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Community and Q&A

Inline Window Installation Detail

jonny_h | Posted in Green Building Techniques on

A quick question regarding window installation details, as I prepare to go ahead with an order of Inline fiberglass windows.

I was reviewing their installation instructions (here: also see drawings here: and found a detail I’ve never seen before: They apparently provide an “installation angle”, which looks to be a (metal?) angle piece that serves as a sill back dam and also fastening for the bottom edge of the window frame.  The sides and top are fastened with straps as seems more typical.  This installation method is unfamiliar, but seems reasonable, at least.

However, I’m trying to install my windows as “in-betweenies” in a wall with exterior foam insulation.  I’ve seen dozens of window details for all kinds of different situations, and I’m trying to synthesize something that’ll work for my situation out of all those.  Notably, I’d like to avoid plywood window bucks / Dudley boxes, since I’m working with existing rough openings that I don’t want to shrink — already going to tilt-turn windows is reducing my glass area with the wide frames.

I’ve attached a quick sketch of my thoughts on a sill detail.  My exterior insulation will be 2 layers of 1.5″ polyiso, so I’m thinking I can go with the “inset picture frame” approach with flat 2×4’s in the outer layer of foam (and the inner layer providing a thermal break) — with the “picture frame” on the outer layer, it provides something for trim to fasten through to, and supports the outer edge of the window frame.  I can provide a tapered subsill, the Inline installation angle / back dam, and then wrap sill flashing tape from the surface of my polyiso, over the picture frame, subsill, and around the installation angle.

The big question is, what else happens at the sill?  I’ve sketched in what would basically be a field-bent metal sill plate that would extend out over the rainscreen gap and in front of the siding / trim, but the Inline window frame profile doesn’t seem to provide a “nice” place to install this (unlike some European windows I’ve seen that have a little groove specifically for aluminum sill pans).  Inline does provide some snap-in frame extensions of varying widths, but it’s unclear how well sealed they are (or if, in fact, it matters).  Or, I’ve seen some details for unflanged windows that just involve face-taping (or “zero reveal” taping) the frame to the sill flashing tape — but I’d still want something to direct bulk water coming down the window to out in front of the siding.

Also, with the installation angle to the interior, and from Inline’s installation drawings, it looks like the primary air and water barrier for these windows is on the interior side, with some caulk (and presumably hopes and wishes) on the exterior side — making the whole area around the window (1/4″ – 1/2″ all around) just an open, uninsulated space that they expect to be drainable to the exterior.  This seems like a big hole in an otherwise reasonably well insulated assembly with expensive well insulated windows.  The Inline instructions state “Perimeter cavities – Between window frames and rough opening.  Insulate continuous around inner perimeter of window with low expansion foam or fiber type insulation.” but it’s unclear what they mean by the “inner perimeter” — especially since there’s that installation angle at the sill, so from the interior there’s no gap exposed to insulate (and their drawing doesn’t show any insulation in the sill detail).  Do I just stuff something moisture-insensitive like rockwool in there and hope for the best?

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  1. Expert Member
    NICK KEENAN | | #1

    I'll chime in because no one else is.

    I don't like the way the window frame isn't rigidly attached to the drainage plane. Windows can be under a lot of force with strong winds, and it's very hard to maintain an air- and water-tight seal if the pieces are moving relative to each other even just a little.

    How is the flashing that extends over the window trim attached? Is it just glued to the window above and the flashing below with sealant? Does it go past the edges of the trim piece where it can be nailed to the sheathing? If it's not firmly attached it's going to develop leaks as well. But the only place I can see putting a fastener without causing leaks is either the back of the dam or beyond the edges of the window.

    I would bevel the top of the window trim to match the slope of the dam and give continuous support to the flashing.

  2. Expert Member
    NICK KEENAN | | #2

    And how is the flashing terminated at the corners?

  3. jonny_h | | #3

    Hi DCContrarian, thanks for chiming in -- you've hit on exactly some of the concerns I have!

    The Inline instructions have the metal flashing just stuck in between the shims and the window unit, and caulked at the exterior -- so the stackup is like
    [rough opening / tapered subsill / "installation angle" back dam]
    [Subsill flashing membrane]
    [Shims and gap that can drain into the rainscreen gap]
    [metal sill flashing]
    [window frame caulked to metal sill flashing]

    I suppose for mechanically fastening the sill pan, it could be nailed through to the subsill under the window frame -- depending on that being mostly out of the weather, and the subsill flashing membrane being something that self-seals around fastener penetrations. At the front, instead of just forming a drip edge in front of the trim, I guess it could wrap entirely around the bottom trim piece. Good call on beveling the top edge of that trim piece -- that's a small and easy to implement detail that'll likely make everything just a bit smoother.

    >how is the flashing terminated at the corners?

    That's a good question -- on a broader sense, I'd live to see more corner details in general. All's fine and good with a wall section, or a sill or head flashing detail, but the corners are always the weakest and trickiest to implement! I think with a metal sill flashing like this, I'd need to bend up the ends to form end dams, maybe 1/4" to 1/2" high, at the sides of the window frame. Those would end up behind the window trim frame extensions , which would make the trim work a little bit fussy.

    1. Expert Member
      NICK KEENAN | | #4

      I'm thinking that the order of assembly would be flash the opening, install the window, apply bottom piece of trim, then the metal flashing/sill extension, and then the side and top pieces of trim. With that order I don't see anything you can do other than just slide the metal under the sill of the window.

      1. jonny_h | | #5

        That sounds about right -- since it's not a flanged window, it'd probably be possible to install the metal flashing before the window to get it a little more "sandwiched" in there, but still depending on the rough opening being well-flashed too.

        Another option, rather than field-fabricating a metal flashing / sill extension and wood jamb / head frame extensions, would be to use the Inline accessories. They only provide a few details drawings of these, example attached -- it's unclear though that the extensions would actually provide a good seal / positive drainage outward, so even with these extensions the window is dependent on a well-flashed and drainable rough opening. Also, the sill, head, and jamb extensions all protrude the same amount, so there's no way to have the sill extend further to a drip edge outboard of the trim.

  4. Expert Member
    Akos | | #6

    I don't see any benefit to the metal L inner bracket, I would stick to standard metal clips for mounting. There is a pretty big cavity between the metal clips and the frame which is very
    hard to seal up afterwards. I find it best to fill this gap with foam before the window goes in.

    The fiberglass ones I've installed had a groove on the face for a metal sill which makes it much simpler.

    If you can make the exterior sill extension work (even if it means moving the windows outboard a bit) with the siding+insulation depth you have, it will be a much easier and cleaner install. These are typically sealed to the window frame with caulk. Won't be 100% waterproof but sealed well enough that the WRB layer underneath can handle the minor leaks.

    The lack of drip edge is an issue, you can add in a small L shaped metal drip edge to the bottom of the sill extender.

    Unlike with vinyl, spray foam sticks quite well to fiberglass, foaming the window in place would get you pretty good seal. I prefer to tape the inside to the rough opening with flex flashing tape for a better seal and redundancy. Flex caulk over the foam works as well.

  5. Expert Member
    NICK KEENAN | | #7

    What I don't like about the sill extender is it isn't tied into the window flashing, so water can get behind it.

    Here's my thought on tying the sill extender into the window flashing: flash the opening normally. Then cut a piece of self-adhesive flashing tape as long as the width of the window. Score the backing tape longitudinally, leaving one side was wide as the sill protrudes. Remove the backing from the other side, and apply it to the bottom of the window opening with the sticky side on the bottom and the side with the backing still on sticking out. Install the window. Put the sill extender into place, remove the backing, and push the sill extender up into the tape. Then install the furring and trim.

    Then the question is what to do at the corners. Option 1 is do nothing, just have the sill extender end. Option 2 is have the sill extender bend up and go along the side for a bit, maybe as high as the sill is thick. In that case it's easy enough to have the flashing tape follow. Option 3 is the bottom trim piece goes under the side trim and the sill extension goes between the bottom trim and the side trim pieces. The flashing tape could follow in that case as well.

  6. jonny_h | | #8

    Thanks for the info, Akos! Out of curiosity, who made the fiberglass windows with the groove for a metal sill?

    > If you can make the exterior sill extension work...
    Are you suggesting getting the fiberglass frame extension piece for just the sill, or for the entire frame? I have flexibility in the mounting location, we were just going for an "in-betweenie" mounted in a place that looked reasonable and could use standard wood sizes for trim and frame extensions -- so it could be adjusted to use the fiberglass frame extenders. Would it be better to get the front of the frame extender flush with the face of the trim, or set back 1/4 - 1/2"? It seems like setting it back a bit would be a more forgiving installation -- and come to think of it, the head and jamb trim could be rabbeted to cover the frame extension, while the sill could get a small metal drip edge -- maybe even just a length of a standard prebent thing. How does that sound?


    1. Expert Member
      Akos | | #9

      They were Inline's standard (non tilt and turn) windows.

      The extension should be all the way around, this saves you having to make the pieces plus it will match the window color exactly.

      I would set the windows back just a bit from the edge if you have trim. Rabbeted can also work. Just make sure that you are not creating any pockets where water will sit.

      You can also go trimless have the extensions stick out a bit past the siding. Requires a bit more care with exact siding lengths but gives a very clean look.

      Whichever way you go, make sure there is a head flashing either directly above the window or above the top trim. The head flashing should extend all the way back to your WRB and be lapped by it. This detail is important for any window that will see a lot of rain exposure.

  7. jonny_h | | #10

    Akos and DCContrarian, thanks for the help -- I've revised my sill detail to use the Inline frame extender piece and a small metal drip edge. The frame extender to frame and drip edge to frame extender joints would be sealed with sealant, but it wouldn't particularly matter if there were minor leaks here because the rough opening is sloped, covered with Extoseal, and drainable to the rainscreen space. On the interior side, there's a bit of a thermal bridge weakness around the edges, but maybe the window just gets shimmed up a bit more from the sloped subsill to provide room for expanding foam. Haven't drawn the head / jamb details yet, but for the head imagine this detail flipped over, with a head flashing taped to the polyiso and extending over the head trim, and an appropriate flashing tape connecting the polyiso down to the window frame. Thoughts?

    1. jonny_h | | #11

      Sorry for the noise, just a quick bump on this -- anyone have thoughts on this in-betweenie flangeless window detail? Thanks!

  8. Expert Member
    Akos | | #12

    I think it looks good. Make sure with your sequencing so that you can apply sealants and SPF. I prefer to leave enough space to be able to foam from the inside.

    If your rough openings are anything like the 100 year old ones I've seen, they will be very far from square or level. Make sure you have enough space to adjust the windows to level them out.

    Tilt and turns are unforgiving to any twist of the frame, you might need to shim to get your rough opening flat.

    I don't think the extra bit of foam under the interior jambs is doing anything, I can't see any real thermal bridge there unless you are installing a metal sill pan.

    1. jonny_h | | #13

      Thanks for the input! Yeah, the openings aren't *terrible*, but they're certainly not perfect either, so we're leaving some space for adjustments.

      1. LaciB | | #14

        I am considering a similar set up with Inline Titlturn windows. I’m in CT. Wondering if you got your windows and where you are on the install.

        1. jonny_h | | #15

          Hi Andy,
          I've placed my order with Inline, but the windows won't be delivered til probably sometime towards the end of October. I can updated then with how the actual install goes!

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