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under-slab insulation thickness

Post in 'DIY and General non-hearth advice' started by jklingel, Nov 30, 2007.

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  1. jklingel

    jklingel Feeling the Heat

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    It is pretty standard to use 2" of blue foam under slabs up here, and I gather other cold-ish places. My heat loss SS sure has a big number next to the slab loss when using only 2" of foam. Does anyone have any experience w/ using 4"? Or, perhaps, 2" and a layer of this "new" 1/2" thick SlabShield (and others) that are supposed to be the "new ticket". Haven't crossed that bridge yet; still researching the stuff. Thnks. j

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  2. jklingel

    jklingel Feeling the Heat

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    BTW: I just found out that the president's house at UAF has 4" of extruded poly under it, and apparently the radiant floor heat circuit for the basement rarely turns on. On another note, "so much for this bubble-wrap stuff that some folks are saying to use under slabs. This is a good read, if you are considering using it. http://www.blueridgecompany.com/docu...e Bursts.pdf
  3. titan

    titan Minister of Fire

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    In my experience, that bubble wrap stuff is junk.2" of polystyrene seems to be the norm around here,but like you mentioned,4" might be worth the extra$ in Alaska.As long as the r-value beneath your radiant heat source is higher than the r-value of the materials above it......the heat has to rise upwards.
  4. WILDSOURDOUGH

    WILDSOURDOUGH New Member

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    I used 2" Pink foam and the bubble wrap (silver, bubble and a white layer) with my pex system-
    second winter with it- it's great to walk around in socks in the basement with 85 degree concrete floors !
  5. jklingel

    jklingel Feeling the Heat

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    I second the warm feet deal. My sons and I installed radiant on one of their places 2 yrs ago. Sold me instantly. For kicks, I have been playing w/ the SS some more, and spending an extra $2K on blue foam will save me $530/year on oil, at $2.90 a gallon. Burning wood most of the time is going to negate that savings quite a bit, sure, but I only PLAN on burning lots of wood. God may have other plans for me.....
  6. WILDSOURDOUGH

    WILDSOURDOUGH New Member

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    jklingel-
    yes, but insulation- for the price- is always a great investment, it's always worth it, expecially in new construction.
  7. jklingel

    jklingel Feeling the Heat

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    Wild: I hear ya. I am leaning toward "a tad excessive" (according to the SS), as materials today are the only fixed expense, and I have rarely seen any prices, esp energy, drop over time. I was laughed at for building my first place so thick-walled, until fuel prices slammed up. Oh, wait.... this just in.... "OPEC and EXXON Mobile, etc, have gotten together and agreed that the public has been cheated for the last 5 yrs, so they are dropping the price of a barrel of oil to $50, and will increase that by only 3% per year for the next 15 yrs." What a bunch of nice people! I guess guilt just overwhelmed them.....
  8. WILDSOURDOUGH

    WILDSOURDOUGH New Member

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    jklingle---

    I did follow the advice Radient Floor Co. of Vermont. In their manual it said to leave a 'Heat Sink' in the middle of the basement floor uninsulated. My foundation dimensions are 36 X 65, so I left a 14 X 20 area in the middle of the floor W/O pink board and only put down the bubble stuff. The idea is that the heat will sink into the center and warm the peastone/sand/ground. This acts as a storage, and as the rest of the floor cools- will release the heat back up and out.

    I put a piece of pex (capped off) under the floor before I poured. With an older Radio Shack wired theromomtor in it, and the OWB running it reads between 90-110 degrees under the floor near where the 6 runs of 3/4" pex exit the floor.
    This is at an outside wall- a don't know what the center of the floor is. All I know is that the floor is plenty (silly) warm- and it's freezing outside. Works for me (and wife is :) ).

    Did not want to put pex into floor joists upstairs- so we put a put a Lopi in this year- now the whole house is toasty warm, and using very little wood to do it is even sweeter.
  9. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    As others have noted the bubble stuff is completely worthless.

    There's no such thing as too much insulation (within reason), and especially when the insulation is being put in place as a once-only change (under a slab), with no option to upgrade later.

    You also want a good water barrier, to protect the insulation from groundwater infiltration. It's obviously not going to stay "dry," but you want to prevent the groundwater from having an easy time of flowing through, between the insulation and the slab.

    That would be offset by the loss to the groundwater. Heat doesn't tend to "stay put" underground. If it did, you could easily freeze the soil with a geothermal system.

    Joe
  10. jklingel

    jklingel Feeling the Heat

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    Regarding not insulating completely under a slab: Radiant Floor of Vermont must have plenty of oil company stocks in their portfolio, if that is in fact what they advised. I am sure that "hole" in the middle of the floor is a heat sink: Heat just sinks in there and goes to China. Extending that concept, why not leave a lot of insulation out of your walls (ceiling, too) and put an old truck near every wall to absorb and give back? I thiink maybe there was something lost in translation there. Never in a hundred years would I do that. Are you sure they did not mean putting a mass above the well-insulated concrete (same idea as water storage tanks)? That I could understand. People do that with solar heat instead of opening windows to cool the house. Hmmm. This one needs some thought; I just don't see it. Blind? Could be; I've been there before....
  11. Rhone

    Rhone Minister of Fire

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    Actually, that hole in the middle doesn't act like a heat sink (well maybe in Fairbanks!). I read about it somewhere, I'll hav e to find it. What it acts like is unlimited storage.

    I think what you're missing is that the Earth is an insulator. It may take I'm guessing 2-3' of earth to insulate as well as 1" of insulation BUT, when the heat has to travel through 10'+ of earth to get to the outside cold of your foundation it's a better insulator than 2" of insulation. I'll have to spend the time & research to find it cause I was really interested the first time I read it which was years ago. Anyway, I read about that about leaving the center open, which turns the entire earth into heat storage since it has to go through so much to escape and earth has an R-Value. But, again... probably not practical in Fairbanks.
  12. WILDSOURDOUGH

    WILDSOURDOUGH New Member

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    You got it !
    The earth (dirt) is a insulator, a storage medium, a natural humidifier/dehumidifier...and more.

    In most places in the lower 48, six to eight feet underground- the tempature remains fairly constant year round, we all know about this. (Sorry to Alaskans and Canadians with your permafrost, your outta luck....till it melts.)
    That is why, in my Insulated Concrete Form foundation/basement- even without heat- it will never get down below the mid 40s. I know- we did it last winter for a few days to see where it would stop dropping.

    2 cats, a dog, two adults home in the evenings- and the heat from a running refrigarator--- outside temps=10/20s
    Inside basement=45+/-.

    Right now (12/10/07- 8:35pm) Outside temp= 11.3, Inside temp=69.5 w/radient floor on- total of six logs added today.
    Total cost to heat 4000+sq/ft today (estimated)= $ 5.00.
    (Now even running the Lopi- haven't in four days- it would cook us out).

    If you read what Browning wrote...you would think that this big uninsulated space under my foundation would act like a drain- sucking the heat down into the ground and off to China.... well, from experience, I can tell you it doesn't work like that. It works just as it is described... Heat go's into the earth under the foundation and as the slab cools ( the heat is off for 48 hours or more ), more stored heat is transfered to the slab from the earth. We are so pleased with how little it takes to heat this house. As a side note- We have awful soil (clay) with a high water table... so even with the vapor barriers- I think of the water under the slab as more heat storage medium...doesen't have any noticable effect and I don't worry about it.
    ***Bet ya a dollar that those who say that the bubble insulation is junk have no real experence with it***
    (Like it so much, and had leftover- used it to insulate the insides of my gables !)

    I am not an engineer-(thank you Jah) but from what I and I see-Insulation works and using the thermal mass of the earth works.
  13. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    No, it transfers it horizontally, in the ground water.

    Wet earth is not an insulator, no matter how good an insulator dry earth might be.


    How are you measuring this?

    Bubble insulation works in open air. It does not work underground. That's been demonstrated in every single unbiased test that's been done, to the best of my knowledge. There are a lot of resources on this available on and off the Internet.

    The best insulation in terms of cost/benefit was, IIRC, the cores of quality insulated steel doors (the scraps from where they punch window openings out of them).

    Joe
  14. WILDSOURDOUGH

    WILDSOURDOUGH New Member

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    Not going to argue the science of it... just stating my experience with it.
  15. Rhone

    Rhone Minister of Fire

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    Well, this has been interesting. What I found, heat does not like to go down (probably obvious). Also obvious the sides of the slab/foundation must be insulated which is a given. The question is, does one need to insulate the entire foundation floor or not. If you have a high water table or your house is situated on ledge your heat loss can be 10x more and a definite yes your entire floor should be insulated. Otherwise worse case scenario in the coldest continental state if one puts in a radiant floor heating in an uninsulated basement floor and covers it with carpet & pad they will lose 25% of the heat to the ground but in more mild climate with a tile floor one can expect to lose a little over 10%. That's with just outside insulation and none under the entire basement floor.

    So the question then becomes what about insulating just the perimeter and leaving the center open for those not on ledge or high water table? Up to the owner there isn't a clear winner. Insulating the entire floor will speed up reaction time as you're not dragging the earth with you and does save you energy (it's really not much). But, leaving it open costs less up front and will be a better buffer against heat & cold spikes as you do have more mass.
  16. jklingel

    jklingel Feeling the Heat

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    Now this is getting very interesting. Does anyone have any references for using the earth as a heater? I am not going to state that whatever I say has any particular authority, 'cause I ain't got any. However, having taken physics and spent my life dinking w/ "science stuff" (like many of us here) I am still not convinced about this earth-heater thing, and here is why. For one, if this method really works, why not use NO under-slab insulation and have even more heat coming back at night? Same concept, to me, just a larger version. The heat loss calc equation is: heat loss = reciprocal of R-value times the surface area times the temp differential, between and of two adjacent objects. Q=UA(delta T). Period. Heat transfer is independent of earth's meager gravitational force, ie heat does not go "directionally", but rather "to cold". (Matter in a higher state of excitation ("warmer", higher entropy) gives off energy in the form of heat to matter in a lower state ("cooler", lower entropy)). Period. Up/down/sideways matters not. What DOES go up are warmer liquids/gasses, because they are less dense than their cooler partner and therefore effected less by gravity. A concrete slab can only get heat from the earth if that earth is warmer than it. If a slab cools to 44 degrees and the ground is 45, the ground will warm the slab. Until that time, the warmer slab is warming the earth. Certainly, earth warmed by someone's wood boiler will eventually "give back" some heat if/when the slab gets cooler than it, but I don't think I want to send all those btu's down there to warm it, primarily because I can not control how far down AND sideways the heat will go. Heat goes to cold. Period. I've been told that a foot of dry earth has an R of 4-ish, about what 1" of Extruded Poly has. Do I want to heat a huge mass of earth below my slab so that I get SOME of it back? No. I don't. Analogy: The more money you pay for certain tax write-offs, the more you get back, but it is not a good investment over-all, (just looking at the money part). That said, would I heat a bunch of earth in a box ON TOP of my insulated slab? Sure, just like I'd heat water through which to run a coil to heat DHW, etc. The difference is, I can pretty well control that heat transfer, and keep most of it in my house. I wouldn't look at that system as saving me anything, but rather shifting the time when I get that heat into the house. Heating the earth is a limitless venture; you could run a heat coil down there and heat it all day, but I bet you'd get a very small percentage of it back at night. The Chinese are my buddies, but I am not going to send heat their way unnecessarily. Does that make sense? That last point is the critical one to me: how much do you spend to get how much back? AND, BTW.... we could extend that hypothesis and say "Well, I opened my door to heat the air around my house, so that I can get it back tonight." Assuming no wind and no Brownian motion in the air molecules to move the heat from right around the house, that would be the same concept as heating the earth, would it not? Finally,if you are sitting on a 'thermal", like in Yellowstone Park, by all means don't insulate one iota; let the earth do it all. So: Somebody respond and tell me where you think my reasoning is in error. I think it is ok, but then I tend to be prejudiced.... thnx. j
  17. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    j, that's about the long and short of it. Heat doesn't "go up." It will go wherever there is a lower level of heat, at the rate allowed by the material it is in. Heat will not just politely sit there under a slab, waiting to heat your house for you.

    If you really want "dry" thermal storage under your house, build a two-story basement, fully insulated, and then fill it half-way with sand, pouring a slab over that.

    A water tank will do it with less cost.

    Joe
  18. Rhone

    Rhone Minister of Fire

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    Well, that's how you determine conductive heat loss but energy turns into conductive, radiant, and convective. Radiant energy is a form of light, and only happens at the surface of an object. So any of the floors energy that turns into radiant is going to radiate up and hit objects and the ceiling where it will be absorbed and turned into convetive, radiant, and conductive energy. The part that turns into convection (hot air), is that hot air going to want to flow down through the earth? Nope, it's going to rise up and from there probably flow up to the next floor none will be lost to the earth.

    So, any of the heat that turns into convection heat will have no loss to the earth. Any that turns into radiant energy, will also have almost no loss to the earth. The only heat form of the 3 that can have loss is conductive, which is mostly lost at the perimeter which we've got taken care of leaving earth is an insulator (as long as it's dry anyway). I agree that, there is heat loss but it's not as much as I'm getting the sense people are thinking it is.
  19. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Conductive, however, is the most efficient.

    Perfectly dry earth is a marginally-decent insulator, but I doubt that's what's under most foundations.

    Joe
  20. Rhone

    Rhone Minister of Fire

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    Here's the info I could find about it, could be bogus who knows. In summary soil & climate conditions make it a crapshoot at best. MOST of the conductive energy is lost at the perimeter of the building, but we've got that insulated. The report then continues to say not insulating the floor would cost $14.73/year in Colorado and in Canada $40/year in year 2000 dollars of heating extra. That's for an entire floor to be insulated vs. none at all. So, insulating the outside of the basement floor and leaving the middle open would amount to a savings of a fraction of the $14.73/year or $40/year (again in year 2000 heating dollars). Won't amount to as much as I think people believe, but with energy costs today I'd sure choose to insulate the whole thing.

    http://www.radiantpanelassociation.org/i4a/pages/index.cfm?pageid=420

    Another study by Montana that came out in March of this year showed that, vertical insulation around the perimeter below frost line was better energy savings than doing the entire floor with R10. Also, that vertical R10 insulation around the perimeter amounted to a savings of $321 whereas fully insulating the perimeter AND floor with R10 amounted to $393... a savings of $72 (0.3% annual savings). They did not test the perimeter + just the outside edges of the basement floor but, would be a fraction of that.

    http://www.energycodes.gov/implement/pdfs/MTslab-edge_2006IECC.pdf
  21. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Look at lifetime costs, not just what it saves per year.

    Particularly as fuel goes up.

    Also, the heat loss is only part of it. In order to overcome it, the water in the tubing needs to be hotter. The prime efficiency benefit to radiant is the low water temp; anything that raises the temp is attacking the efficiency of the system, directly.

    The small cost of insulation is not much compared to the loss in efficiency.

    But the real issue I had was with the claim that not insulating would improve efficiency, which is completely bogus.

    Joe
  22. jklingel

    jklingel Feeling the Heat

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    Rhone: Good articles. Here is another point of view from a guy on the Garage Journal. I definitely need to run this buy an engineer familiar w/ our conditions. At the very least, it appears that one needs to insulate the footer wall down several feet, perhaps the footer horizontally, and some distance XX around the perimeter of the slab, underneath it. I am glad I am not building for a year or so; much to be learned. I will say this, though: I now have a slab w/ EPS vertically outside the footer wall and about 4' in, under the slab, and you can not walk around on the slab when it is 40 below; not without bunny boots, anyway.

    "Take a look at this information from Radiantec. I am looking to do the same thing. I've got to believe they've done their homework.
    Their specific insulation methods are on pages 6 and 7 of the Design and Construction brochure.
    http://www.radiantec.com/pdf/Construction.pdf"
  23. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    I was trying to think of a diplomatic way to say this, but I can't... any company advocating open systems for radiant (ie, combining radiant with domestic hot water, with no heat exchanger) is grossly incompetent. Aside from wear on the equipment, all sorts of nasties can grow in the heating tubes during the summer, then get pumped into your drinking water when you fire up the heat in the fall.

    I can't imagine anyone professional being willing to advocate such a thing. I went to grade school with a little girl who was crippled by diseases she got from a contaminated well. It's just not something you play around with. Heating water and domestic water are always separate.

    Joe
  24. jklingel

    jklingel Feeling the Heat

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    Joe: I did not look at the file other than the bit about insulating under a slab. So THAT is what an "open" system means. Good God! Who in the (insert some nasty word) would ever mix those waters? That is crazy for sure.
  25. Rhone

    Rhone Minister of Fire

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    Apples to oranges. Contamination of wells and open systems have nothing in common. What causes contaminated wells is when the well head is left open, or its seals have deteriorated bugs/leaves get into into the well and die in the water leaving organic rich food for bacteria to feed on and that contaminates your well. An open system didn't cause the contamination, you'd be in just as much trouble closed or open. I personally don't see a problem with open systems, I don't think it any more health risks than closed. My thinking, and I could be wrong is that if there's going to be any bacteria it's going to exist in the hot water tank that's the place it stagnates most, is constantly given fresh oxygen rich water and, won't matter with a system that's open or closed. Even hot water tanks at 140F have a 25% likelyhood of bacteria infection. But, you can't normally run 140F through your pipes that's scolding temps and, code requires a mixing valve to tone it down below temps that can kill bacteria. Everything I see says that the case. As for the cold water, my sister lives in a condo on the 5th floor and cold water goes through 500 ft of PEX before reaching her. I have an open system, and for me to drink cold water it goes through 500 ft of PEX before reaching me. Is there any difference? Sure, mine zig-zags cause it's open, but we both have 500 ft of PEX so what's the difference? Why aren't all these people in skyscrapers or with plumbing at great distances getting sick? Why is it so common today to have huge PEX runs from each appliance going to manifolds in ones basement if bacteria infestations are so common in long stretches of PEX pipe? That's why I think open or closed isn't going to matter.

    The Radiantec site, the owner of that company is Bob Starr. He's the one who invented running PEX tubing through a thick insulated slab for heat storage, buffering, etc. which is what we're talking about and he invented it in the early 80's. The purpose was a means for a simple way to combine active & passive solar which he called a hybrid design. Anyway today almost all solar designs are based on his invention. So, I hope he knows what he's talking about as what we're talking about here, he invented! :)
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