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Community and Q&A

Detailing Windows in Assembly With Rainscreen and Exterior Foam Insulation

jonny_h | Posted in Green Building Techniques on

For those following along at home, my Inline windows are finally scheduled for delivery, a month late >_<.  While reviewing our installation details (attached), we came up with a couple questions:

First, on the sill, there will be a sloped subsill & back dam protected by a waterproof membrane, with the window supported on sloped shims.  At the jamb and head, I’ll maintain continuity of my water barrier by taping from the window frame extensions to the surface of my exterior foam (which is my water barrier), and the interior of the window will be sealed all the way around (tape or sealant or expanding foam).  At the exterior sill, however, I’ve seen varying opinions on what to do — it seems that some people tape this with a vapor-open tape, providing a continuous air & water barrier and assuming that if any water gets into the space, it’ll be a small amount and can dry through the vapor-open tape.  Other people seem to leave the sill entirely open to the exterior, and insulate with something fluffy and non-moisture-sensitive (ie mineral wool), so that any amount of water finding its way into this space can drain.  Still other details seem to mostly seal this area but leave some small weep holes or something.  Any thoughts on this?

Second, the window trim will be supported by furring strips, then there will be Coravent above and below the window to ventilate the furred-out siding rainscreen space.  How is the edge of the siding over the Coravent secured?  Is it sufficient on, say, a 5″ wide piece of siding to screw the upper 2″ to furring strips and leave the lower 3″ over the Coravent unsupported?  Put some screws into Coravent despite it not really being designed for structural support?  Rip down strips of Coravent to narrower widths so only 1-2″ of the siding is unsupported?


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  1. Expert Member


    Both the head and sill sections confuse me. Why is the cor-a-vent there? don't see any vent opening for it to protect in either case.

    At the head I would hold the siding up 1/2" over the metal flashing and run the vertical furring strips down to provide support, with cor-a-vent strips placed in between them.

    At the sill I would not vent the cavity at all. My feeling is the heightened chance of bulk water intrusion offset any advantage that venting provides. For the details to work, either as drawn or as I suggest, the trim around them needs to be 2"x material. It isn't realistic to profile it as they have shown leaving about a 1/4" overlap.

    1. jonny_h | | #2

      Hi Malcom,
      Thanks for the reply! The details aren't perfectly drawn, we were sketching things out yesterday; I did intend for there to be a small space between the siding and the trim to provide ventilation. I'm basically taking guidance from the Hammer and Hand manual ( but the part I missed that you pointed out is that at the windows, the furring is continuous to the edge of the siding and the Cor-a-vent is cut into strips between the furring. I think my mind was on the H&H "bottom of wall" detail, which shows continuous Cor-a-vent; now that you've pointed it out I see their "top of window" detail shows exactly what you described. That perfectly answers my second question! Thanks!

      >At the sill I would not vent the cavity at all.

      Do you mean that you wouldn't vent the siding rainscreen cavity (ie omit the Cor-a-vent and a gap between the siding and trim at the sill), or you wouldn't vent the cavity between the window and the rough opening (ie you would continuously tape all around the window at the exterior), or both?

      1. Expert Member
        MALCOLM TAYLOR | | #3


        I wouldn't vent the cavity below the window at the sill. That area is a particularly vulnerable part of the building envelope. I prefer to concentrate on making sure that no water can get in, rather than prioritizing good ventilation of the rain-screen cavity behind. if you stagger the battens below the window, or leave gaps, you will still get some air movement around the window.

        I don't vent the top of any of the cavities, so there isn't much difference in those full height ones and the ones that terminate at the window sill anyway.

        One small point on the head: If you hold the siding up to create a vent opening, make sure to put tall end-dams on the head-flashing to stop water getting in around the sides.

        Sorry I don't know enough about flashing un-flanged windows to be any help there.

  2. jonny_h | | #4

    Yay, the windows arrived today!

    Now that that's out of the way, I'd like to bring up the tape-the-sill-or-not question again -- I still haven't seen a really conclusive answer on this (maybe there isn't one).

    In this article ( and several others I've seen, they do tape the sill, which is also sloped and back-dammed, and say they expect the vapor-open tape to pass any moisture that gets back there.

    In other articles and videos, I've seen the sill left un-taped, and not taping at the sill is pretty much the rule for flanged windows -- but with flanged windows, the untaped sill flange is leaving maybe 1/16" max gap open, vs probably 1/4-1/2" gap around an unflanged window.

    My fear on leaving un-taped is that "stuff" -- water doing capillary things, bugs, etc. can get back there. My fear on taping is that a drainage failure above -- flashings, tapes, or the window unit itself -- might leak more than a couple drops of water that'd evaporate, and that my carefully sloped and back-dammed sill will just form a little pond. Maybe neither of these fears are realistic (or maybe both are).

    Maybe I should just tape it with a couple of coffee stirrers to provide "weep holes"? :D

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