The window instructions say to put the fins on the outside of the sheathing, but if a combined sheathing/siding product is being used (LP SmartSide 38 - model 27874), should the fins go on the outside or inside of it?

Instructions for the window are at https://images.thdstatic.com/catalog/pdfImages/9b/9bb798d8-fee7-40ac-8f82-ba091e2511bd.pdf if helpful for reference.

Looking at the second to last diagram on page 5 of https://images.thdstatic.com/catalog/pdfImages/0b/0b06104c-ffa0-4bc8-ae91-c6a4b0c34eae.pdf it seems like it should be under the LP SmartSide but I wanted to confirm as I've never installed windows before.

Also, trim is planned to be put around the window if that matters.

  • 1
    From that diagram on pg 5, it looks like it should be under the Smart Side (I have no experience with that product). If on the outside of the Smart Side - that creates complications in flashing and the final appearance of the siding, at least IMM.
    – SteveSh
    Dec 21, 2022 at 16:16

1 Answer 1


According to the Smart side instructions, the window nailing fins are nailed to the framing members. Then the Water barrier if used. Then the window is flashed and properly caulked where needed. Then the trim and on to the finished product, ( In your case the Smart Side ). For proper nailing you have to be sure you have the framing around the window opening spaced properly so as to have a nailing surface for the end of the board. This is dependent on your trim size.

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