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Is radiant barrier necessary for staple-up radiant heat?

user-1079403 | Posted in General Questions on

I took a local builder’s advice and put R-19 batts under the radiant PEX, rather than a radiant barrier like the foil bubble foil, but after reading more I’m worried I made the wrong move. The heated floor is above a half crawl/half basement space. Thanks to this forum I air sealed and insulated the band joist so the crawl/basement is about low 50’s when it is single digits outside

Now I’m worried that I got bad advice but I hate to think about pulling out all the insulation to put foil on top (particularly given we just installed it). is it going to kill my efficiency e.g., will all the heat just sit in the insulation? I thought about puting the barrier under the batts but that would seem to be a waste.

Any advice is much appreciated.

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  1. GBA Editor
    Martin Holladay | | #1

    You're right that R-19 batts weren't the best choice under your PEX tubing. And you're right that the R-value is low.

    However, I wouldn't bother to pull out all of the insulation that is already installed. If you want more R-value and better performance, I recommend that you install a continuous layer of rigid polyisocyanurate foam under the joists. Seal the seams carefully with high quality tape.

  2. user-1079403 | | #2

    Thanks Martin (as noted, I took your advice on an earlier post and spray foamed the band joist and air sealed the floor (there were lots of penetrations to seal).

    Your thought on rigid foam is exactly what i was thinking. Funny thing is that the designer I'm working with suggested rolling a continuous radiant barrier under the joists. His thought is that it will catch any radiant heat that gets through the insulation but more importantly create an air seal that improves the batt performance. I doubt the radiant side would work at all since it will be up against the insulation (maybe a slight gap in places, but if the primary benefit is in the air seal it would be a lot cheaper than rigid foam board.

    If i would have put the radiant barrier on top of the insulation would that have been all I needed (i feel some remorse here)

    Thanks again for all your help - your articles and info on this blog are some of the best I've found.

  3. GBA Editor
    Martin Holladay | | #3

    Q. "If I would have put the radiant barrier on top of the insulation would that have been all I needed?"

    A. No. A radiant barrier alone, even with an air gap facing the radiant barrier, would not have been all you needed. According to ASHRAE Fundamentals, the R-value of an air space faced with a low-e material (that is, a radiant barrier), with heat flowing downward, varies from R-2.5 to R-4. That is much less than the R-19 fiberglass you installed.

  4. user-1079403 | | #4

    As always thanks Martin.

  5. Ax5D3whCtB | | #5

    As a side note, the placement of the radiant barrier is actually irrelevant to functionality in terms of which side of the conductive insulation it is placed on. It will work equally effectively located on either the interior or exterior surface, although as you note, placement can sometimes be used to gain other benefits as well. This is somewhat counterintuitive but quite true.

    Remember that what a radiant barrier does is reflect radiated heat - something which conductive insulation does not do very well at all. Placing on the exterior reflects it back through the material, just as easily as it passed through on it's way out. While some absorbtion does take place, the overall heat loss to the exterior is the same.

  6. GBA Editor
    Martin Holladay | | #6

    Your explanation is not quite accurate.

    First of all, a negligible amount of radiant energy makes it through an R-19 batt. Radiant energy travels well through a vacuum or through air, but not through solid objects.

    Second, it's not quite true that "what a radiant barrier does is reflect radiated heat." It's more accurate to say that, when placed near an air space, a radiant barrier can (slightly) raise the R-value of the air space it faces.

  7. user-1079403 | | #7

    Martin, I assume that the by increasing the insulation R value with say the rigid foam under the joists, the radiant heat would migrate to the lower R value area i.e., the subfloor and into the building envelope. So in the end probably better off really insulating to a high R value that messing with the radiant barrier foil/bubble stuff?

  8. GBA Editor
    Martin Holladay | | #8

    You're asking the wrong question. You're asking about which direction "the radiant heat" will migrate. But there isn't a special category of heat called "radiant heat." Heat is heat. The idea is that heat flows via three heat flow mechanisms -- but it's still just ordinary heat, not a special kind of heat.

    The PEX tubing is warm, and if the tubing is warmer than adjacent objects, heat will flow from the tubing toward cooler objects nearby. Insulation slows heat flow, so if you have a lot of insulation under the PEX tubing, then the flow of heat downward will occur very slowly. If you have less (or no) insulation above, then the heat will flow more rapidly toward the subfloor above, and the subfloor will get warmer.

  9. user-1079403 | | #9

    Sorry, yes that is what I meant - the heat from the radiant system rather than the heat radiation. Thanks for that.

    I'm going the rigid polyisocyanurate route and create as much R value as possible.

    Thanks again for all your valuable insight and patience with people on the forms who have little knowledge but lots of curiosity.

  10. John Holscher | | #10

    Our new house has 9.25" I-joists with staple-up pex and Watts Radiant FlexPlate transfer plates. I have yet to put any insulation. Of course we have electrical wiring and plumbing running throught he joists.
    What's my best option for insulating?

  11. GBA Editor
    Martin Holladay | | #11

    Is this a basement ceiling? If so, is the basement finished living space or unconditioned space?

    Ordinary fiberglass batts can be used for this application. If you really want to avoid heat loss to the basement below, you can supplement the fiberglass batts with a continuous layer of foil-faced polyisocyanurate foam installed on the underside of the joists.

    For more information, see:

    Insulating under a hydronic radiant heat under subfloor between joists system

    Heat Transfer Plates for Radiant heat under floor joists

    Reflective barrier and insulation for radiant heat

    Lessons Learned the Hard Way

  12. Expert Member
    Dana Dorsett | | #12

    The R-value that make sense varies- "value rolls" of kraft-faced R13 is sufficient for zone-isolation over a conditioned basement, not so much if it's over a vented crawlspace. Unfaced R19s snugged up there would save you the cost of an ignition barrier for the facers, if you don't plan on finishing the ceiling.

    I-joists are sometimes a PITA since standard width batts won't fill the space edge to edge, but batts designed for steel studs will work fine.

    Tell us more about how the room below will be used, and how it is (or isn't) insulated/vented.

  13. John Holscher | | #13

    Yes, it's a basement ceiling and it's an unfinished basement. The basement walls are constructed using Thermomass 2" XPS insulation between 2-4" thick whythes of concrete. the floor is insulated with 2" of XPS below. I've got a workshop and wine storage area in the basement.
    I see that reflectix recommends a double layer of their foil faeced bubble wrap . One between the joists at half the joist depth and one layer across the bottom edge of the joists.
    What do you think?

  14. GBA Editor
    Martin Holladay | | #14

    I think real insulation -- the kind with an R-value -- is a much better investment than bubble wrap.

  15. Craig1967 | | #15

    I've got a 960 square foot house. Basement walls are 8' with 6' below grade. House is very well insulated and buttoned up tight. We are near the river so flooding precludes gas to heating, so our only option is electric heat.

    I built a 230 gallon hot water boiler/heater that is wood fired. In my test runs the water has gotten 170 degrees+ very consistently. My burn chamber will last about 6-8 hours so I plan on stoking it twice a day.

    I bought all the components for a radiant heat system. Pumps, manifolds, pex, connectors, etc.. I am ready to install.

    I am going to put the pex tubing under the floor joists of the house, and in the joist spaces above the basement.

    Two questions to begin with. What is the max temp I should let my water get to? Is 170 to high for pex?

    If I regulate the water temp at 150 degrees will heat transfer plates make a measurable amount of difference in heating capabilities?

    I really appreciate the time and effort for the answers on this thread. I have learned alot just reading here.



    1. Expert Member
      Akos | | #16


      The water temp limit with floor heat is always the transfer rate and the max surface temperature. There is not max limit per say.

      Before you go full out on piping, it would be good to figure out your heat load. As you say your house is "well insulated and buttoned up", a full surface install might not be the best as the floor will never get hot enough to get that warm toes feeling.

      You are better off installing only in high traffic areas (kitchen, bathroom, mudroom hallways, under larger windows), this will be nicer on the feet and cost you much less in plumbing parts.

      I generally find heat plate install to be pretty time consuming plus it puts the pipes close to flooring nails. Either do a simple staple up to the sides of the joists or go with something like Ultra-fin (they have high temp install specs). I like the ultra fin as it is quicker to install and much shorter piping than the staple up.

      If you have hardwood floor, it is important for your thermostat to have a floor heat sensor, you want to avoid going above 28C.

      Without some insulation the floor heat will overheat the basement, it is good to put in some batts under the pipes. If you have the heat loss for each space you can figure out how much the basement needs (for every BTU put into the 1st floor, the basement would get around 1/2BTU without insulation) and insulate accordingly.

    2. this_page_left_blank | | #17

      Different grades of PEX have different temperature ratings. You might find some that can handle 170. I think a bigger concern is what the flooring can handle. Most wood floor manufacturers specify the floor must not get above 90, sometimes even lower. I believe that the higher the temperature, the lower the efficiency as well.

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