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Reflective barrier and insulation for radiant heat

Michael Schlee | Posted in General Questions on

Hey all, looking for thoughts on my heating upgrade. After reading and reading I think I know enough to ask questions (but not have the right answers) so i figured the voices of experience could help.

I’m in the process of installing radiant heat in my 1,800 sft single level home in Long Island NY. It will be PEX in the joist bays over a crawl space and basement. the crawl space has a concrete floor over a vapor barrier and the basement is unfinished without any underslab/exterior insulation. I’m trying to figure out the best way to insulate so here are my thoughts on the process

– air seal. either air seal the floor to stop leakage btw the crawl/basement and the living space, or around the band joist to stop air coming into the crawl/basement, or both?

– aluminum plates. i’ll use the thin kind rather than the thicker type primarily to spread the heat more evenly and improve responsiveness.

– reflective barrier. place in the joists approx 2 inches down from the pex tubing.

– insulation. probably go with the JM comfort therm r-19 stuff (enclosed in a plastic type wrap mainly as not to have bare fiberglass exposed as i spend some time in the basement workshop)

so a few questions (in addition to the obvious question of am i doing something wrong:
– should i air seal floor or band joist? i was thinking both as i probably want to avoid cold air being drawn up to the living space and also want to minimize air movement in the crawl/basement if i’m going to use batts beneath the PEX
– do i need both the reflector plates and the reflective barrier? or could i just do pex clamps to hold in place and then the reflective barrier to push as much heat up to the floor as possible
– since I’m insulating btw the crawl/basement and living space, do i need to insulate around the band joist (I was thinking I could just spray foam the band to both air seal and insulate in one go)

your thoughts are very much appreciated and thank you for all the great info on this site I’ve read so far.

Mike

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Replies

  1. Zac McCormick | | #1

    Don't do the clamps, that is an accepted practice referred to as a "suspended tube" system the main advantage being that it allows for more even heating the drawback is however that it requires much higher temperatures due to it's utilizing convection to transfer the heat to the floor as opposed to a plated system that uses conduction as the main heat conveyor. There will be some heat striping that you can feel by running your hand across the floor but I really doubt you would notice in an everyday living environment. Use lot's of plates to keep the temperature low and you can get the most efficiency from all kinds of great heating sources like solar & geo, gas really likes low temperatures too.

  2. Michael Chandler | | #2

    Air seal both
    Use copper two hole tubing straps to pull the PEX tight to the sub floor
    Cover that with cheap aluminum flashing then cover it all with spray foam and sheet rock the workshop ceiling to separate the foam from the workshop keep the heat in the floor
    If you have Nat gas look at an AO Smith, Vertex water heater
    No potable water in the floor, equal length tubes, lots can go wrong here.
    See if you can get good local help from someone who's done it successfully before.
    https://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/stuff-i-learned-joe-lstiburek-s-house-part-1

  3. GBA Editor
    Martin Holladay | | #3

    Michael,
    You definitely want to air-seal and insulate your rim joist areas. This is not optional.

    Assuming that your basement and crawl space are within the conditioned envelope of your home, it isn't necessary to air seal your floor.

  4. Michael Schlee | | #4

    Thanks all. Michael, i have propane but yes the vertex is what i'm looking at. i have someone who is advising but I'll do the work myself.

    Martin, would the quickest approach be to spray a few inches of foam around the band joist to air seal and insulate. one issue i struggle with is that sill plate. house is about 30 years old and the gasket (for lack of a better term) between the concrete blocks and sill plate is not in the best condition and missing completely in some areas. however i really cant spray foam over it to make sure we can inspect for termites (they are an issue in my area but i have no problem thus far however I've only been in the house about a year) . What is the best way to insulate the sill plate or can i skip it? the basement walls and crawl will not be in the conditioned envelope as they are not insulated so probably around 55 degrees (but with the vapor barrier/concrete floors and good dehumidifier it is nice).

  5. GBA Editor
    Martin Holladay | | #5

    Michael,
    Q. "Would the quickest approach be to spray a few inches of foam around the band joist to air seal and insulate?"

    A. Yes.

    Q. "What is the best way to insulate the sill plate, or can I skip it?"

    A. If you need to leave the sill exposed for termite inspection, then clearly it can't be insulated. The best approach would be to clean out the crack between the concrete and the sill (using a utility knife, if necessary, to trim back the old gasket) and then to seal the crack with high quality caulk.

    If possible, use spray foam or air-sealed rigid foam to insulate the rim joist above the mudsill.

  6. Michael Schlee | | #6

    thanks Martin. i'll spray foam the band and caulk the sill as you note (i may cut strips of rigid foam to put over the exposed holes of the concrete block and calk it up)

    sounds like Michael and Zac differ on the need for radiant plates. i've been digging around and found a study done that seems to suggest that they are a good idea. it is ASHRAE Research Project 1036 "Develop Simplified Methodology to Determine Heat Transfer Design Impacts Associated with Common Installation Alternatives for Radiant Conduit" in the study they conclude that "The primary result from these four configurations is that the heat transfer plates increase the heat that is transferred to the occupied by space by between 160% and 172%, depending on where the insulation was positioned [below the plates]". They used the extruded plates that may be more effective than the thin sheets that staple up, but it appears that there is some evidence to support the usage of plates. In their experiement the plates increased the heat flux from about 23 to 46 btus (i'm averaging).

    However, they used blue board insulation and there is no mention of a reflective barrier. I wonder if the same results would occur if they used reflective substance on top of the insulation?

    would love your thoughts on this because it could not only save me some initial cash, it could allow me to run water temperatures lower to acheive the same flux thereby saving my propane usage as well- and thanks again for all the help.

  7. Aj Builder, Upstate NY Zone 6a | | #7

    Michael, I installed one of the first propane radiant staple up foil insulated floors near me twenty years ago. It works very well. It also uses lots of propane. A pellet stove might lower energy costs for you but there are weekly chores involved.

  8. Michael Schlee | | #8

    since the focus of the question is now on heat transfer plates, i'll start a separate thread (i think that is proper etiquette)

    thanks everyone for the advise - very helpful

  9. Bill Bradbury | | #9

    Based only on direct observation of systems I have worked on or installed, the flat panel radiator systems outperform any radiant system.
    It seems like this is for two reasons;
    1 flat panels have convective tubes in the back so you get radiant and convective heating,
    2 there is no increase in comfort from heating the framing of your house.
    I only install radiant heat in bathrooms and even there I recommend a hydronic towel warmer/radiator.

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