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Filling Gap Between Wall Sheathing and Concrete Foundation Before Liquid Flashing

Nat . | Posted in General Questions on

See attached sketch for clarity.

I’ve got zip-r. I plan on liquid flashing the zip to my concrete basement wall. The way things have worked out in the field the zip-r often sits up to 1/2″ above the concrete. I don’t want to try and bridge that gap with the liquid flash, I want to fill it to a reasonable depth first. The joint is a bit deep (1.5″) to fill.

I’m considering:
– Put 2 layers of 5/8 backer rod in there
– 2 layers of 5/8 backer rod plus a bead of polyu caulk to help hold it in place and form a better profile for the liquid flash to ride on
– Canned spray foam to fill the gap and liquid flash over that.

Any better ideas or opinion on the above?

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  1. GBA Editor
    Kiley Jacques | | #1

    Because this is a vulnerable transition with dissimilar materials prone to movement, most high-performance home builders use a belt-and-suspenders approach. Your idea of a double layer of 5/8 backer rod and a bead of polyu caulk followed by liquid flashing is reasonable. For redundancy, consider applying canned spray foam to the interior of the sheathing where the framing attaches to the ZIP. (Note that you will want to make sure the materials you choose are compatible.)

  2. Nat . | | #2

    Is the reason to not use the spray foam detail because of it's inability to expand/contract? I'd have the liquid flash over it which is my air seal so really it's about filling the gap and providing a substrate for the liquid flash. The spray foam would be a lot easier/faster.

  3. Andrew C | | #3

    There is an article at JLC this month (Jul/Aug 2020) that does close to what you are proposing: backer rod first, then caulk to get a better bead shape. They applied and tooled PolyWall Joint Filler 2200, which was part of their overall vapor permeable liquid WRB. IE, they used
    "...the Blue Barrier system to the walls. There are three components to Poly Wall’s system: Joint Filler 2200, a thick gun-applied structural adhesive and detail sealant; Flash ‘N Wrap 2400, a thick trowel-applied flashing for windows and doors; and Liquid Wrap 2300, a roller-applied or spray-applied covering that serves as a vapor-permeable membrane that protects against air leakage and bulk water. All three formulations are based on the same chemistry: STPE (silyl terminated polyether)." This is one approach to making sure all the materials are compatible.

    Personally, I'd avoid the spray foam on the outside in this case..

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