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Will squeezing backer rod into gap around vinyl windows warp them, and should I caulk around the exterior at all?

etting | Posted in General Questions on
The instructions for my SmartSide panel siding call for a minimum 3/16″ gap between the window frame and the trim, and then sealant applied in that gap.  The instructions for my JeldWen vinyl windows say, “For a finished look and additional protection, seal this gap on the sides with backer rod and sealant.”  Simonton Windows says, ” Seal the gap with elastomeric sealant and backer rod when necessary.”  Given the net leaning of these recommendations, I was all set to seal around my windows, but Martin’s post in 2011 makes sense to me: “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.”
What do you recommend?  Have sealants improved enough in the last 8 years to change your thinking? I’m in central Arizona with 24″ overhangs on the eaves and 18″ overhangs on the gable ends of a one-story house.
If I do apply sealant in the gaps, which are 3/4″ deep, the Dynaflex 230 instructions recommend a backer rod for any depths above 1/2″.  My gaps actually vary from 3/16″ to perhaps a bit over 1/4″ wide.  I understand from a reference at that my closed-cell backer rod should be 1/8″ larger than the gap, so I was planning to buy 3/8″ rod, but it will have to be squished pretty hard into the 3/16″ gaps.  If there any risk that compressing it in that much will warp the vinyl window frames?

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  1. etting | | #1

    I can't find any way to email GBA to let them know that I couldn't see what I was typing in the Description box when I first loaded the form, nor on the two times I edited my question, because the form is using a white font on white. I got around the first problem by typing in a Notepad sort of application and pasting it in, then I edited my question by highlighting the whole box in blue so that the white showed. If you're trying to reply to my question, these tricks may help. If you're on the GBA staff, this is my way to let you know there's something wrong. Interestingly, this Reply box and the Title box are behaving normally.

  2. Expert Member


    The cladding and trim represent your first layer of protection agains bulk water intrusion, and I don't think the problems caused by deliberately allowing water to penetrate it are outweighed by the drying potential gaps afford - especially if the wall includes a rain-screen behind. It's mute point here anyway. Sealing the gap is required by code.

    Smartside's recommendation aside, the trim is only going to expand and contract around where it is pinned by the fasteners. Leave an 1/8" gap, which doesn't need backer rod, and then caulk it.

  3. etting | | #3

    Thank you, Malcolm. The trim is all installed, so it's too late for an 1/8" gap. Do you think the backer rod will put too much pressure on the vinyl window frame if I squish it in? I know regular expanding foam, not the minimal kind for windows and doors, on the interior can, but I have no idea how the amounts of pressure compare.

  4. ohioandy | | #4

    Jeff, I wouldn't worry about backer rod distorting adjacent vinyl window frames. These gaps can vary considerably; in thinner places just tug on the backer rod as you install it, which will stretch it out. At super-wide spots, twist two together. I don't think the rod, even when compressed to the point that you have to shove it in with a poker, has enough rebound force to affect the window.

  5. Jon_R | | #5

    Don't just use any random caulk - I'd use a +100/-50% (like Dymonic 100).

  6. etting | | #6

    Thank you, Andy and Jon.

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