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Community and Q&A

Exterior siding and window jambs: sealant or backer rod and sealant?

gtmtx | Posted in General Questions on

We were reviewing some details in our office the other day and I noticed we had been showing backer rod and sealant between a “flanged” window and siding/trim. It get me thinking about all the articles I have seen in JLC, FHB, etc and never recall seeing the backer rod. Even Martin’s recent details in FHB don’t show it. Then I checked several window manufacturers and Andersen was the only one I found so far that says backer rod and sealant. Of course, they do say to leave 1/4″ gap, which to my knowledge is the smallest backer rod available.

So, what is everyone doing? I typically see 1/8″ gaps with sealant only against siding (fiber-cement) and stucco. With a properly installed/flashed window, it really shouldn’t matter except to provide for expansive/contraction and additional air sealing. I would expect a good quality sealant ($5+ tube) to perform with an 1/8″ gap and no backer rod. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    Here's my vote: don't install any caulk at all between the siding and the window frame or window trim. The siding is not part of your air barrier, and it's better to let this small crack drain freely and dry quickly.

  2. dankolbert | | #2

    I agree. At some point the caulk will fail, and will then trap more moisture than it keeps out. We generally try to avoid using caulk on the exterior surface of a building for precisely that reason. If you do caulk, make sure someone will be responsible for maintenance.

  3. gtmtx | | #3

    I hear what you are saying and generally agree, but there are some standards that say otherwise. Here is one I found (page 7) saying to use backer rod sealant.

    The issue we encounter on the design side say for multi-family projects, which are commercial and have the corresponding liability, is that you get someone on the the other side of the table saying we "always" do it this way and you (Architect in this example) are saying to do something that is potential not industry standard or at least common practice. And as we know, common practice can be wrong. If FHB, This Old House, etc are building the project, I am not too worried, but unfortunately that makes up a small % and is not the "standard" of care, which seems to get lower and lower each generation for everyone.


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