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4″ of Roxul Comfortboard with open-joint rainscreen and metal flashing details

lherbert | Posted in Energy Efficiency and Durability on

I’m midway through a project that is partially clad with Standing seam metal roofing, and partially clad with open joint siding. The windows are plywood bucks with sealed units installed directly into them (no manufactured frames), which I’m nervous about the details of, but the client got a great price on some triple paned units, so I intend on sealing the bucks directly to the WRB with tapes, and using other standard flashing details, including setting blocks and glazing tapes to position the glass in the unit. I think I mostly figured that portion out.

But my question is:

#1) How do I transition the Open Joint UV resistant WRB which is installed on the exterior of the 4″ or Roxul Comfortboard (directly underneath under the open joint siding) to the tyvek WRB which is supposed to be underneath the Roxul for the metal clad sections of the house? Do I just bring the tyvek up through a crack in the roxul, and use an appropriate tape to join it with the UV resistant WRB?

This is probably confusing. Which is why I’m asking the question!


#2) Should I even bother transitioning the TYVEK into the UV resistant WRB or should I just wrap the entire building in TYVEK underneath the 4″ of roxul, flash everything appropriately, and then for the wall portions that are open joint, add another layer of the UV WRB just purely for the aesthetic of the black behind the open joint? Effectively there would be two WRBs on this portion of the wall, but is that really a big issue?

#3) Or should I just put all the WRB (both the tyvek and the UV resistant one) on top of the roxul the whole way around the building? I don’t think this is recommend by Roxul, but I imagine it probably wouldn’t hurt.

Now if someone can figure out what I’m talking about I would be impressed.

Finally, does anyone have any advice or links about setting the sealed units directly into the plywood bucks? Drawing details on direct setting sealed units would be great!

I’d appreciate the help!


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  1. GBA Editor
    Martin Holladay | | #1

    I'm not sure what "midway through a project" means, but I hope that construction hasn't started. The questions you raise are fairly fundamental, and should be figured out before construction begins.

    It's essential that you and every worker on the site know where your WRB is, and that this WRB be integrated with your window flashing. My own vote would probably be to go with Option #2 -- wrapping the entire building with Tyvek, and making this Tyvek layer your WRB, with a secondary layer of housewrap under your open-joint siding (and with the understanding that this secondary layer of housewrap is not your WRB). But this approach will only work if you have figured out how to integrate your window flashing with the Tyvek.

    Have you ever put together a site-built window? I have. It sounds like you want to insert your IGUs directly into plywood window bucks to make a site-built fixed window. Plywood is not a good choice for this. When I make a site-built window, I prefer to use white cedar or pine. In any case, the tricky detail is at the sill. You need to transition from a level sill (where the IGU is set) to an exterior sloping sill. That usually requires 5/4 or thicker material, and some planning.

    Good luck.

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