likely to be lower than 100F.

I just finished "fixing" my staple up system by adding thermofin plates. I then skipped the radiant barrier as instructed by those much wiser here on this site. I used Roxul Stone Wool Insulation to insulate. Extra R value for depth. I believe 6" is R23 and 8" is R30. This fit nicely in a 2x10 joist bay. It is compression fit, dense, doesn't grow mold/mildew as it is non organic and will dry if wet. Oh yeah it is a fire barrier as well, as it will not burn. I used the R30 in the box sill (after spray foaming any gaps/cracks). I cut a 9" piece off the end of one bat, layed it in the box sill, then used the remaining 31" to insulate the first 31" into the room of each bay. The rest I used R23. Very happy with the product. It is about 2x the cost of fiberglass, but worth it and if you skip the RB.

I am very proud of you!