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Radiant floor insulation (not slab)

Post in 'The Boiler Room - Wood Boilers and Furnaces' started by Mike T, Oct 9, 2013.

  1. Mike T

    Mike T Member

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    Hi guys-
    I have a large room that has a good radiant floor system. I followed the advice of the installer and merely put up that radiant silver material on thwe bottom of the joists to contain the heat etc. My boiler has a hard time keeping up when it gets real cold, so I use a big wood stove in that room too.

    As you may guess, I know I have too much heat loss to the unheated basement. So I have been thinking about three possible solutions:

    1) Remove the radiant wrap, insert non faced or paper faced insulation, reapply.
    2) Same, but insert foil backed insulation
    3) This would be easiest- Put blueboard over the entire basement ceiling, leaving the radiant wrap in place under it.

    Opinions from the insulation pros?

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  2. bmblank

    bmblank Minister of Fire

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    I'd leave the wrap up and put insulation up underneath it. The best radiant effect comes from directing as much of the radiant heat upwards. With the radiant wrap below the insulation, the extra energy not directed upwards will have to go through the insulation, get reflected, then go through the insulation again instead of just being directed upwards in the first place. The extra insulation is just to keep the non radiant heat from falling prey to a gigantic heat sink.
    BoilerMan likes this.
  3. Fred61

    Fred61 Minister of Fire

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    It's not insulated correctly as it is. The reflective surface should be just below the pex tube and then insulation below that but that might not correct the situation. Did your installer do a heat loss calc. before the installation? Is the pex on stand-offs or is it stapled tight to the floor underlayment? Why do you define your system as a "good" system if it doesn't heat the intended space? What temperature water are you running?

    I would check the water temperature and I would install aluminum heat transfer plates on as much of the tubing as you can reach and then insulate correctly. Your installer deserves some scrutiny if this is his normal line of work.
  4. BoilerMan

    BoilerMan Minister of Fire

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    What is the tube spacing? As Fred said, what temp are you running and how is the pex attached?

    I'd be inclined to put blue board under the whole thing as well, but we need to know more before you can quote me on that.

    TS
  5. JP11

    JP11 Minister of Fire

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    Just as a comparison..

    I have NO transfer plates at all. The tubing is 8" on center, so three runs per 24" bay between trusses.

    Pex stapled to subfloor with wide staples for room to play.. lays flat against floor.

    1" of air space beneath pipes. then 2" foil faced hard foam board set into each truss bay. Worked well. taped end seams of foam. ends of the wall where the pipes come down and 'jump' bays,... I covered the ends with bubble foil faced 'refletex'

    worked pretty good for me. I was fortunate plumber let me use his 800 dollar wirsbo stapler.

    JP
  6. Mike T

    Mike T Member

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    Ok Guys-
    The pex is stapled directly to the subfloor and there are two lines in each joist bay. They lines are spread pout in each bay. The design came from a well respected engineer on the pex, loops, etc.
    The water temp going out is 140+. This is a large addition that this is heating.

    My second plumber is the one who said to just staple up the foil over the bays. I know its not the best way now, but not then.
    What I did in an older section of the home is to use the regular insulation, and then put the wrap up under that against the joists.
    I wanted to improve that further, so I removed the old regular insulation and put foil backed up, with the foil next to the pex, and restapled the radiant wrap.
    This is probably overkill.
    Just trying to see if should do it this way again or if there is an easier way.
    Thanks!
  7. bmblank

    bmblank Minister of Fire

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    The foil definitely needs to be up against the pex. When its at the bottom of the joists you're probably getting minimal radiant reflection. It'll still insulate slightly, but the radiant is the target.
    What i would do is put the reflector up against the pex and then put insulation up below that to pervert the heat sink effect of the unconditioned basement.
  8. thecontrolguy

    thecontrolguy Member

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    IMHO, the foil (or commonly called the "radiant barrier") should be mounted close to the heat source, but not touching. The key is a trapped layer of air in between. This prevents convective losses from the warm pipes to the air below, through touching the radiant barrier. This is often used in an attic under the roof deck, but not touching it. Foil on foam is a great solution insofar as the radiant barrier faces the warm side and the backing board supports the foil and provides the insulation value to further reduce infrared conduction through the radiant barrier and losses to the cold backside.
  9. bmblank

    bmblank Minister of Fire

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    I guess i could agree, but i sort of disagree. In the case of purely reflecting radiant heat i could understand having a thermal barrier in there. On the other hand, much like those aluminum sections the pex clips into, there is more area to broadcast the radiant heat, much better heat transfer.
    In my foil its not just one layer, its a bubble wrap, so the air gap is built in. I expect very little conductive heat transfer between one side and the other.
    At what point the convective heat losses overcome the radiant heat gains... Got me...
  10. Floydian

    Floydian Feeling the Heat

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    A radiant barrier will not work if the foil is up against the tubing, no way, no how. This would be conductive heat transfer and .25" of bubble insulation with some foil will provide very little real R value in this case. Now if properly installed with an air gap(1 to 2") below the tubing a radiant barrier can provide a real world R value of 1 to 2 assuming this radiant barrier never gets any dust on it. When it does its ability to act as a radiant barrier is basically done for.

    An install with the tubing stapled to the subfloor is basically a suspended tube install and in this case you would want the air space below the tubing to help even out the floor temp. Probably worth a radiant barrier with this approach as long as there is a real R15+ below it for a heated basement. With an unheated basement below I would want no less than R21, probably more.

    An install with aluminum heat transfer plates does not need or even want a radiant barrier. The insulation goes right up against the plates and there is no radiant energy present, only conduction and mostly upwards which is exactly what we want.

    More on radiant http://healthyheating.com/Radiant-Theory/Radiant-Theory-pg1.htm#.Ul6ERlCsh8E and radiant barriers http://www.greenbuildingadvisor.com/blogs/dept/musings/radiant-barriers-solution-search-problem

    Noah
    Fred61 and ewdudley like this.
  11. tom in maine

    tom in maine Minister of Fire

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    Radiant barrier is not enough.
    Install some real insulation underneath it.
    R-19 fiberglass would be fine in this situation.
    It is not critical to move the barrier against the tubing if it is not already there.

    Radiant heat radiates in all directions. If you are trying to move heat into the space above the tubing, going through the floor, there must be more insulation underneath it than above it.
    Radiant barrier companies have done an amazing job of making their product sound like it is all you need.
    A radiant barrier is not necessary with adequate insulation of any type.
    You will not radiate heat through R19 fiberglass or R10 polyiso foam.

    140F might be too low a temperature in the really cold weather. You might have to bump up the temp. Probably not with adequate insulation, though.
  12. Fred61

    Fred61 Minister of Fire

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    Tom, I don't often disagree with you and perhaps you probably just left this out of your post and I'm probably just picking fly sh!t out of the pepper by bringing it up.

    When radiant hits a dark surface or what's called an absorber it converts to heat, in this case, the insulation. It will heat the air in the cavity and then conduct up through the floor, perhaps having the same effect as the radiant would have produced. I don't know! It's like my sun deck with snow on it. The low winter sun hits the snow and it reflects into the house through the windows. When it's shoveled off the dark floor of the deck converts the radiant sunshine and the floor gets warm and does not send any radiant heat through my windows. So that being said, I would still opt for a reflective surface below the tube. In my opinion, there's enough other absorbers in the bay.
    BoilerMan likes this.
  13. DaveBP

    DaveBP Minister of Fire

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    And remember that as soon as there is the slightest layer of dust on that shiny heat reflective surface it becomes almost worthless. So don't forget to get in there and dust out your joist bays every few years.
  14. tom in maine

    tom in maine Minister of Fire

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    We could spend a lot of time talking about radiant barriers and the physics of it. Bottom line is this:
    go stand next to your woodstove when it is operating. It is radiating heat.
    Place a piece of foil faced bubble wrap between you and it. It will reflect the heat and also get hot on your side.
    Do the same thing with 2" of foam insulation. It will not get hot.(Unless it catches on fire!)
    6" of fiberglass, the same thing, if there is any facer on it, you will probably not feel any heat.

    Heat transfer is not going to get sorted out on this forum in a few pithy comments.
    I can offer one that I mentioned earlier though, the radiant barrier industry (if that is what it is) has done a good job of obfuscating the issue
    and making their wares seem really good. Caveat Emptor!
    BoilerMan likes this.
  15. Fred61

    Fred61 Minister of Fire

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    I didn't use bubble wrap. I used a fiber re-enforced aluminized mylar and 6 inch fiberglass below that. The combination gives me a "warm feeling", pardon the pun.

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