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No under slab insulation

Big Will
Big Will Member Posts: 394
I have customer that installed tubing in the floor anticipating future radiant. They have used the forced air system that was installed for two years. Last fall they had a competitor install a Solar Phoenix with Taco X-blocks for the radiant. He told me that for the three months he ran it his propane consumption almost tripled. Note: consumption not cost. So he brought me in for a second opinion. I have followed this contractor a few times and the work is constantly bad. In this case he made some piping errors that would cause the system to run far longer than necessary. Also the solar panels are facing due west. Now I am not a solar contractor at all so I am not sure but I thought they were supposed to face south. I was referred to the homeowner through a solar contractor I work with so he will be dealing with the problems with the solar system. The one thing I am unsure of is the lack of slab insulation. The edges are exposed above grade about six inches all the way around the house.  Also the slab is completely uninsulated. In his favor is our climate and his location. First the house is in Sonoma California. It rarely falls below 38 degrees and usually very briefly in the early morning. The ground is fairly flat with a very deep stream near the house so I am assuming he has little or no ground water near the surface in the winter. However I do not know how to calculate the loss when their is no slab insulation and although I do not approve of the work that was done I question weather the mistakes that were made would consume that amount of fuel. consumption. About the house. It is about 2100 square feet of ranch style home. Slab on grade single level. The system that was running before the radiant install is a Bryant single stage 90% furnace with sub-grade duct.


  • No underslab insulation

    That's too bad that there's no insulation, but what I know of this condition is that heat will flow into the ground until it stabilizes.  It should then perform normally, as though insulation had been installed.

    But from what you are telling me, it's almost like it's an experiment that would prove the above wrong, that in fact, heat was being lost to the ground.

    I would have said a high water table or improper underslab drainage, but you seem to dispel this.
    Often wrong, never in doubt.
  • SpeyFitter
    SpeyFitter Member Posts: 422
    Dense Ground

    Remember that ground/dirt is much more dense than air. This means it can suck up a lot more BTUH's before you will actually see some performance in the tubing actually putting some heat into the space. It's too bad there's no insulation. It's bad enough you get the deer in the headlights look when you suggest to a contractor to put some slab edge insulation, but when you don't put any insulation under the slab/ & pipes you might as well use that tubing for the source loop of your geothermal (water) to air heat pump and stick with forced air.
    Class 'A' Gas Fitter - Certified Hydronic Systems Designer - Journeyman Plumber
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    I am not a heating professional.

    But I do have a house that has radiant heating on a slab-at-grade that I suspect is not insulated. The house was built in around 1950. The reason I suspect it was not insulated, at least around the sides, is that the edge of the slab is exposed all the way around, and except on the coldest days, any snow near the house for a foot or two melts within a day. I may be wrong about this. In any case, I looked up why such slabs should be insulated in this fine book, pages 22-25.

    In my case, the water table is about 6 feet down from the slab. It seems that for slab at grade radiant heating, most of the heat is lost out the edges and that the heat going directly down is considerably less, especially if you run nearly continuously the circulators; i.e., using outdoor reset, so the temperature of the slab does not vary too much. The reason is that the earth does not conduct heat very well, especially for longer distances. While I have not thought to do it, it might be practical to insulate the slab, after the fact, around the outside edges. Of course, insulating underneath would be impractical.

    There are other possibilities, depending on what you want to spend. Radiant panels in the walls or ceiling come to mind.

    But if your fuel costs tripled going from hot air to hot water, I would suspect you need professional help, and I am not speaking of psychological help.
  • hot_rod
    hot_rod Member Posts: 15,033
    edge loss can be a huge number

    run a load calc with and without underslab and edge insulation to see the difference. That will show you where all the heat is traveling to.

    Take an infrared camera to the job in the heating season to show them where the energy loss is. There are ways to add edge and perimeter insulation to an existing building. You need to bend a flashing to cover it, however

    This is how I am adding 2" of insulation to my shop foundation. This is the back before I poured a patio slab against the building. With a metal sided building that flashing detail looks fairly good.

    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    a few factors

    you're in a "perpetual shoulder season". lots of on/off heating cycles where you would heat for awhile, then not heat again for awhile, perhaps days. High mass kind of stinks for that, because to start from cold you have to "charge" the mass up first. then if you lose all your demands, you bleed that heat out and you have to charge it up again the next time. Basically every time you stop heating and let the slab cool to "normal" you lose all that heat. in a cold climate that is probably only a few times during shoulder seasons, then you're heating consistently all winter. In your case, not so much.

    So I imagine one problem is all the continual recharging of your mass, which in this case with no insulation includes a lot of ground mass in addition to the concrete. That's a big no no. I have no idea if that explains it "all", but it's definitely a contributing factor unless I've missed my mark completely.

    edge insulation is important though, definitely and should be added if he wants to heat this slab. Too late to help with the ground issue.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • zacmobile
    zacmobile Member Posts: 211
    edge loss

    slab edge loss is huge, much more than underslab on slab on grade installations. I've taken a couple of in-depth heat loss calculation courses over the last 2 years and they were a real eye opener as to how much you actually lose through the edge, it can be as high as 150btuh per foot sometimes more. if you do get some insulation on make sure you get it down well below the frost line as heat will travel diagonally too.
  • Mark Eatherton
    Mark Eatherton Member Posts: 5,837
    Potential solution to missing under slab insulation...

    I recently started doing Infrared Surveys for a company that uses Urethane foam to jack up slabs that have settled. I go in and show them where it is OK to drill. For obvious reasons, I can't show them where NOT to drill. Not enough chalk and or magic marker in the world for that operation.

    Any way, they had me go to the mountains the other day to mark up a slab in the basement of a home. Unfortunately, my IR camera was being held hostage (whole nother story) by the US PO, so I had to resort to the use of my non contact IR thermometer. Somehow, and it could very well have been my partially dislexic mind, one of the OK marks ended up being DIRECTLY over the top of one of the tubes. The foam contractor hit it perfectly. Maybe BP should hire him to help with their little issue in the GoM...

    So, I had to shift gears, and perform jack hammer surgery to repair the tube after they had already injected the foam to raise the slab (had dropped 2 inches), and much to my surprise, the slab, which had previously been insulated with bubble foil bubble, was now well insulated with high density urethane foam.

    So, in my minds eye, it MIGHT be possible to partially insulate these uninsulated slabs. I envision drilling a series of holes, close enough to allow water to be fed into one hole, while applying a water vacuum to an adjacent hole to create a small cavity. Then, fill the void with the urethane foam, and instant insulation.

    The good thing about this debacle is that when the slab jack contractor called me and said "So, is the homeowner going to get a big bill for the tubing repair?" I was able to tell him about my guarantee. They hired me to AVOID hitting tubing, and I took responsibility for the hit and repaired it at no cost. I lost money that day, but gained 2 customers for life, and that covered my losses.

    The foam company is Concrete Stabilization Technologies. Their web site is

    There may still be hope after all :-=)

    It's not so much a case of "You got what you paid for", as it is a matter of "You DIDN'T get what you DIDN'T pay for, and you're NOT going to get what you thought you were in the way of comfort". Borrowed from Heatboy.
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    One reason for mod|con and outdoor reset...

    "Basically every time you stop heating and let the slab cool to "normal" you lose all that heat."

    That is why if you have a mod|con boiler with outdoor reset, you can diddle the reset curve so the circulator is running almost all the time. The slab (and the earth under it) never gets too hot, and it never gets to cool down. It is better if you have ECM type of motor for the circulators to cut down the electrical load.
  • NRT_Rob
    NRT_Rob Member Posts: 1,013
    that's true

    but in a place like sonoma, legitimate heating demands are not that consistent. reset won't help that much because we're not talking about the time between "cycles" without reset: we're talking about losing a heat demand and not getting it back for a few days because you're "around" the temperature where you start needing heat.

    it's like being in "season start up", but over, and over, and over again.

    this is a working theory anyway, I haven't tested it, but I think it's reasonable.
    Rob Brown
    Designer for Rockport Mechanical
    in beautiful Rockport Maine.
  • cattledog
    cattledog Member Posts: 60
    uninsulated slab on grade heat loss

    Hot rod was correct that the way to get at this is through the heat loss calculation. I'm not sure this will support zacmobiles 150 btuh per linear foot estimate for your climate.

    City of Seattle energy code estimate slab perimeter losses at (.73 btu/h x deltaT) per linear foot. For equipment sizing in Northwest climate they specify a design delta T of 46 which gives 33.6 btu/hr per linear foot of slab edge. Sonoma may be similar.

    Area slab losses are difficult to calculate and are smaller than edge losses, but I have seen estimates of two or three btu/hr/ft2 at unspecified temperatures.

    Compare slab losses to the rest of the envelope, which may be between 10 and 20 btu/hr/ft2 for modern construction.

    Your customer may be seeing 3X the propane consumption with the radiant floor instead of the forced air but it's probably not due to the uninsulated slab.
  • Gordy
    Gordy Member Posts: 9,514
    Agree with Rob

     With the lack of perimeter insulation, and underslab insulation the slab is charging alot of mass, AKA the earth. The benifit of insulation under, and  around the slab would lead to a more responsive output. With a shoulder season  scenerio, or any scenerio for that matter as Rob stated. 

    Does the space overshoot once set point is achieved? Be a good indicator that a lot of mass is getting heated up then bleeding off energy for a long period of time.

    The next thing as mentioned above is type of soil the denser the soil the worse it gets. Throw in ground water, and it gets worse yet.

  • eluv8
    eluv8 Member Posts: 174
    My two cents

    The way I figure it,

    A BTU is a BTU. So, the heat is going somewhere. Up the stack,  higher avg room temperature than the forced air due to overshoot and or open windows to cool the room, ground water or large rock heat sinks, perimeter losses. My guess they have issues with most if not all of these concerns.

     An accurate heat loss would be a good start analyzing what the building should be using with an uninsulated slab with 6" of exposed perimeter. The calculation could also show what potential savings benefit could be achieved with varying thicknesses of insulation.

     A thermal scan would help locate/prove the largest losses and give you an idea of where to start as well. A thermal scan (Picture) can speak volumes to the customer, and lends credibility to the proposed solutions.

    If I had to guess based on standard practices,  if it snowed there, they could still grow spring flowers around the foundation in the middle of the winter.

    If I were the contractor and the heat loss calculations supported my theory,

    (1) I would recommend first insulating the perimeter edge and down at least 2 ft if not 3ft..

    (2) Propose a condensing boiler to take advantage of the frequent cold starts, or an alternative green heat source such as geothermal. LP is not an inexpensive fuel source for heating.

    (3) I would install a Tekmar or similar control system capable of outdoor and indoor reset capabilities with a minimum of outdoor reset with a very aggressive reset schedule, temp boost and warm weather shut down.
  • eluv8
    eluv8 Member Posts: 174
    edited June 2010
    double post

    I always wondered how that happened. Now I know.
  • CMadatMe
    CMadatMe Member Posts: 3,085
    No Under Slab or Perimeter

    Through experience take the heat loss and double it. The easiest thing to do is find yourself some radiant software and run the loss with and without. You can easily get free software from radiant manufacturers. There is no need to get thermal cameras and drive yourself crazy unless you feel there is a leak somewhere. Look at the job as though there is no radiant and start from scratch. Make sure the right amount of loops are there, make sure the right water temps are running, make sure the heating plant is sized properly. If you find that all of that is good with a properly installed radiant system turn around and reverse it as though there is no insulation. Is the heating plant properly sized, are there enough tubing in the floor to carry the btu load, etc.

    Back in the day when we began doing radiant (late 1980's) the first thing we did when we went out to a job because a homeowner called to say the boiler would never stop running was to look and see if there was a snow line approx 2 feet out away from the foundation or the flowers/plants were still green. Running losses the general rule of thumb from those losses is that the heat loss doubles.
    "The bitter taste of a poor installation remains much longer than the sweet taste of the lowest price."
  • Big Will
    Big Will Member Posts: 394
    NO reset was one of the problems

    The X-blocks came with outdoor sensors but they were discarded by the installer. They were running on set-point control only. I find it hard to believe that dry clay will absorb as much heat as the house but that is why I posted. Also the Phoenix was set to 110 deg and so were both the x-blocks. I think running this cool they would never satisfy on a cold day with a lot of the heat going into the earth. That and the system temp would never get higher than 105 with a source at 110 and a heat exchanger.  
  • Gordy
    Gordy Member Posts: 9,514
    If you have ever owned

     A water bed, and after you set it up, and filled it, and before it reached say 82* tried to sleep on it. It sucks the body heat right out of you. That is what the earth is doing to the slab. Clay is a dense soil. Being dry is a plus but it is still a better conducter than an insulator. The hurt is as Rob stated long off periods in a shoulder type of climate.

  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Thermal conductivity...

    I looked up the thermal conductivity of various things in a copy of the Handbook of Chemistry and Physics.

    Material       Conductivity

    Aluminum    0.480 to 0.504

    Brass          0.204 to 0.260

    Brick, red     0.0015

    Copper        0.918 to 1.00

    earth's crust 0.004 (average)

    sand, dry     0.00093

    Now, for the earth's crust, I imagine if you are below the water table, the operant thing to measure is the flow rate of the water, not the conductivity of the earth itself. So it is better to insuate with something, even red brick or dry sand (if you can keep it dry),  but the earth's crust is only about 3x worse than red brick.
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