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Community and Q&A

Parking a Car on an Insulated Slab

Trevor Lambert | Posted in General Questions on

I’m in the early stages of planning a multipurpose outbuilding. This will include a fully conditioned workshop, storm shelter, animal barn, storage, solar collection/distribution and garage. Maybe a cold room as well.  I will probably have the garage portion partially conditioned, that is to say it will be conditioned but not as strictly as the rest of the building. I will do maintenance on cars, tractors, etc. in there, so sometimes it will be brought to room temperature, but most of the time it will just be maintained in the 10-25degC range. The animal barn part will be a lean-to, getting some heating from the main building but not sharing ventilation air.

For the purposes of conditioning, an insulated slab is obviously best. I’m biased toward a floating slab on an EPS raft, due to my experience with it in my house. Would that be a bad idea for the garage portion? Should I look at having a separate slab for the garage section? Can a slab on EPS or mineral wool stand up to the weight of a typical car, point loads of about 1000lbs?

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Replies

  1. GBA Editor
    Kiley Jacques | | #1

    Hi Trevor,

    Martin Holladay talks about the compressive strength of mineral wool insulation for a concrete slab in this article: Sub-Slab Mineral Wool. It might be useful information to consider.

    1. Drew Baden | | #4

      I am doing something similar in my pole barn where I ’ve put insulation only under the loft area. You can see my progress at 4:40 into the video. https://youtu.be/sVXPemTDM7I I doubt you’d have a problem parking a car on it with 4-5 inches of reinforced concrete over your insulation. Just don’t park a loaded concrete truck on it! Let us know what you decide. We’re all learning here.

  2. Trevor Lambert | | #2

    I'm starting to get a complex because the titles of my questions keep getting changed. I don't mind a little editorializing, but in this case it actually obfuscates the main point of my query. I'm not so much interested in comparing EPS to mineral wool under a slab, but any kind of insulation vs none; i.e. whether it's feasible or reasonable to drive a car on a slab on insulation.

    1. GBA Editor
      Kiley Jacques | | #6

      Hi Trevor,

      The reason we change titles is to make them straightforward for online searches. They come up more often if they are simple and have key words. I apologize if I obfuscated your meaning. Always feel free to shoot me a note if you feel that way, and we can settle on a title together.

      1. Sam S | | #9

        It's probably more reasonable for the editors to reach out if they want to change a title, rather than requiring question writers to notice their posts have been changed and then figure out how to reach out.

        Alternatively, if an editor changes a post, they should include a note to that effect. It's a bit unethical to change what someone has written without notifying them, and leaving no trace of the edit.

        For example, the title on this question topic has changed again. Did Trevor change it? Did you change it? All I know is that Trevor's name is still listed next to it, implying he wrote every word.

    2. DCContrarian | | #11

      Two complaints about this practice, as a reader:
      1. It makes it hard to follow a thread.
      2. When the title is changed the thread is marked as updated and pushes up in the list of recent topics. This makes it hard to see which threads are recently updated.

      Is there any thought given to updating the forum software? I love the content but there are better ways of presenting it.

  3. Expert Member
    Akos | | #3

    You can run your setup through the calculator here:

    https://www.buildingsguide.com/calculators/structural/FCSGSG/

    Most like you'll be fine with a standard thickness with some rebar, but I would go for higher PSI foam.

  4. Roger Berry | | #5

    Trevor,

    My garage slab is 5" poured over 3" reclaimed XPS that I believe to be 25 psi stock. I had the concrete contractor add polyester reinforcement fiber for belt and suspenders reason. My tractor is only 5500 with implements on it, so not much more than the wife's car. Three sides are floating with edge insulation. The slab to apron joint (also foam isolated) is the only solid contact point between slab and foundation. There are bent rebars that tie into the slab at each door opening to secure any movement. I have one small stress crack by one door opening. It is other wise faultless over the rest of the slab.

    If you go with rebar and possibly fiber as an additional control you shouldn't experience any noticeable issues. I would worry more about being very attentive to compacting the soil under the foam if you are disturbing things a lot. I had a grade change to fix both sides of the garage perimeter as well as the footing excavation. I believe the soil under the garage slab has remained stable, I can't say as much about the compacting done is a few areas outside the foundation. A few places settled almost 3" over the course of two years despite being compacted in lifts. These were the deepest fill areas though.

    If you have really big equipment and are concerned more about movement than heat loss, maybe skipping the insulation would be warranted. It sounds like you aren't dealing with a combine though.

  5. Keith Gustafson | | #7

    Unless by tractor you mean combine harvester, there is no problem.

    I had a full floating slab with wire on 2 inches of foam. Only hairline crack on an inside corner.

    Not over thick
    No extra reinforcement

    One tends to think of foam as weak because it dents if you step on it, but so does sand

  6. Expert Member
    Zephyr7 | | #8

    The concrete is going to spread out those point loads from your car over a much larger area. I'd put remesh into the slab, and some rerod too, and if you're really worried, go up to the commerical 6" thickness. MAKE SURE your reinforcement is placed correctly near the center of the slab, not pushed down during the pour to near the bottom (happens all the time when contractors are too lazy to put in chairs or other supports for the reinforcement).

    If you look at highway overpasses, you'll see that they use the concrete road bed to span between support girders, usually for several feet per span. This is just thick reinforced concrete. Your slab is no different, if designed correctly it can spread the load out over a large enough area that the insulating foam will be fine. Remember that even 25PSI rated foam is good for 3,600 pounds per square foot! Square law is your friend here.

    I would use 25 PSI foam here though, not the 10 PSI stuff.

    Bill

  7. Maximilian T | | #10

    +1 on a note or something when the titles of posts change. Recently had an experience where I couldn’t find an old post of mine (and wasn’t getting email notifications when there were new replies!) due to this... editors here do a great job but not a fan of this practice.

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