B1004U: do NOT use reflective panels in leiu of plates. they are not equivalent substitutions. All they can do is reflect radiant energy: this is not even in the same league as conductive heat transfer.

What Rob said!

Claims that a reflective panel "does the same job as" heat spreaders are patently false. Without contact with either the PEX or the sub floor it does nothing to thermally short-circuit the insulating air between tubing & wood.

The academic experimental proof of what providing a heat conduction path between the PEX & floor does for you:

http://www.ngml.ksu.edu/research/project_archive/project_archive_publications/ashrae/1036_final_report.pdf

Scroll down to figure 14: Heat conduction plates DOUBLE the heat flux into the room at a fixed water temperature.

What that means in the real-world is lower water temps to deliver the heat, and typically double-digit improvements in system efficiency.

There's no way even using a perfect gold infra-red mirror for a radiant barrier is going to get you anything like that. The air around the PEX forms an insulating boundary layer. To get the same heat out using radiation-transfer alone takes temps tens of degrees-F higher, lowering the boiler's combustion efficiency and increasing distribution losses in the primary loop & manifolds, etc. Making direct contact with a refractory metal between a significant surface area of the PEX and the subfloor short-circuits the boundary layer, and the primary heat transfer method become conducted heat. (About twice the heat as when using radiated + convective heat transfer alone.)

Heat transfer via conduction is very effective, far more so than radiation at low temps. You can hold your hand 1/2" away from a pipe or pan containing 140F water indefinitely without enough heat transfer to damage your skin, but don't try to grasp it firmly for any length of time. Put your hand with a 1/2" between the pan of 140F water and a radiant barrier with another 1/2" gap your hand will be warmer, but still not cooked.

Skip the RB, go for the heat transfer plates. Using R21-22 "cathedral ceiling" batts will be more effective than R19 low-density batts, and will perform about as well as R19 + RB. The heat-transfer plates are key to keeping the efficiency up- just from a heat-loss through the flloor insulation point of view, lowering the necessary water temp from 150F to 120F in a radiant floor over a 50F basement or crawlspace is about the same as going from R19 to R25 on the insulation (which I s'pose you could do.) But the difference in whole-system efficiency at the lower temps will be 10% or more (much more, if using a condensing boiler.)

...and the truth with set you free!

Think of doing it right as a good investment. In today's economic climate, saving fuel is more important than ever.

Don't be tempted by those who tell you how easy and cheap it can be. Remembering there is no such thing as a free lunch.

OK, i feel like i have learned a lot, albeit been lost a few times by professional jargon i am unfamiliar with. It is clear to me then that I do not need the RB in either application, the Icenene insulation over my drafty crawlspace, or the high density FB bats over the basement. So, i am left with one final question that I am a bit unclear on, and it gets back to Rob's earlier point:

Do i need to "protect" the pex tubing from the spray-in Icenene so it does not get in the gaps? These seems a bit tricky to me as installing a clean barrier there, particularly if it is cardboard, seems a bit clumsy and imperfect to me. Would i just use a strip 9" wide or so and staple it up on either side of the plates as smooth as possible? Is this typical? Has anyone seen this done before? What about putting in a very minimal FG bat right up against the plates and tubing, then spraying it in?

Or, how about this: it sounds like icenene is not too common. What if i just skipped it altogether (it would be cheaper i imagine) and just went with a high density FB bat as the insulation over my crawlspace too? I just worry about the draftieness especially because it it is a POORLY insulated addition. I am going to have to address the walls and ceiling in the fall to get some decent insulation there as well. By the way, i am in Salt Lake City Utah. Hot summers, moderate to cold (not Canada cold) winters...

Icynene really IS a more-expensive way to go, and using fiberglass (even high-density type) under the floor is bound to be less expensive, with better bang-per-buck value. As much as I like foam insulation, in warm-side-up applications fiberglass batting really isn't all that bad, even at high differences in warm/cold side temperatures. (Cold side up, as in attic floors, convection currents within the fiberglass cause the R-value to drop dramatically, since the warm side air will rise to the top. FG SUCKS in attics for heating dominated climates, since the times you need it the most is when it's performance is worst.)

You may still want to consider using icynene to insulate and seal the band joist & foundation sill everywhere- that draft problem may simply dissappear if you do. Even in Salt Lake I'd think 3-4" of foam on the band joists would be well worth the investment. (It's a heluva lot less material than doing 5.5" under the radiant floor everywhere too!)

In order to deal with losses from drafts/infiltration in the fiberglass, foam-seal all wood/wood or wood/concrete seams the ends of any joist bays you're using (a good idea no matter what. DOW Great Stuff or similar is good enough.) Then snug the FG right up against your tubing/plates Use either PERFORATED radiant barrier or permeable house wrap stapled to the underside of the studs as an air-barrier/retarder. Perforated RB will give you some insulation benefit, will allow a small amount of infiltration air, but not exactly a "draft", and allow sufficient air & vapor-transfer to limit condensation issues on the wood. Housewrap woiud seal the fiberglass from air infiltration somewhat more completely , yet without trapping water vapor that could condense on the wood joists. DON'T use polyethylene or similar plastic sheeting.

It's a somewhat separate project, but the sealing any crawlspace ventilation and putting a vapor retarder on the floor is really the right thing to do. Once you have R19+ of floor insulation, the ends of the joists are now much colder than than in an un-insulated floor. For at least part of the season you'll almost surely have the exposed wood at/near the dew point. Even if the space remains low humidity without the vapor barrier, radon & other soil gases are probably worth avoiding. Sealing crawlspace vents is an energy-saver both winter & summer, even with an insulated floor, since it effectively earth-couples the structure to 50F+ soil rather than allowing hot summer breezes or cool winter drafts to add to the heating/cooling loads. Sealing the crawlspace & basement reduces whole-house air infiltration a great deal- it's the right thing to do. Air-sealing both the foundation & attic levels is usually much more important than what you do in the middle! Seal any plumbing & stack chases that run the full height in the building as well. (Use the appropriate materials if working around any combustion-exhaust though.) If you have atmospheric-drafted HW or heating appliances in the basement, always verify combustion-air supply and backdrafting issues any time you upgrade the air-tightness of the house.

Dana, if memory serves some codes REQUIRE a vented crawlspace. having a vapor retarder on the floor again risks trapping moisture and creating mold in the crawl... what is done to keep the house from rotting down if you vapor barrier the floor and seal up the crawl?

Codes have been changing after a bunch of DOE studies in the '90s demonstrated (particularly in the humid east),  the summertime condensation of ventilation air in the crawl is a bigger problem than trapped moisture infiltrating in from the conditioned space in winter.   Traditional codes requiring crawlspace ventilation have been based on imperfect theory and no science- we've learned a lot in the last coupla decades or so.  Whether and when crawlspace ventilation is beneficial (from a mold point of view) is highly variable with climate zone, yet the code standards tended to be universal.  From an energy-consumption point of view ventilating crawlspaces is a net loss- converting it to conditioned or semiconditioned space is a net win in both AC & heating dominated climates.

Any location/season where the outdoor air temps are above the ground temperature, introducing ventilation air into the cooler crawlspace ADDS, rather than removes humidity from the crawlspace, especially with an insulated floor above it, which keeps the crawlspace closer to ground temp. It's mostly true in the un-insulated floor case too, but the temperature break-even point is somewhat higher. In Salt Lake the average daily temps are higher than the ground temps for most of the year, but since it's a relatively dry climate ventilating the crawlspaces (even with insulated floor above) doesn't generate a HUGE mold potential the way it does in the gulf states.

Vapor pressure from the soil is a significant secondary issue, but can be largely mitigated by vapor retarders on the ground.  In humid areas (but probably not in SLC), some amount of ventilation air injected from conditioned space may be necessary.  The goal of the vapor retarder is to keep the moisture trapped in the soil.  By semi-conditioning the crawlspace air with a small amount of air from conditioned space during the cooling season it stays dry. Depending on climate it may/may-not be important to convert the crawl more fully into conditioned space by insulating the crawlspace walls as well as sealing them, but in dry not-so-cold Salt Lake it's probably not as much an issue as it is in GA or AL.

The ground never (ever) behaves as a hygric buffer to dry out or moderate the humidity in the crawl- it's always a water vapor source, not a sink, so the vapor retarder is ALWAYS a good idea whether the crawl is vented, unvented, conditioned space or not.  It's the same as putting a vapor retarder in your basement slab, but skipping the slab itself.  (Soil gases aren't your friends, don't invite 'em in.) 

Outdoor air can be a humidity source or sink depending on the immediate weather of the minute, but in most of the US (outside of the southwest) it is a humidity source many more hours of the year than it's a humidity sink.  The hours when outdoor air has actual drying capability correlates highly with the hours when the ventilation air would be much colder than conditioned air, making ventilation a significant heat loss.

References:

Some older stuff from the epihphany in '90s:  http://www.fpl.fs.fed.us/documnts/pdf1994/rose94a.pdf

more recent thinking, with construction recommendations & pretty pictures: 

http://www.buildingscience.com/documents/reports/rr-0401-conditioned-crawl-space-construction-performance-and-codes

http://www.buildingscience.com/documents/insights/bsi-009-new-light-in-crawlspaces/files/bsi-009_crawlspace.pdf/attachment_download/attachedFile

Thanks a ton for all the great info!

Dana,

As I have looked into vapor retarder on the floor of the crawl space, i have heard more and more folks recommend it. Do you have a specific product you recommend, or is polyurethane clear plastic enough?

Thanks!

High density polyethylene sheeting is pretty much the same from any vendor, more-or-less.  If you use something  that's thick/rugged enough that it can be used under slabs and I'm sure it'll be tough enough for use without the slab.  (Worst case, if it's some inferior product that gets brittle and flaky you may have to replace it in a decade two.)  Just don't use anything thinner than 6mils (0.006") and you'll be good-to-go.  Clear is fine- coloring agents might even weaken it a bit.