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Foundation to sill sealing

jmidway22 | Posted in General Questions on

Great articles and resources on the foundation to mudsill / sheathing details. I have started trying to incorporate those into my build or at least the principles.

I am running into a question of how to cover the exterior peel/stick or even fluid applied product that transitions from the sheathing down onto the concrete foundation? I want to run down atleast 1-2″ onto my foundation covering the crack under the mudsill but with the hardie spec, they stop the starter strip at the bottom of the osb sheathing or bottom of mudsill. So that would leave my peal and stick exposed the 1-2″ below the bottom course of hardie lap siding.

My ideas were to either use flashing to extend down over that and cover (like a simple drip edge at the bottom) or to somehow lower the base course of lap siding to hang lower than the peal/stick. But with that solution I would be afraid of unsupported lap siding that hung too far unsupported and could be more easily broken.

Here is the general detail that I am talking about except I am adding the peal and stick. Sure there is a great solution I am naive about.

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  1. Expert Member
    RICHARD EVANS | | #1


    There a lot of options here:

    Have you considered installing the Hardie siding on vertical 1x3 strapping? There are many benefits to having a rainscreen gap behind your siding. You can run the 1x3s long and over over sill tape/liquid flash.

    Another option would be to tape the sheathing to the sill before standing the walls and then tape the sill to the foundation on the inside. You can also use sealant or a gasket or both to seal the bottomed plate to the foundation.

    One final thought: you might get away with covering the tape with synthetic stucco. However, I've learned from personal experience that it doesn't hold well to tapes with a smooth surface. If you go this route, use a 'fuzzy' tape like Siga Fentrim.

    And a final, final thought... If you are in a cooler climate, consider exterior insulation on your foundation wall.

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Josh.

    As Rick pointed out, there are a lot of options. If you will be insulating the foundation wall on the exterior, the insulation will cover the tape or fluid-flashing and you'll need a finish to cover the insulation. For slab-on-grade, some builders use the sub-slab poly, run under the plate and taped to the sheathing as the air seal.

    Another thing to note, based on the drawing you posted, I've been seeing more and more builders who seal the sheathing to the foundation wall setting the wall back so that the sheathing and foundation wall are flush. This coplanar joint will be easier to tape or use a fluid-applied flashing on.

    We could probably be more helpful if we knew more about your specific assembly.

    1. jmidway22 | | #3

      Thanks for the ideas guys, many of those are great additional options I would want for my house or a more expensive build, like the exterior insulation over foundation and wall assembly or a rain screen but those are not always in the cards for a project or budget. They would allow me to avoid this problem but are a major scope creep. We are zone 4 so some of the insulation ideas are more nice to have than necessity also.

      The detail I showed (actually hardie drawing) is almost identical to the typical build I work with and is standard in the market, sometimes the sheathing doesn't protrude the foundation/mudsill but either way a peel and stick or fluid applied can handle those situations. Of course sill gaskets are applied and usually I am including a 2 beads of caulk with another along the interior at the mudsill/foundation joint.

      But I am just trying to address the situation of this flashing being exposed when bridging the sheathing/wrb down to the foundation given the example detail. Seems anyone installing hardie siding with a typical wall assembly would be dealing with this too if they decided to go down the exterior flashing approach on the mudsill. Good number of articles detailing this type of flashing application to this wall assembly but not how to cover that afterwards, maybe they aren't using lap siding. One other note, I could use trim band board to cover the flashing but frequently that trim is not wanted aesthetically.

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