We are installing 1/2" pex piping on 3/4" subfloor with "sleepers" between piping and then our flooring installed over that.  The rooms under the subfloor are heated usable space.  Joist spaces will have fiberglass insulation.  We were thinking of putting down space blanket (mylar blanket) material as a radiation barrier.  Is this a worthwhile endeavor?  I believe the R-value for this type of application is relatively low (1.1) from what I can find.

Thank you.  I'm relatively new here and apologize if this has been covered before.  I can't seem to figure out how to search for specific thread subjects on here.

Reiner

do not bother. take the money and buy some spray/rigid foam for the ends of the joists to seal them up tight and you'll do more good!

I've heard more than R2.  I'd use the rigid foam with the foil already on it if the price was right - but not spray, it is expensive. Of course with the space below heated, there is no net heat loss possible, only diminished floor heating.

I think Rob was talking about sealing & insulating the ends of the joist bays at the rim-joist/sill to prevent thermal bypassing of the insulation.  It's not that much spray, and a lot less labor content than cutting up rigid foam, THEN sealing the seams with 1 part foam to keep slow convection loops from condensing on and rotting the band joist or sill.

Foil buys you next to nothing here, since the tubing is above the subfloor, which translates into even lower temps. Foil sometimes buys you something when the delta-Ts are high, but here we're talking about sub 10F delta between the sub-floor and the conditioned-space room below.

Sounds like an argument for no insulation in the joist spaces.

Do you have figures for the R value equivalent per $ for foil or foil backed insulation as compared to unfaced or kraft faced insulations? Assuming of course that an air space is present and there are no differences in labor costs.

I don't know how you got there.

The point is, you must insulate and isolate. Foil adds little if anything to the efficiency of heat transfer making a great reflector of radiant heat in the correct application but when placed between the joists it does very little. If dirty/dusty it does nothing. This is more especially true for suspended tube applications as a little over half of the energy transfer will be done by convected currents within the joist space, thus the need to isolate the tubed joist space so super-heater air does not escape to the space below or out the rim-joist.

Nearly all foil products are wasted in radiant heating applications save some engineered products where they are almost always used to conduct rather than reflect radiant energy.

When between conditioned spaces you only need sufficient insulation to be able to control the temps independently, without much interaction. R19 is overkill there- R13 would be plenty. But foil beneath a ~75F subfloor isn't going to be anywhere sufficient for isolating it from a 68F room below on it's own, but provides about zero practical benefit in this application when the necessary fiber is installed. The fiber or other conductive insulation is both necessary & sufficient, whereas the foil is neither.  Remember, there is ~R1 of subfloor between the tubing and the insulation here. 

There are some tortured arguments to be made in a suspended-tube/180F water case, but typically it's less labor and more permanently effective to just add another R6-8 of fiber.  Under aluminum plated staple-ups it's silly-on-a stick, 'cuz the emissivity of the hot aluminum is EXTREMELY low, so the radiant barrier is only dealing with a much much lower radiated heat flux (from the lower temp exposed wood of the subfloor that isn't covered by the plates.)

But under a hot roof deck with less than R30 of fiberglass insulation on the attic floor there is sometimes a measurable benefit.  The radiated heat flux is simply larger than in the 75F subfloor/68F lower room case.

So no hard cost/performance figures with references? (say for other small cavity cases, this one is beaten to death)