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Dudley box question

Boring19 | Posted in General Questions on

I’m installing Non-flanged vinyl windows in Dudley boxes and have a few questions.
I plan on using DuPont straight flash VF around the window in all four outer surfaces of the windows and then to the face of the box to seal off water .
The windows would be installed flush with the face of the nailing flange of the Dudley box.
Does this sound ok or is there a better way?
Window trim to be installed over the window by a 1/4 inch and then caulked
I also have a question about caulking the Dudley box, the article doesn’t show a good shot of the bottom of the box after caulk when being installing.
I would think the bottom of the box should be left free of caulk to allow for drainage except a little around the corners. I would also not tape of the bottom face of the box as well.
Thank you.


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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Gregory,

    One assumption that I'll make with your installation is that you are using exterior rigid foam insulation, the reason for the Dudley Boxes, as your water-resistive barrier. (Otherwise, you'll have to integrate your WRB into the Dudley Box flashing details.)

    You can seal the Dudley box flange to the foam with caulk on all four sides. Install peel-and-stick flashing tape inside the bottom of the box and lap it up the sides. Then install the window. Since it is not flanged, lap peel-and-stick flashing onto the face of the window. Use a J-roller to make sure you get a strong bond. Side pieces first, then the head flashing.

    Most builders do not tape the bottom of the window, but that only helps if you have installed the window on a pitched sill--for example, a piece of beveled siding, that will drain water. Or if you have a back dam, I guess. But a pitch seems necessary to me, to avoid standing water.

    Whether you use one wide strip of tape or two layers, bring the peel-and-stick flashing all the way from the window to the foam.

    I assume that you have seen these articles. If not, they will be helpful:

  2. Boring19 | | #2

    Thank you for the reply Brian, Both of the articles are great, there the reason I decided to take this retrofit on.
    Yes the boxes will be installed into 2 inches of rigid foam.
    This would be my process.
    So three beads of caulk on the back side of all four nailing flange to seal to foam.
    Inside the box I install peel and stick across the bottom and up the sides about 6 inches.
    I then Install the window, flashing the window face making sure to prep the window and assure the flashing is rolled, and make sure the trim will be covered by the flashing.

    I was thinking of flashing the window at the top, bottom, and sides since the box is flat. The installation-tips-for-a-deep-energy-retrofit article appears to me that all four areas of the window being taped, the bottom is only taped from the window to the face board and the face board and doesn't appear to be flashed to the foam. Forgive me if I'm overthinking this, just want to do a quality job. Please make suggestions or corrections as you feel the need.
    Thank you Brian

    1. GBA Editor
      Brian Pontolilo | | #3

      Unless I missed something, I think that is an inconsistency in that article. He says he lays a bead of caulk on all four sides of the box's flange, so I am not sure why he doesn't tape the bottom of the box's flange to the foam. And if he was going to, it should have been the first piece of taped he installed---for a proper lapping sequence. I don't feel entirely confident on this last details and perhaps you'll get some other thoughts, but if you won't have a pitched sill or back dam beneath the window, then I guess you probably should tape the bottom of the window to the Dudley Box to keep water from getting in there.

  3. Boring19 | | #4

    Sorry for the late reply.
    I decided to use a pitched sill and Prosoco liquid flashing .
    I'm going to install the box in the foam, seal the inside corners and the box edges to the foam with joint and seam filler followed by a layer of liquid fast flash over the complete box and onto the foam.
    I will install the window with a 1/2 inch of the face of the window being proud of the box, better for caulking the trim after install.
    I will caulk with joint and seam filler on the sides and top but leave the bottom unflashed.
    Thanks again!

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