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  1. #1
    Join Date
    Jul 2008
    Location
    Thief River Falls, MN
    Posts
    5

    Proper way to insulate basement slab for radiant heat?

    We will be pouring the basement floor next week, and I am getting conflicting information on the best way to insulate under the slab.
    I had a very reputable source tell me that it is actually cheaper to heat a floor that has no insulation under it, because using the ground for heat storage actually causes less run time on the pump over a course of time. He stated that while the pump runs initially longer to heat the slab/ground beneath it, with all that storage you have longer intervals between runs. He also stated with an insulated floor (2" foam) the run times are shorter due to the insulation, but are more frequent because it heats up just the concrete, shuts off, cools down quicker, starts up again, etc.

    I was also told that the 2" foam was the only way to go, nothing less. I need to get this nailed down by next week.

    Any help/input is appreciated.

  2. #2
    Join Date
    Aug 2007
    Location
    MN
    Posts
    2,677
    don't listen to your reputable source! The 2" foam is best IMO. You can go with 1-1 1/2" but it breaks up when you walk on it. I have heard of guys insulating then laying the tubing and pouring a couple inches of sand on top, then pouring the crete, giving them a little more heat sink. But there is a very good chance that if you don't insulate, you will never heat the basement because the heat sink is too large(high water tables are very bad for this.)
    You can't fix stupid

  3. #3
    Join Date
    Aug 2006
    Posts
    112
    hire a pro and let them do their magic. insulation will be done right tube length and spacing will be done right and you most importantly will be comfortable when the project is done.

  4. #4
    Join Date
    Jul 2000
    Location
    Northern Wisconsin
    Posts
    2,005
    Let me throw a wild guess out here........ You're reputable source is the concrete guy.

    I've done hundreds of heat load calculations for infloor (slab) radiant systems and I can tell you from that experience (whether you want to consider it reputable or not) that by changing the R value of the insulation under the floor from ZERO to R-10 you will reduce the heat load answer about in half. If all you're worried about is run time on the pump then no insulation is the way to go. If you're worried about having to get a second job to pay the heating bill I'd suggest putting down the insulation.
    Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

  5. #5
    Join Date
    Jan 2007
    Location
    Minneapolis, MN
    Posts
    415
    4" high density EPS. Thermal breaks at edge. Radon resistant construction while you are at it. There is a construction technique for thermal storage but it is too lengthly for this thread.

  6. #6
    Join Date
    Jul 2008
    Location
    Thief River Falls, MN
    Posts
    5
    Actually it was the geothermal guy. He put up 2 identical buildings, one with 2" foam for insulation and one without. Over the course of a year, the one without the insulation was actually a little cheaper than the one with it. I couldn't believe it. And he is very reputable as well as being the most trusted guy for geothermal in this area.

  7. #7
    Join Date
    Jan 2007
    Location
    Minneapolis, MN
    Posts
    415
    Occupant behavior has a big impact on energy use. With out any real data it's all speculation. Heat travels from warmer to cooler, that a thermal dynamic law. The earth is cooler than the house. Heat will move to heat the cooler earth and will not return to heat your house. But enough of my silly talk, you should go with the reputable guy.

  8. #8
    Join Date
    Dec 2002
    Posts
    77
    I went to some training at the Rehau factory and disticntly remember them discussing this.

    They showed thermal imaging from radiant job sites with and without under slab insulation. It was amazing the amount of heat being apsorbed into the earth. They summarized that double bubble is better than nothing, but the 2" syrofoam is fantastic.

    Keep in mind Rehau does not manufacture insulation, so they have no interest in what product a contractor uses, other than wanting their product properly insulated so the customer is happy.

  9. #9
    Join Date
    Jul 2008
    Location
    Thief River Falls, MN
    Posts
    5
    mlrman,
    Thank you very much for the info. It is good to hear from someone that has seen that kind of information first hand. That is the kind of info I was looking for. I will let you know where we end up-thanks!

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