more radiant heat questions.

Discussion in 'Remodel Forum & Blog' started by Randyj, Feb 4, 2007.

  1. Randyj

    Randyj Master Plumber

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    Geniescience... I appreciate your theoretical explanation of things and would like your opinion/input on my radiant floor design. I read one post which indicated that there are various benefits of laying the PEX tubing then putting the rebar on top v.s. clipping or tying the tubing to the top of the rebar of the slab. I'm weighing the benefits of these methods to decide which way to go. I've got people telling me to use a 5" slab and others saying a 4" slab is plenty sufficient. Too much concrete will make it slow to heat, too little allows lots of problems. I tend to think of heat as "flowing" as if water or electricity. I'm considering using a "heat break" between the I-beams of this "all metal" house and the slab floor as well as between the metal channels for the metal studs and the slab floor....just wondering what thoughts anyone else would have on this. Also, I'm only installing the tubing at this time and will add a thermostat later. I'm assuming I can drill and insert a thermostat probe later which I can seal in with something which will adequately transfer the heat of the floor to the probe.
  2. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    PUt a vapor barrier and insulation under the slab. Keeping water out and insulation will allow the slab to retain the heat.
  3. leejosepho

    leejosepho DIY scratch-pad engineer

    Drilling later would scare me no matter how well I had made previous note of a safe spot! Cast in a short piece of your tubing for later use.
  4. well, ya got me here, Randy. Have you already discussed all this somewhere else that i haven't seen? i'm a bit lost.

    I do know that any rebar in any slab is there to hold the slab up, and that is "Job #1", and when you really really know heat transfer, you won't care a hoot about whether the pex or the rebar is first or second, top or bottom, under or over, since the heat is all gonna equalize itself in short order, in a slab made of concrete. But, on the other hand, where to place rebar inside a slab, so the slab holds together, now that is non-negotiable in many cases. Some things are not worth maximizing at the expense of other things that are critical. And heat transfer is going to occur, regardless of whether the pipe is run under, or over, or weaved in between, the rebar.

    On the same topic, any probe will read the right temperature, when it is encased in ANY material when it is inside the slab. If the probe is surrounded by some material that is not concrete, this surrounding material acquires the temperature of the slab, after a small time delay.

    Since you mentioned I-beams the slab is not your basement foundation, I assume.

    david

    Edit next a.m. - are you talkig about a concrete pour on top of an existing wood floor (subfloor)?
    Last edited: Feb 5, 2007
  5. Randyj

    Randyj Master Plumber

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    Basic design is that which I've barrowed from Heritage all steel homes website http://www.heritagebuildings.com/about/video.asp as jadnashua mentioned it will be slab over vapor barrier and 2" styrofoam. As for the "over/under" question" my interest is how it would affect the strength or cracking of the concrete. I'm looking for the pro's and con's of doing it one way or the other... all thoughts are appreciated.... It's about warm enough now that I can go outside and work on it a couple of hours today. Everything is laid out and ready to form up and pour... got plenty of styrofoam, forms, plastic, & rebar...plumbing is already in the ground and gravel is spread... all that's left to do is finish the job!
  6. not a wet slab expert

    i'm not the person to consult, for a poured slab. I do know enough about heat transfer to tell you that it is not worth "reducing" anything in the slab because of pex piping inside the slab. Heat will flow fast through concrete. Make the slab strong so it doesn't crack; heat transfer will take care of itself since concrete is a heat conductor. Do isolate the slab from the ground; that is worth a lot of planning, and this includes isolating all sides too.

    Isolating something physically is a big big part of what they call "insulation" -- that is why i always talk about making something airtight instead of focusing on the choice of materials. You said you have foam, and an H20 barrier. So far so good. Wet earth won't be in direct contact with the concrete. That is 80% of the necessary work.

    Whether to go with 4" or 5" thick, is going to depend on what is right for the slab. Heat will move around in the slab whether it is thicker or thinner. What would be the right thing to do, if the slab were not heated? That is probably a good indication of what you ought to do, with the pex piping in it too. Certainly not less concrete.

    david
  7. Randyj

    Randyj Master Plumber

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    Think I'll have a small glass of wine and contemplate this whole thingy. I'm wondering whether or not I want to try to "isolate" the metal stud track and base plates of the I-beams from the slab to inhibit heat transfer....and what material might be best for that.
  8. good choice.

    The walls are insulated too, outside the metal frame... correct? If so, then the heat transfer is a good thing, as it spreads heat a bit into walls that would be cold in winter.

    david
  9. Randyj

    Randyj Master Plumber

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    Yep... whole thing wrapped in double bubble (e-foil). Torn between whether or not to add conventional "pink panther" in walls but fairly well convinced to add it to the e-foil under the roof. Roof, not attic will be insulated (radiant barrier + insulation)...
  10. TimL

    TimL New Member

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    Location:
    Whitney Point, NY
    Randy,

    Just wondering if you've tried www.heatinghelp.com's forum

    http://forums.invision.net/index.cfm?CFApp=2

    and Radiant Panel Association's

    http://radnet.groupee.net/eve/forums/a/frm/f/4771065301

    I've read a lot on both and it seems that the consensus on the bubble wrap is that it's a waste of money. The radiant barrier effect can diminish quickly if/when it gets dusty, and the R values aren't really there. They've cited testing that debunks the radiant barrier claims.

    They consistently recommend rigid foam insulation and expanded foam insulation, with the pink stuff coming in around third place.


    Tim
  11. Randyj

    Randyj Master Plumber

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    TIM... I greatly appreciate your post/links. I'll be reading them FOR SURE! Sounds to me like everyone is tooting their own horn. I recall reading about the issue of dust on the foil. However, the priciple of reflecting radiant heat has been working pretty dog gone well in the space program. Sounds like more investigation might be a worthwhile investment. I do know of a few buildings near me that have used it with great success... so, the big question is "who's telling the truth, the whole truth, and nothing but the truth"???? No one wants to lose their market share of a good product....or get replaced by some new fandangled gizmo. I'm all for new products/methods but every new mouse trap will not sell if the old type is still doing it's job cheaper and with fewer problems.
  12. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Dust on a radiant barrier does decrease its efficiency, but as long as it doesn't get really thick, it's not enough to worry about. If things get that dusty, you've got other problems (too much circulation) that should be addressed.

    I've got a radiant barrier in my attic along with a pile of normal cellulose and fiberglass. Prior to the installation of the radiant barrier, in the summertime, the ceiling got quite hot by the end of the day. Now (and it's been maybe 7-years and it's somewhat dusty), it still works the same way.
  13. TimL

    TimL New Member

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    Location:
    Whitney Point, NY
    The discussion on the other boards has dealt primarily w/ radiant barriers being proposed for under slab and underfloor heating. Barrier in the attic to reflect solar gain seems reasonable to me.

    Anyway, I'm not an expert, and I don't have a dog in this fight, but I'll summarize the arguments against radiant barrier in heating as best I can remember:

    Rigid foam, expanded foam, fiberglas, mineral wool -- all these have measurable R value. The expanded foam has great air sealing properties as well.

    Radiant barriers will reflect radiant heat, but require an airgap to do so. Some manufacturers have stated an "equivalent R" factor for their products, but then recommend the product for situations where there is no air gap, like under a slab. The RPA forum has put out some pretty good evidence that these products are not worthwhile in that application.

    Dust will diminish the radiant reflecting qualities. Many of the pros on the other forums believe an underfloor radiant barrier will get dusty quickly, and an installer showed photos of an installation that was heavily coated w/ dust after one year of operation.

    Even with a radiant barrier, real R value and air seal is a must. I doubt anyone would argue if you wanted to use a radiant barrier and the best foam insulation. But if it's a choice of one over the other, my impression is that the majority of the pros on the other forums would go with the real R and save their money on the bubble foil.

    Good luck -- we've been living w/ radiant heat in our addition for a two winters now and it's absolutely great!
  14. great! radiant heat is "great".

    thank you Tim !

    R values as defined by ** and as then tested in labs does not allow radiant heat reflectors to shine as well as another test, defined differently, would do.

    From wikipedia.org "...trying to associate R-values with radiant barriers is difficult and inappropriate. The R-value test does not control the amount of heat transfer due to conduction / radiation respectively. There is no standard test designed to measure the reflection of radiated heat energy alone..." This quote describes only a part of a bigger problem, but it's a bigger discussion than we may want to get into here right now. :)

    Your term "airgap" is good; I have always said it needs to be "surrounded by air". I like your term better. I want to congratulate you on something else too, even more important. You have made it clear that radiant barriers need air to work!

    Under a basement slab, aluminum coated bubble wrap has to be doubled back onto itself so the foil side is doubled and in contact only with itself and air bubbles -- not in direct contact with solids like concrete or the ground. I have found that hard to explain to people. Or, people have found that hard to understand to the point where they do "the right thing". I'll bet that 90% of all foil faced bubble wrap has been improperly installed.

    The same applies to foil faced foam. Where I live, people buy dense foam boards coated with aluminum on both sides, for wall insulation. First of all, there is no gain to be had from two parallel radiant barriers; it is far far better to have the same quantity of metal twice as thick in one layer. Then, people place that foam board so that is in direct contact on one or both sides with other building materials. I have found it very hard to explain why that is not going to help... Or rather, people have not found my explanation easy enough to understand. They install radiant barriers the wrong way, and the manufacturer doesn't print any installation tips.

    Enough ranting. Besides, I don't have the kind of job that requires that I "teach" people how to install things the right way. It's only a tiny little frustration to see it being installed improperly everywhere I go. :)

    You probably have seen solar camping cookers that work by reflecting the sun's energy to a central point. When I was a boy scout, these things fascinated me. They work on cloudy days too. They reflect invisible heat energy. They also reflect some visible energy (i.e. "light" energy). They are mirrors for heat, not light. A mirror on the other hand does not reflect heat, only light.

    No one is going to deny that these things exist and that the phenomenon is known as "radiation". Heat radiation. When you put your hand under a broiler heating element, you feel radiation. Heat is radiating downwards onto your skin. (Meaning: Heat does not rise, it goes in all directions equally.)

    As you pointed out, it is true that some manufacturers fudge the whole issue by talking about an "equivalent R" factor and then they "recommend the product for situations where there is no air gap, like under a slab." Agh! I'll go see the RPA forum you mention.

    Your last line says it's great to have radiant heat. Good to hear it from someone else who's done it.

    I think the ASHRAE has published a definition of "comfort" that mentions that the coldest part of a room conditions your sense of comfort, so a cold floor is a No-No if you want to have a real sense of comfort.

    David
  15. Bob NH

    Bob NH In the Trades

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    The strength of a concrete slab is roughly proportional to the square of the distance from the re-bar to the concrete surface that is in compression (usually the top surface). Thicker is better, except for the weight. Concrete and steel will weigh about 13 pounds per square foot per inch of thickness and the beams and falsework (form supports) must be designed to support it.

    Is the slab designed, or are you winging it to determine the thickness of slab and amount and placement of the steel?

    If the slab is spanning only the space between a pair of I-beams, then the steel should be near the bottom with enough cover between steel and bottom to prevent the steel from breaking out. In a slab in a non-freezing environment, inside, I would want at least 1/2" of cover between steel and the bottom surface of the concrete. An inch would be better. The aggregate (stone) should be smaller than the distance between steel and adjacent surface.

    If the slab is crossing more than 2 beams, then the steel should be at the bottom of the slab between beams, and near the top of the slab where the slab crosses over the beam. In those cases there are usually two layers of steel. In a careful design, the bars or mesh can be bent or curved so that only one layer is necessary.

    The main steel runs transverse to the beams, with at least some steel parallel to the beams.

    Concrete should be only wet enough to allow it to fill the forms and get around the steel. You need to "rod" or vibrate it in some way to get good coverage of the steel. The concrete should stand in a rough pile, like a very thick cake batter, when dumped in the forms, and should be moved around by screeding and vibrating. It should be thick enough to pick up on a shovel. If it runs like pea soup it is too wet. You will know it was too wet (you will know too late) if you get puddles of water on the surface when you finish it.
  16. Randyj

    Randyj Master Plumber

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    Thanks guys for all of your input. I've been piddling with this idea for a few years. Last year I finally packed up, cleaned out, got rid of a heck of alot of stuff, bought a 40 ft. semi trailer to store things in, had it moved to my place on the lake where I'm now living in a 60's house trailer that I'm trying not to do any repairs on, yet the ceiling is falling down and when it rains...it pours inside too. Out back I've got my foundation started, gravel is about 2" thick. I had planned footings all the way around 12" X 24" and 6" I-beams on 16' centers for post & beam construction of an "all metal" house on a monolithic slab. Under each post I planned 24" thick 36" x 36". Where I had to bring in fill dirt it was compacted and I'm drilling 8" holes for piers down to hard undisturbed earth about every 4-5 ft along the footings. The slab will be on 4 mil plastic vaporbarrier and 2" eps styrofoam. The plan for the radiant barrier is as a "house wrap" over the steel skeleton. My original research agrees what what you guys are saying. It needs a minimum or 3/4" air gap. What I was reading from Oak Ridge research labs also says that it must have air space to work. The whole idea of using it under a slab has pretty well been totally debunked and the air space issue preculdes its use when sandwiched between solids. Another issue which I just ran across and it's one of those things that I intuitively knew but didn't know why I knew it or couldn't explain it was covered in my readings from Oak Ridge and that is "thermal bridges" which relates to the question I put to Geniescience about isolating the metal from the slab. The issue appears to be more important in controlling where the heat is or goes. The research pretty well reveals that maximum prevention of heat transfer thru walls will be greatly enhanced by covering points of contact such as metal sheeting to the perlins or metal wall studs with 1" styrofoam and that it doesn't have to cover the whole wall, just the point along which the sheeting contacts the structural members....so, now I'm tinkering with the idea of gluing slices of styrofoam on the outside of metal studs and I-beams then covering these with the double bubble.
    Also, I've been pretty deep in the john bridges ceramic tile forums. I've taken the issue of "uncoupling" pretty seriously and feel that this can be used to great advantage to prevent cracks in the slab. My floor plan is an L-shape because of property rights & constraints by flood levels of the lake. I'll have two 32 x 32' slabs and one 24' x 32' slab. My idea is to have a "floating slab" 4" thick 1/2" rebar 12" spaced which rests completely on styrofoam and sits on a ledge of the footings and is totally isolated by styrofoam. As always, comments appreciated..... I greatly appreciated Bob NH's advice on the placement of re-bar. I've got a son-in-law who is a concrete guy but I don't exactly trust his expertise. I do plan to use a very dry mix also to minimize cracking. (very rough sketch below but you get the idea)...

    [​IMG]
  17. foam = air = all foams = good as air

    nice project!

    is it clear that a radiant barrier with foam on both sides, under a slab, is the way to get both a rigid stucture and a radiant barrier that has air gap around it?

    Since foam is air. The molecules, holding the little air pockets together, transmit little heat. Just like fiberglass molecules don't transmit a lot of heat. All foams, isocyanurate (R8), phenolic (R7), polyurethane (R6) act as trapped air and they are rigid too. Best of both worlds.

    It does work to use a foil-faced bubble material, or foil faced rigid foam board, as a radiant barrier under a slab, when placed on another layer of foam or bubble. Since the radiant barrier is surrounded by mostly air spaces and not by conductive solids.

    I'm saying that foam is good, and radiant barriers are good, and both work together without canceling each other out. Seems you like bubble material best, and that works too. I have seen slabs poured on top of bubble. But remember to add one more layer of almost-air product (either bubble or foam) for the radiant barrier to perform the function it was designed to do.

    david

    p.s. avoid calling foam "styrofoam", and don't use polystyrene (R5) since it is the weakest.
    Last edited: Feb 19, 2007
  18. Randyj

    Randyj Master Plumber

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    Thanks for the suggestion David. I've pretty well got everything I need to start pouring my slab as originally planned. I don't think I'll be adding the radiant barrier under the slab. It probably will work as you describe but the added expense is questionable. I'm nickle and dimeing tis project so have not purchased any materials to go any further. As I am ready to pour the footings I intend to order the pex tubing soon. Whether to buy it locally or online is a good question. The only online stores I've even discussed it with are on ****. If there's a good source I need to know about I'd appreciate knowing. I've got several links that have been posted but haven't done the price comparisons yet. Probably will not even get serious about ordering until the footings are poured. I'm doing all by myself except that I will have a couple of guys here when I pour the slabs.
    I'm not necessarily locked into the bubble wrap thing but don't have a source for other types of radiant barrier. I need to investigate that seriously as I get closer to completing the slab. I'll have quite a bit of time as I'll have to get the red iron and assemble that before I'm ready for it....and I'll have to have the metal for the roof and outside walls also. The original idea was to use steel lapped siding (looks like vinyl siding) which should give me the 3/4" minimum air gap. I'd be happy to spend a few dollars more for something other than the bubble wrap if it will do the same job better. Not sure what else is on the market but it's about time for me to check it out.
  19. about the pex: i would buy locally since the guy you buy from will be your advisor about the fittings and other things.

    david
  20. Randyj

    Randyj Master Plumber

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    thanks... cost will be a serious factor....but I'll pay a reasonable amount more to buy locally.
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