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Slab on grade questions

Daniel G. | Posted in Energy Efficiency and Durability on

I am looking into purchasing a home built in 1973 and want to make sure I do my homework prior to taking the plunge. The garage and den are both at ground level on a slab. The rest of the first floor is built above a crawlspace. The den currently has carpet but I would want to replace it with laminate or wood flooring. Do I need to be concerned about moisture coming through the slab? Is there a way to tell if the slab has a vapor barrier or insulation underneath?

If I add a vapor barrier (and possibly insulation) above the slab, where does the moisture go? It seems like it would move to the walls instead.

If part of the crawlspace is adjacent to the den and garage, does that present any problems with encapsulating the crawlspace in the future?

Thanks for any help you can provide.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Daniel,
    Q. "Do I need to be concerned about moisture coming through the slab?"

    A. Yes.

    Q. "Is there a way to tell if the slab has a vapor barrier or insulation underneath?"

    A. Yes. You can drill a hole through the slab in an inconspicuous spot.

    Q. "If I add a vapor barrier (and possibly insulation) above the slab, where does the moisture go?"

    A. The moisture stays where it belongs -- in the soil. In general, soil is damp, and the interior of your house should be dry. You want to keep it that way; that's why people install a vapor barrier between the indoors and the soil below.

    Q. "If part of the crawlspace is adjacent to the den and garage, does that present any problems with encapsulating the crawlspace in the future?"

    A. Not really. For more information on this topic, see Building an Unvented Crawl Space.

  2. Daniel G. | | #2

    So do I need to insulate over the slab if there is nothing underneath (I am in Zone 3A)? Would the order go slab --> vapor barrier --> insulation --> floor (with sleepers or plywood between insulation and floor)?

    Will a vapor barrier on top of the concrete cause the slab to stay wet? Will this cause any problems? Would leaving the insulation out help warm and dry the concrete?

    I just want to make sure I do whatever I need to do while the old floor is up and don't kick myself afterward.

    Thanks

  3. User avatar GBA Editor
    Martin Holladay | | #3

    Daniel,
    Yes, you have the order of all the layers correct. The usual method calls for attaching a layer of 3/4-inch plywood on top of the rigid foam by fastening the plywood through the foam to the concrete below with TapCon screws.

    Don't worry if the concrete is damp. The reason you are installing the vapor barrier and rigid foam is to separate your interior layers from the concrete.

    For more information, see:

    Fine Homebuilding: The Stay-Dry, No-Mold Finished Basement

    Fine Homebuilding Q&A: Finishing a basement floor

    GBA Q&A: Basement floor insulation retrofit

    GBA Q&A: Floating plywood floor

    Green Basement Renovation

    The High Cost of Deep-Energy Retrofits

    Fixing a Wet Basement

  4. Daniel G. | | #4

    Is there a preferred way to transition between the new thicker floor and the doorways to the existing lower slab floor in the garage and screened-in porch? Would a reducer threshold suffice? Between the insulation and the flooring it seems like it would be sitting 1.5" above the doorway.

    From some of the comments in the above links it seems that a floating laminate (or plywood) floor could be installed directly over the foam without having to use TapCon screws. Are there any drawbacks to this method versus securing an additional layer of plywood using TapCon screws?

    Thanks

  5. Mark Fredericks | | #5

    Martin, thanks for the links you provided!
    I see that The Stay Dry No-Mold Finished Basement by Andy Engel suggests a vapor open assembly for the walls and slab, including the choice of flooring. This is confusing when we hear elsewhere to seal the slab with a VB before laying insulation and subfloor. Can you comment on how we should choose between these 2 approaches? (vapor open/closed)

  6. User avatar GBA Editor
    Martin Holladay | | #6

    Mark,
    Andy Engel is wrong about the need for vapor-permeable assemblies. The polyethylene is installed to stop vapor diffusion.

    You absolutely do not want to encourage vapor diffusion through your slab. Why invite soil moisture into your house?

  7. Mark Fredericks | | #7

    Thanks Martin, I just needed to hear that it isn't a wise strategy.

  8. Daniel G. | | #8

    Can rigid mineral wool insulation be used under the subfloor rather than foam?

  9. Nate G | | #9

    No. It's too squishy and it's not a vapor barrier.

  10. User avatar GBA Editor
    Martin Holladay | | #10

    Daniel,
    Some green builders are now using the densest available panels of mineral wool as sub-slab insulation. (You still need a polyethylene vapor barrier, of course.) This method is experimental, and is still not supported by mineral wool manufacturers.

    I recently spoke to a Roxul representative who said that Roxul is conducting its own tests to gather data on this approach. Eventually the company hopes to be able to issue guidelines on the method, but for now they are silent.

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