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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. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Bacteria aren't going to propagate to dangerous levels inside flowing water. The issue with open systems is that the water sits stagnant for months during the summer. Then you fire up the system when it gets cold, and you are dosed with the bacteria. Months of growing time, not the short trip through a PEX pipe inside the house.

    That's an amusing notion.

    Joe

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  2. Rhone

    Rhone Minister of Fire

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    Ah... that's the key. I would agree, but their design is unique in that in summer you have the cold water run through the radiant slab/floor instead of heat helping you cool the building. Running the garden hose, taking a shower, in summer will run cold water through the PEX cooling the building saving you cooling costs, while at the same time pre-heating the replacement water to the hot water tank & shower saving you heating $. Pretty ingenius, prevents stagnation too.
  3. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Last I heard, that was not part of their design except if someone specifically requested it. And radiant cooling can also cause health issues by creating condensation and mold. Radiant cooling needs controls to limit its operation in response to the humidity, so the radiant panel never goes below the dew point. Even if you add the controls, now you've just isolated the water again by shutting off the slab.

    Joe
  4. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Exactly. People are used to thinking that "heat rises" because they are used to heat in fluids. A hot fluid loses density, and rises in comparison to a colder, denser fluid. The heat precipitates it, but is not the direct cause.

    Heat will always flow towards cold, regardless of the direction. That includes down into the earth.

    Joe
  5. Rhone

    Rhone Minister of Fire

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    I'm pretty sure it's a part of their open direct system design since, not doing it would cause stagnation and it's only a $15 part. But, I did find I made a mistake after reading http://www.radiantec.com/faq/faq04.php#4a it only puts cold water through the floors in summer when you're using hot water in summer, working as a hot water pre-heater. That being the case, the cooling feature doesn't offer much cooling (only doing it when hot water is called), and little chance for humidity build up.

    In that link
    "What about condensation during the summer months?
    Because the tubes are enclosed within the floor, humid air does not reach them and condensation will not occur. You will not get enough cooling to present a "whole house" moisture problem. In an extremely humid climate, air conditioning will be required and this will help to dehumidify the air as well."
  6. slowzuki

    slowzuki Feeling the Heat

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    I spent a good deal of time doing heat transfer calcs when designing my slab since I didn't have any money and I live in Canada. For my money I put in tons of drainage down to 6 feet, a vapour barrier, built up a 2 foot deep coarse bed of sand then my slab. I insulated with 3" of XPS around the entire perimeter down 2 feet. I went out with 3" XPS a foot, then out with 1.5" to 4 feet.

    Because it is a shop primarily, I will never be raising it much above the mean ground temperature in my area, so my heat loss to the ground, even though its called a semi infinite solid, is minimal compared to the conductive and convective loss to the -30 right next to the edge of my slab.

    In my apartment, I insulated with 1" XPS below the slab so I'd have quicker control over the temperature.

    Dry ground is a fairly good insulator, this is evidenced by the permafrost that forms under hockey arenas, they have to put 6" plus of XPS under the ice so the ground won't freeze. Also, you can freeze the ground with a geothermal system, thats why they have minimum sizes and you can't just bury it at the frost line.
  7. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    A ball valve is hardly a legitimate part of a system, in terms of switching modes. What if you have a hot day and a cool night? Have to run down to the basement and turn the valve?

    Condensation forms on the slab, not the tubing. See, this is what I'm talking about. They play word games instead of giving the straight answers that any professional would. "Oh you won't get any condensation on the tubing."

    Joe
  8. Rhone

    Rhone Minister of Fire

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    Man, I'm regretting ever saying that now in this example the floor is 100F the ceiling 72F which way is the heat going to flow?
  9. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Into the 50-degree soil, if there's no insulation.

    Oh, some will go up, too. But conduction directly to the soil is a far more efficient transfer method than convection or radiation.

    Let's put it this way: you come in from the cold and I offer you one of those hand warmers. I tell you that you can hold your hands a few inches over it, or you can put your hand on the table and I'll put the hand warmer on top of it. Which is going to warm your hand most effectively?

    Joe
  10. slowzuki

    slowzuki Feeling the Heat

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    I'll hit a double whammy here, pre-heating your domestic hotwater with a slab in the summer won't tip the scales to whether you get condensation, but it is likely a waste of pumping power. It is such an insignificant amount of energy taking your say 50 F water to maybe 70 F, for an average of what, 300 gals per day hotwater use in a busy family? A solar preheat loop would make much more sense.

    Second, in dry soil, the ground temp stratifies over time so you aren't directly losing to the 50 F ground. If you have springs or high ground water, you will be losing to 50 F ground though!!!
  11. Rhone

    Rhone Minister of Fire

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    There will NOT be condensation forming on the slab running cold water through it the few times a day in summer you use hot water. Running 55F water through a cement slab for 30 minutes of showers in the morning will NOT lower the dew point in the slab to enough to cause condensation on the surface. Neither will it when I wash my hands a few times during the night, nor wash my dishes where it runs the hot water for a couple minutes. It takes around 24 hours to charge up a slab, you think it can cool to dew point levels with sporadic use of hot water during the day in summer? Again, we're talking about a slab or earth here that doesn't exactly heat or cool instantly. But, can put stress on ones pump but if you have city water you don't have a pump to wear out.

    About flicking that valve. I can't remember ever needing the AC during the day and heating system at night, you and I don't live in Texas. If you're having to do it, I recommend insulation. You know what I do, I pick a day to put my AC away and after that I don't put it back in. If I have a strange day or two that I might need it, I survive. Would rather flick a valve but they tell you it doesn't offer much cooling so it's not like flicking it is going to relieve you of heat in a 95F humid day especially since it only works when using hot water in summer it's not like you're going to turn on your hot water faucet and keep it running for 24 hours to try to cool the slab.

    I've come up with 2 studies showing hardly any loss to the soil (which is R1 per foot) yet, you keep talking about how efficient it is at wasting energy. You did read the articles I posted earlier about heat loss to earth right? You did notice both showed hardly anything? How about the study where the difference between insulating the perimeter vs. perimeter & floor with R10 amounted to an additional 0.3% savings? Well, 0.3% is $2.16/year for me and tells me Earth is no super efficient medium. Will cost me over $800 to do it. Conductive is not the most efficient, the human body is most sensative to radiant so, the more you increase the mean radiant temperature the more comfortable you'll feel when compared to trying to increase other energy types and, it depends on the material.

    Anyway, it's evident this has hit the sarcastic level and time for me to depart from this topic. I think I've made the point as long as your perimeter is insulated earth loses very little energy and, in one report shows it to be around 0.3%. Think I've made the point that an open system will work and think it no different than closed but that's just my opinion I can see people who won't like it. There is NO way of getting a many ton slab to reach dew-point levels and causing condensation by running cold water through it a little bit sporadically during the day. Lastly, having to flick that valve for heating mode is fine. Most do not live in an environment they turn the AC on during the day and heat at night (insulation will help if not) so won't be any problem.

    Cheers!
  12. WILDSOURDOUGH

    WILDSOURDOUGH New Member

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    Rhonemas- "http://www.energycodes.gov/implement/pdfs/MTslab-edge_2006IECC.pdf"... That is a great source, and more vindacation of 'Radientec' ideas of leaving a percentage of slab uninsulated, including a thermal break at the periameter meeting with the footings, and the importance of good (tecfoil) vapor barrers.

    Since I built with ICFs- the interior and exterior insulation was covered. 'Grace ice + water sheld' also was used to cover the exterior foundation walls to the footings. Two layers of 'tecfoil' used as a thermal break at the interior slab perimiter. Effectivly have 3 layers of vapor barrers under the entire slab- 2 plastic and one of tecfoil. I did follow Radientec idea of uninsulated core of the slab ( Glad I did !)- can heat up slab as usual till friday morning- then go away for the weekend, slab is still warm (not hot) on a monday afternoon ! I do run 'very hot' (130+/- degree) water from the OWB thru the pex- slab heats up to 105 degrees, but oddly enough- never any more that that.

    I did not like the idea of mixing floor heating and domestic water. I use my OWB to heat the floor only- and a super insulated 'Marathon' water heater for domestic HW.

    As I stated in previous post: I can't completly explain the 'science' of it- but it 'sounded good' to me to build it this way... I added a few extra things...(two thermal breaks, to the footings water sheld, 1800' 3/4 id pex...and tecfoil). It works so far. But to each, his own way.
  13. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    As I've said, my point was not that failing to insulate will destroy your system's ability to function. Although it's still a silly waste. Much like not insulating the walls of your house.

    The point was that not insulating will not give you a benefit, which is what was claimed.

    You heat a slab to over a hundred degrees, and you think it's because of the lack of insulation that it stays hot? It would stay hot longer if you had it fully insulated.

    Cannot overcome losses to the soil.

    Joe
  14. slowzuki

    slowzuki Feeling the Heat

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    Joe the main point here is that in some areas of the country, it is not foolish to leave the middle of a slab uninsulated. If you are on rock or high water table, then yes, it is crazy to leave it uninsulated. If you are in the south with a high average ground temp, and good dry sandy soil, then its a no brainer to leave out the insulation in the middle.

    I put about 3000$ of XPS into the ground around my slab, it would have been almost 2000$ more to get the middle for 0.3% of my heat loss. 2000$ can buy me a lot more than 0.3% reduction elsewhere such as windows, doors, an extra foot of blown in an attic, etc. But if I have high ground water or rock and am losing 20% through my slab, I know where my money would go!
  15. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Water tables shift.

    You only get one chance to do the under-slab insulation, for obvious reasons. Attic insulation can always be added, later.

    And, again, the main issue I had was with the idea that not insulating will improve efficiency. As if it were a good thing to do, not just a cost-cutting measure.

    Joe
  16. WILDSOURDOUGH

    WILDSOURDOUGH New Member

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    "You heat a slab to over a hundred degrees, and you think it’s because of the lack of insulation that it stays hot? It would stay hot longer if you had it fully insulated."

    1. Yes- foundation slab is heated to 100+ degrees. (and I like it very, very much !)
    2. No lack of insulation- just not insulated 'to your liking'.
    3. Is 'fully insulated'- just that I chose to use 'tecfoil' in the center, which has a 'low' insulation value compared to pink or blue foam.
    4. I believe that (with proper care taken-in new construction) some people can use the 'earth under their homes' as a heat storage medium, which you do not believe.
    5. My proof is my basement floor- heated to 100 degrees, then left without additional heat input to unheated air exposure for 96 hours....and is still 'warm'. Under these conditions (no additional heat applied)- the floor should be cool to cold- not warm to the touch- so I tell you it works. I do not say- " it’s because of the lack of insulation that it stays hot? " I say it is because of the proper use of the properties of heat storage and release of the earth under my slab that it is able to maintain this warmth. You have no proof that "It would stay hot longer if you had it fully insulated." And the truth is I do not either... but what my floor does do- works !

    Hell, I would have used 'Earth Sheltering' if the wife had not wanted a 'Traditional' looking home. The Earth is the least expensive and best insulator anyone can get, and it's natural properties of give and take with heat and humitity are unsupasses. I've known a few 'earth sheltered' homeowners- never- ever have I heard one of them say- "it's cold". But: to each- his own
  17. BrownianHeatingTech

    BrownianHeatingTech Minister of Fire

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    Physics disagrees.

    Earth-sheltered homes work because the temp below the frost line is a pretty consistent 50-55 degrees. You just heat that little bit from there to 70, all year 'round. They don't work because of any sort of "heat storage in the earth" or somesuch.

    I've never seen an earth-sheltered home that wasn't heavily insulated.

    Joe
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