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Community and Q&A

Insulation Under a Heated Slab

74VQqN7vqw | Posted in General Questions on

I live in Sterling Alaska on the kenai penninsula. The lot I am building on has 1 to 1 1/2 feet of top soil then I am into a good gravel base that is at least 10 to 15 feet deep. Some tell me the gravel base is a good heat sink and when heated would support keeping the slab warm others tell me to insulate under the slab so of coarse I want to do what is best. I have also read where some insulate except for an area in the center where they don’t insulate? Very confusing when trying to understand the best option. According to my other posting everyone is telling me to insulate. What type insulation, what thickness and should I leave an opening in the center of the slab. The Square footage of the slab is roughly 1800 sg/ft.

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  1. GBA Editor
    Martin Holladay | | #1

    Your question is usually answered by energy modeling. If you don't want to do your own modeling, you'll have to hire a consultant to model your house.

    The last time you asked, several GBA readers provided some advice. I don't think that if you ask the question a second time, you'll get a different answer.

    The last time you asked, I said you need a continuous layer of horizontal insulation under your slab. That means "no hole in the middle of the house."

    The last time you asked, I said I wouldn't advise less than 6 inches of foam under a heated slab of a house in Alaska. You might see a trend here -- my advice hasn't changed.

  2. davidmeiland | | #2

    To which I would add... use XPS foam insulation. I would probably use 2 layers of 4" with staggered seams. Same thing vertically at the perimeter.

    I can't imagine any reason for a hole in the center.

  3. 5C8rvfuWev | | #3

    William, I guess you don't like the answers you're getting in the other thread. The only way you can change the advice would be to change what you mean by the word "need" ... as in "Do I NEED subslab insulation ...." No, I guess you don't. Not even in Alaska, if you're very rich (to pay the price for energy that you waste as its price goes up and up and ...) or don't mind getting very very cold (like people in tents in Antarctica). The answers you've been getting relate to "best practice" and it's all anyone can answer who is informed.

    I sympathize with your confustion. Notice my Zone (in my sig.). Here in Georgia I'm continually met by builders who say I don't need Insulation in a crawlspace or under a slab. Or more than R11 in the walls. Or more than batts in the attic. "This is the south," they say. "Y'all don't need much insulation, but we put it in for people." (Let me point out that I forget all about construction folk who think they can ignore code as easily as they can ignore building science!)

    You should know that even in Zone 3A I'm intending to use subslab insulation (R-10 perimeter and exterior, R-5 in the center). I'm also looking to build R30 walls and R50 attic.

    Why? Because 10-20 years from now I'm going to NEED energy costs I can manage to pay. For me -- even if you ignore the environment and all building science has to offer -- it's that simple. A contractor says gravel and dirt are good enough; experts tell you he's wrong and should be dumped. Yes, you have to make a decision. If you have the money, it should be easy. If you don't, my suggestion would be to wait, save what you need before you do something less than you "need."

    Best wishes,

  4. jklingel | | #4

    William: Pay heed, bro. Sterling ain't no hot springs. Insulate like a mad dog. P, EOS. I'd never use less than 6" down there. That is MHO. "The earth is a good heat sink...." Yes, it is. It will take all the heat you can sink in there, and send it right on down to China. PS: EPS seems to be getting much more attention than XPS. Check w/ CCHRC in Frb, and I bet they lean toward EPS below grade. That is what I have heard from there. Good luck, insulate well, and call me when the kings are in; we'll do fish, eh?

  5. user-869687 | | #5

    Here's another vote for EPS. XPS is in the doghouse until they resolve the blowing gas impacts.

    John K, you have stumped me with your abbreviations.

    P, EOS.

  6. 74VQqN7vqw | | #6


    I am listening to your advise and I do appreciate it. The reason for the second question series was in response to one of the folks kind enough to answer my question that wanted a little more information about my location here in alaska. Certainly didn't mean to bring out the impatience and negative feedback from some. Thanks, I will insulate double layer, and I will look into the EPS. Thanks, great site and I do appreciate and respect your feedback. As far as fishing goes your on I am at Real Alaskan Cabin and RV

  7. jklingel | | #7

    TJ: "Period. End of Story." That is my little creation from long ago; maybe a bit esoteric? Sorry about that. j

  8. Perry525 | | #8

    The way to go is, install a strip

    foundation and build to at least ground

    level, then infill with gravel, level,

    install at least 8 inches of tightly butted

    polyurethane foam sheet, cover with

    plastic membrane taking membrane to

    the outside of the base, lay steel mesh

    reinforcement, then lay out and tie pex

    under floor heating to reinforcement,

    doubling up round the perimeter, raise

    perimeter wall by 4 inches, add a two

    inch thick upstand round the inside of

    the wall of polystyrene to stop the heat

    from the concrete slab leaking

    sideways, infill with 4 inches of

    concrete to make working base.
    This will raise the home high enough to

    stop rising damp and avoid water

    ingress and you will slow the downward

    and outward heat loss to an

    immeasurable level, much reducing the

    cost of spent of

    insulation will be recovered in a couple

    of years.

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