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Community and Q&A

Zip liquid flash ONLY for foundation to sill plate

blamus20 | Posted in Green Products and Materials on

I have searched and read many threads on the discussion of using sill seal/owen foam/EPDM gaskets + some type of sealant/liquid flashing between foundation and the sill plate. But I can’t find a discussion on skipping the sill seal equivalent material and JUST using a liquid flash, which would obviously achieve the air sealing. 

My contractor wants to use zip liquid flash only and skip the gasket. Is that a good idea? or do I need both? 

He said he can either just apply liquid flash to the outside connection after the plate is on, or put the liquid flash between the plate and the concrete (and presumably also liquid flash the outside after). Can liquid flash replace the “solid” gasket used in this way?

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  1. brian_wiley | | #1

    Hi, Bernard. While I believe that you’d be okay from an air sealing perspective, there is the benefit of the gasket acting as a capillary break between the concrete and the sill plate.

    Here’s an article that discusses that in more detail:

  2. blamus20 | | #2

    Thanks Brian, needing a capillary break is fair enough. If that's the case, would coating the top of the concrete with liquid flash before putting the sill plates down be recommended? I.e. using liquid flash as the sill seal, and the liquid flash would therefore be working as the capillary break as well as the air tight seal. I wonder if the weight of the building would compromise the effectiveness of the liquid flash?

    1. AJ__ | | #3

      Sill gasket is far cheaper and faster to apply

    2. Patrick_OSullivan | | #5

      Since we're talking about applying something to concrete to provide a capillary break, look at documentation about capillary breaks on footings:

      However, the top of a foundation can be quite irregular. Foam sill seal helps conform to these irregularities. You'd potentially need *a lot* of liquid flash on top of foundation such that embedding the sill in the liquid flash would provide both a consistent capillary break and air barrier.

      I don't understand why someone wouldn't use foam sill seal + sealant, or one of the more purpose built EPDM products.

      For what it's worth, my foundation has foam sill seal for a capillary break and then a bead (fillet) of liquid flash between the foundation and a piece of Boral that protects the bottom edge of my Zip R-sheathing.

  3. brian_wiley | | #4

    I’d be hard pressed to say with certainty if zip liquid flash would work in that capacity. Common sense would suggest that it would, but it seems like a bit of an untested gamble, and a particularly expensive one at that when you compare it to the cost of a doubled-up layer of Owens Corning sill gasket and some acoustical sealant.

    I’m not quite sure why your contractor is so adverse to a sill gasket, but if it were me, I’d keep the liquid seal for the zip-to-concrete transition where it’s been tested.

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