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Basement slab minimum insulation and best bathroom subfloor

patrick1 | Posted in General Questions on

I’m remodelling the basement of my Ottawa, Ontario house and trying to choose the best insulation approach.  I’ve read the “No-mold finished basement” and “What’s the Best Basement Flooring System” articles.

Our plan is to insulate the walls with 2″ of spray-foam.   The floor is more of a challenge since head height is a significant constraint for us.  For the floor, I was hoping to get away with a layer of Delta-FL, followed by 1/2″ of XPS foam, then 7/16″ OSB.   Will the 1/2″ of foam be enough to prevent condensation on the floor during the summer?

I’m also at a loss as far as what to do for the basement bathroom subfloor.  We definitely want to use tile.  I’ve read on various forums that OSB over foam is not rigid enough to prevent tile cracking, but pre-made panel manufacturers (e.g., Dricore R+, Amdry subloor, barricade) say that you can tile over their products as long as they’re tapconned to the slab.  I would like to have some insulation under the tile if possible so the floor is not so cold, but I also don’t want to have to redo it because of cracking.

Hoping for some suggestions here.

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  1. Expert Member

    For advice on all things tile related, especially substrates, I'd ask over at John Bridges:

    1. patrick1 | | #3

      Thanks for the pointer. Lots of good advice. General consensus seems to be going with a manufacturer approved solution such as Wedi board.

  2. GBA Editor
    Martin Holladay | | #2

    Here is a link to a previous Q&A thread:
    "Ceramic tiles over rigid foam."

    If you Google, you'll find various Q&A forums where builders report success with either Kerdi or Wedi systems -- but you need to get an OK from Kerdi or Wedi.

    Here is a link to a web forum:

    On that page, a reader points out that Schlüter-Kerdi-Board in the U.K. used to recommend their products for floors -- I'll cut an paste the info below:

    Use of Schlüter-KERDI-BOARD in floor areas

    To install Schlüter-KERDI-BOARD in 5, 9, 12.5 and 19 mm thicknesses the substrate must be level, ready to bear weight and free of all substances that may weaken the bond. Any levelling must be completed prior to installing Schlüter-KERDI-BOARD.
    The adhesive must bond well with the substrate and mechanically set in the anchoring fleece of Schlüter-KERDI-BOARD. Standard dry set mortars as defined by EN 12004 are suitable for most substrates. Otherwise, select another suitable adhesive, carefully checking for any incompatibilities of materials.
    Apply thin-bed mortar over the substrate with a notched trowel and fully embed the anchoring fleece of the Schlüter-KERDI-BOARD panels in the adhesive bed. Tightly abut the individual panels and align them appropriately. Observe the curing times of all materials.
    To install Schlüter-KERDI-BOARD in thicknesses of 28 mm or more, the substrate must be sufficiently load bearing. Over such substrates, Schlüter-KERDI-BOARD can be installed with dabs of thin-bed mortar or other suitable mortar, followed by height alignment. To ensure proper weight bearing of the covering, place the dabs of mortar closely together. Apply thin-bed mortar to the joints at the edges of the panels and ensure they are supported by dabs of mortar from below.
    Use an edge strip to rule out the buildup of tensions in Schlüter-KERDI-BOARD and the tile covering in edge areas.
    The tiles (minimum size: 5 x 5 cm) can be installed immediately after fully embedding the Schlüter-KERDI-BOARD panels, using a dry setting mortar that meets the requirements of the covering. If using dabs of mortar, allow them to cure first for full weight bearing of the substrate. Choose a notched trowel to match the tile format. The curing times of the thin-bed mortar must be observed.
    Observe the applicable technical standards for movement joints used in field definition and edge and connection joints.

    1. patrick1 | | #4

      Thanks for the reference Martin. I managed to find a couple of PDFs giving Schulter's detail for use of Kerdi board on the floor. See attachments. Unfortunately they no longer seem to approve this approach. I contacted Schluter Canada and they said the use of Kerdi board on the floor is untested and would not have a warranty. So looks like I'll have to go with Wedi. It's too bad because Kerdi board is way easier to get around here and Schluter seems to have a more comprehensive set of products and instructional resources.

      What are your thoughts about the minimum foam thickness for the rest of the basement? Dana Dorsett provided some good advice for the question here:

      But his advice was for climate zone 5 and Ottawa is in climate zone 6 I think. Not sure what our subsoil temperatures would be. Also not quite sure why the focus is always on condensation during the summer. Couldn't condensation occur in the winter as well? Is it because there's not enough humidity in the air during the winter to raise the dewpoint enough to create condensation?

  3. cingwersen | | #5

    We use both products on a weekly basis. Side by side it is clear why Wedi is much better suited for being used on the floor as it is much stiffer. Kerdi board dents very easily.

    We have also had good luck using Ditra membrane (Ditra, Ditra XL or Ditra Heat) overtop of Dricore which has been tapconned or hilti'd to the concrete.

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