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To Caulk or Not to Caulk? Siding to Window Trim Intersection

Patrick_OSullivan | Posted in General Questions on

New Jersey. Climate zone 4A. Zip R sheathing as air and water control layer. 3/8″ Rain screen over Zip.

Hello! Using LP SmartSide on my home and, quite honestly, don’t want to caulk it where it intersects window trim. I have a rain screen and a robust water control layer below that. Therefore, I’d rather not install something that will just degrade/fail in the future. Cut ends of SmartSide will be painted.

My feeling is that, particularly because of the rain screen, this joint will see plenty of drying and caulk might actually be a hindrance, rather than a help, over time.

Disclaimer: Manufacturer may recommend caulk, but I suspect that’s because 99% of installs are not over a rain screen and they are trying to prevent trapped water against cut edges. My theory is that this is less of a concern with a rain screen. I’d rather do the ‘right’ thing than the theoretically warrantied thing.

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  1. paul_iowa | | #1

    If you were to go this route you'd definitely want to make sure you're painting the back sides several inches in. The backside is raw OSB or fiber product, depending on what LP line you're using. And one thing to note with LP is the the degree of expansion. Your uniform gap next to openings and corners will expand and shrink. I've used smooth LP lap on several projects and haven't had any issues at the ends with failed caulks. LP specs a C920 caulk. Best of luck.

  2. Expert Member
    Michael Maines | | #2

    I would contact their technical service department. I have had several situations where they will allow a non-standard installation if they understand the bigger picture. I doubt they would go for it in your case, but it's worth asking.

  3. Expert Member
    Akos | | #3

    My approach to caulking joints is that if it prevent water from getting in, but doesn't prevent water from getting out, then I caulk.

    For example, side of trim can be caulked. Any water running down behind the siding can continue to run down.

    Bottom/top trim. Not caulked, or caulked with drainage gaps. Sealant there can get in the way of water draining out.

    Also when caulking lap siding only do the top surface, never around the bottom to the next row.

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