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Community and Q&A

Sealing the sill

user-2895420 | Posted in General Questions on

Our “final home” foundation has been poured and we are thinking ahead to when we air and water seal the Zip-R sheathing over the sill to the foundation. Our plan was to use Zip tape for this application, but unfortunately, the concrete forms that were used were brick pattern forms (as that’s all the subcontractor had). The brick pattern surface is covered with ridges and fake mortar lines, which will make air sealing difficult.

Are we best off trying to stick the Zip tape to the concrete the best we can, and then following up with a liquid flashing product to seal any remaining voids? Or would we be better off exclusively using a liquid flashing product (and no tape)?

Second, there are several kinds and thicknesses of EPDM to choose from to use for a sill seal. Plain black and white faced, as well as a variety of thicknesses. Does it matter which EPDM we use as a sill seal?

Thanks in advance!

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  1. Expert Member
    RICHARD EVANS | | #1


    This appears to be a common problem for high performance builders. I would totally opt for a liquid flash as few tapes seem to stick to concrete well. For ultimate durability, you could opt for the liquid flash that requires mesh to bridge the gap.

    If you opt for zip flashing, Huber recommends that you do not put the their liquid flash over their tape unless you use an "approved primer". You can put zip tape over zip liquid flash however.

    I don't have any first hand experience with the EPDM gaskets but these seem to be popular:

  2. GBA Editor
    Martin Holladay | | #2

    To air seal the crack between the foundation and the pressure-treated mudsill, I would use an EPDM gasket from Conservation Technology, or ordinary sill seal (flexible foam) plus caulk.

    To air seal the crack between the Zip sheathing and the pressure-treated mudsill, I would use acoustical sealant, caulk, or construction adhesive (applied to the mudsill before the Zip sheathing is nailed to the framing).

    This approach avoids the problem of trying to adhere tape to the brick pattern forms.

    1. AC200 | | #8

      That's a practical and cost effective solution. Any concern about the exposed edge wicking up water?

  3. Stockwell | | #3

    Note that Nancy is using Zip-R. If you follow Martin's advice about sealing to the sill, make sure whatever product you use is compatible with the foam in the Zip-R. Some just eat right through it. I am going to be using Prosoco Joint and Seam filler for this purpose on my own build. Once again, you would have to check on compatibility. If you use tape, use a primer made for the tape. A number of the high performance tapes have approved primers available that are used on things like OSB or rough concrete. It's a small world Nancy--I think you are building your house one street over from where I am renting!

  4. CarsonZone5B | | #4

    I had the same question. Huber's instructions seem to make no mention of air sealing between the sheathing and foundation. To make it worse for ZipR, the sheathing is often inches out of plane with the foundation and zip liquid flash does not list foam as a suitable substrate, so it's not clear that sealing the bottom of the foam to concrete is acceptable. My plan is to seal metal flashing along the bottom of the foam to the concrete, with a few small gaps of foam between in some places that will hopefully not matter if I liquid flash over.

    1. Expert Member
      MALCOLM TAYLOR | | #6


      With either exterior foam or Zip-R it's a lot easier transition if you set the wall framing in on the foundation so the outsides remain (relatively) in the same plane. The bottom can then be protected by the sill-gasket and you only have to deal with the transition on the outside.

  5. bigred | | #7

    What I did on my house was to wrap the bottom of each Zip-r panel with zip tape. Huber told me that all I needed to do was to carefully wipe the zip tape with DNA or acetone and then the liquid flash would easily stick to the tape. So after wiping the tape down, I liquid flashed the zip panel to the slab, taking the liquid flash up on the panel, down under the panel and onto the slab making a continuous barrier.

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