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Community and Q&A

How to Get the Look of Traditional Trim Using PVC or Boral on Flangeless Windows

mpsterner | Posted in GBA Pro Help on

I am building a pretty good house (zone 7) right now. Wall system has 2×6 wall with interior service cavity and 4″ of exterior Rockwool (image attached).

My windows will be installed in the 2×6 wall at the sheathing level, along with the WRB. I then need to create extension jambs to extend beyond the 4″ of Rockwool and 3/4″ rainscreen strapping to my cladding.

I am struggling with how to detail the exterior extension jambs with traditional trim using cellular PVC for a more classic look. 

Rockwool has a good resource for many installations, including flange and non-flange windows, but they rely on a metal sill and do not go into any detail on how to complete the extension jambs for good drainage. Relevant window pages from the pdf are attached.

Fine Homebuilding covers exactly what I am after with flanged windows here:, however, they only show the traditional-style trim using flanged windows. Since the flanged windows protrude from the wall, they create a nice box to frame your extension jambs around, then simply fastening the extension jambs to the wall using brackets. This would be done before the exterior Rockwool. The attached image from Fine Homebuilding shows exactly what I am after but instead with a flanged window.

I’d like to use European-style tilt-turns which will not have flanges. 

How do I execute traditional looking trim with cellular PVC or Boral Truexterior trim for my extension jambs and trim and create a good seal and drainage?

Thank you.

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  1. creativedestruction | | #1


    How about a full-depth 3/4" plywood window buck? Fluid applied flash for the outer projecting half? The beauty of the flangeless frames is you can set them wherever you want within the opening depth. Could choose based on the preferred trim stock width.

    1. mpsterner | | #2

      Hi Jason,

      Given that I am not already using a liquid-applied WRB or something like Zip sheathing with an integrated WRB, I would have to change other elements of the design to accommodate the addition of a window buck and the liquid flashing.

      My concern with this method is that it would add [I think] significant cost and make it so that I don't have a way to do the thick sloped sill of a traditional exterior window casement.

      The attached image from JLC demonstrates the issue nicely. There is clear information on how to do what I am after with a flange, but if you imagine this image without the flange and its related protrusion from the window frame, it is difficult to imagine how to interface the extension jamb with the window.

      Another good resource for good installation detail with a flanged window:

      All the install challenges with my existing wall system seem to be coming from using flangeless.

      Thank you!

      1. creativedestruction | | #3

        The sloped sill can still be achieved. Thorsten Clupp describes how he did so at this thread #53 (different wall assembly, but similar idea)

        More slope is less risk of course, so I imagine fluid applied is not required, just easier to install well, more so than tapes or SAM flashing.

        Honestly, I think the buck will make things easier. Cost issue is tricky with the price of ply these days...have to balance the labor side and fuss with material costs.

      2. creativedestruction | | #4

        Side note, I'd seen that JLC detail previously and thought: are the exterior extensions glued on, or held there by the peel and stick and wishful thinking?

        1. mpsterner | | #5

          Hey Jason,
          I REALLY like this option, especially with fluid applied WRB.

          I am concerned that I then still have to frame out extension jambs inside of the plywood box that interact with the face of the window frame, for that finished jamb look. This would be cellular PVC, Boral Truex or some other product not affected greatly by water.

          It strikes me that I am doing the same thing twice then when I could have just installed the window and flashed it with the WRB and then applied the extension jamb after-the-fact.

          I hear you though... I am wondering the same thing looking at that JLC article... There is no REAL waterproof connection between the extension jambs and the window face. It seems that you're relying on either caulk (though that is not shown), or just accepting that some amount of water will work up behind the extension jambs and then be taken care of by a [hopefully] solid WRB.

          That has always seemed to be the weak point of that approach to the installation. I do think that the concern is limited by having the flanges and mostly the protrusion outward into the jamb extension area. Water hitting the window ultimately drains down the face of the window, hits that frame that protrudes behind the sheathing plan and, in theory, drips right to the angled extension jamb without trying to sneak back behind the E.J. and down the WRB layer.

          Perhaps that is wishful thinking and just relying on hopes and dreams???

  2. creativedestruction | | #6

    "There is no REAL waterproof connection between the extension jambs and the window face."

    Right -- there's nothing to connect to at all in their detail. The way I see it, mid-wall window with exterior mineral wool leaves you with 2 options, either the full ply buck, or more beefy 2x4 exterior furring to give you something to fasten the extension jambs. Either way, the extension jambs and trim are the last part of the install. They're just cladding, they needn't be watertight. Hydrophobic materials, sure. Watershedding, great. Solid support backing, flashed and drained--absolute necessity.

  3. mpsterner | | #7

    So, given that this is all outboard of the WRB, the question is, does it matter that the extension jamb doesn't have a waterproof connection to the window or WRB itself?

    This approach is written about from JLC and Fine Homebuilding, both extraordinarily reputable sources.

    Even the Building Science Corp drawings don't show a waterproof connection with the extension jamb.

    1. creativedestruction | | #8

      Correct. The extension jambs, acting as cladding, can not achieve a true waterproof connection fully exposed to weather. The ply buck I'm suggesting, however, would need air and watertight connection both to the WRB and the window frame. Alternatively, explore a 2x4 furring extension jamb connection, where it all stays outboard of the WRB/air barrier.

  4. maine_tyler | | #9

    Every situation—and window—is different, but if I'm understanding your concern correctly, I don't think it's really the issue you think it is. The finish jamb just butts to the window frame. No? How to attach depends on a few other details (rainscreen?), but it's kind of a separate issue and would exists even with a flanged window.

    Have you seen this entry in Eric's blog?

    In it is an embedded Hammer & Hand video that may be of interest, and I linked to a specific time stamp to get to the point:

    I think the sill (especially if not using metal) is really the trickiest part.
    I did a similar thing using some Boral glued together to create a thicker 'traditional' sill. But I wasn't getting paid by the hour and it was a lot of finicky work. It involved setting the window up on an 2x to gain some height difference between the bottom of the window and the face of the siding. I can find some photos if it's of interest, but fair warning that it's probably not a production-build-friendly detail.

    1. maine_tyler | | #15
      1. mpsterner | | #16

        Thanks for that link Tyler!

        The Hammer and Hand option, which looks to be what they used, is no doubt robust. It is also the most intensive option. Furthermore, for a traditional looking jamb and trim detail, the same questions persist, what do the jambs/sill interface with (as described below in the available Zola frames), do they get caulked to the window wood frame face, to the window aluminum cladding, etc.

        The metal sill makes it nice and neat because you just interface with the routed wood drip edge from underneath. That still begs the question though what they do on the sides and head–are they using 1x material and just butting up to the window frame?

        Perhaps I would simplify everything by just using a metal sill and then adding the thicker sill look only at the trim level, underneath the metal sill.

  5. mpsterner | | #10

    GBA must be optimizing for search engines or something as I see the post title has changed. The title is not terribly relevant to the subject without keywords like "exterior insulation" and "extension jambs," but I appreciate you all finding the post and responding!

    Jason, to your point "Alternatively, explore a 2x4 furring extension jamb connection, where it all stays outboard of the WRB/air barrier." ... This is ultimately what I was trying to do, just skipping the 2x4 and instead achieving the same function through something that is not water sensitive. Mounting the 2x4 furring would still require that I clad it with a finished product, wouldn't it? What advantage would that have?

    The concept came from an article that Building Science did for the Department of Energy on retrofits. Page 73 is where the window detail starts, they go into pretty good detail on it (on a flanged window):

    They're keeping the extension jambs completely outboard of the WRB. They also add some caulk/sealant between the window and extension jamb for some additional sealing.

    This method seems like a lot less work than building a buck. That said, the window buck concept does seem more robust.

    Tyler - Yes, that is my understanding. It just butts up. I am mostly just trying to understand if that is really all there is to it and if the protrusion of a flanged window covering the small bit of extension jamb is providing additional protection or if you could just do the exact same thing with a flangeless window?

    To your point "How to attach depends on a few other details (rainscreen?), but it's kind of a separate issue and would exists even with a flanged window." That is detailed in the Building Science / Department of Energy article I linked to earlier in this comment. It is basically a pre-built jamb extension box that is then bracketed onto the wall outboard of the WRB.

    I guess the question is just whether or not that is a good idea. It does seem a little lacking in its robustness, however, it is also documented from Building Science, Fine Homebuilding and JLC (with flanges only though).

    Love the GBA article from Aarow Building and Jake Bruton. That said, it only shows a great installation for those and not how to detail the extension jambs. I would be curious if he was doing exterior insulation rather than the Zip R-Sheathing how he would have the extension jambs interface with the window?

    In the example of, this is awfully similar to what I am looking at. The only difference is that they're using a metal sill at the base that would hook underneath the window. They don't show the extension jambs after and how they interact with the window and he doesn't mention whether those extension jambs would be caulked or if they're basically just butted up against the window frame...

    1. maine_tyler | | #11

      Regarding my comment about setting the window up on a 2x... you can ignore that since I had double stud walls with plywood bucks. With ext. rockwool it's not the same issue.

      I'm not a professional builder so can't say how its usually done, but for me it was just a matter of butting and caulking. (I had some windows with flanges and some without, which were actually repurposed replacement windows)

      The key, as mentioned above, is that the weather barrier is behind that all and that the system can drain.

      "It is basically a pre-built jamb extension box"
      That is what I did, and then used my rain screen strapping (strategically placed) to attach to. Small trim screws right through the boral into the side of 3/4" SPF. Plastic shims used before screwing to keep things straight.
      Like I said, finicky, but plenty strong for trim. Overall, it's admittedly not the most robust set-up, but that mostly stems from use of the Boral itself (weak) and not the attachments.

      The design really relied on the pieces being assembled beforehand as a unit. There would be little holding them in place if it wasn't all joined as a box-unit.

      Pictures of the buck before install, then one mostly in on a flanged window, then one without flanges. Note not all the trim (like the face pine) is attached in the pic yet.

      The last pic it's butted and caulked, though that window does have a bit of a protrusion which may give the appearance of having flanges.

    2. Andy_ | | #22

      "It just butts up."

      "I am mostly just trying to understand if that is really all there is to it..."

      "...and if the protrusion of a flanged window covering the small bit of extension jamb is providing additional protection..."
      Not really.

      "...or if you could just do the exact same thing with a flangeless window?"

  6. Expert Member
    PETER G ENGLE PE | | #12

    I may be missing something here, but I'm not sure what all the fuss is about. Nothing connects the window frame with the WRB. That's the job of the flange in a flanged window. In a storefront-style flangeless window, you waterproof the opening - window buck, finished jambs & sills, whatever. The window floats in the waterproof (and airtight) opening. The window is fastened in place and shimmed to fit. Then the window is made waterproof and airtight using backer rod and caulk between the sides of the window and the opening. Tool the caulk to make a slightly concave surface. Fill the rest of the gap with low-rise foam and you're done. Yes, the water and air barriers depend on the caulk, so you use a very high-quality sealant. I've worked on high-rise buildings where this joint detail has held up for 20+ years, but it does eventually need to be removed and replaced as a normal part of scheduled building maintenance. With a pitched sill and backdams, there is little to go wrong. Plus, you can replace the window without any disruption of the primary WRB. Just cut the caulk and go. The FHB detail for a flanged window from the OP makes window replacement hard and expensive.

    1. maine_tyler | | #13

      "The FHB detail for a flanged window from the OP makes window replacement hard and expensive"

      you mean because it's flanged?

      1. mpsterner | | #14

        Thanks Peter!

        I understand waterproofing the rough opening and installing the flangeless window following the approach described here:

        That said, then comes the extension jambs. In this case a finished material that is meant to look the same as the 1x4 exterior casement for a traditional look.

        What I need to understand is what do I do with the extension jambs? Do they interface with the wood frame of the window or do they bump up to the face of the aluminum cladding? Then do they get just caulked in place and bracketed to the wall on the outside of the WRB? Then I can flash the extension jamb at the head as well?

        I've attached the Zola window profiles that are available for the different installation methods. You can get different levels of aluminum cladding coverage.

        Zola's drawings (also attached) all show a metal sill interfacing with the router drip edge at the bottom of the window but they don't show how to do a thick angled sill out of PVC or similar material for a traditional look and what that would interface with.

        1. maine_tyler | | #18

          Isn't that first image with a thicker sill?

          1. mpsterner | | #19

            Oh yes, I created that though. I am using that to sort of ask is that how I should do it or do I have the jambs interface with the aluminum clad face of the window.

  7. Jon_Lawrence | | #17


    I have flangeless windows and 3” of exterior Rockwool, so my situation is similar to yours. Here is what I did:

    I added strips of beveled cedar to the sill to create a slope. I then taped the sill and 6” up each side of the jamb with Pro Clima Extoseal stretch tape. The window frames are flush with the sheathing and I taped the windows to the Zip to maintain continuity of my air and weather barriers. It was a good six months after the windows were installed before the Rockwool (not easy to find a Rockwool installer) and cladding so I was able to confirm that there were no leaks. Next, we built a window buck out of Azek with a sloped sill that had a kurf cut underneath for a capillary break. This buck is temporary held in place with a few finish nails. Then we added head flashing and taped it to the Zip, then the Rockwool, furring strips and vent strips. The buck was then secured to the furring strips. Finally, the casing and crown were added to complete the install.

    1. mpsterner | | #21

      Thanks so much Jonathan! This looks super close, pretty much identical to what I am doing.

      Do you have any additional close-ups of the rough openings, the jambs, the trim, etc.? I'd like to see more on what you did.

      It appears you also use a Coravent material or similar for venting the space below and above the window?

      I'd also like to better understand how the Azek meets the window? Is it against the window frame just enough to cover up the tape or is the outside of the jamb the dimension of the window frame and it makes full contact with the jamb material? Did you caulk it against the window or just bump it to the face? Did you have uPVC or aluminum clad windows?

      Thanks so much for sharing your example.

      1. Jon_Lawrence | | #23


        I found some more pictures including the mock-up that I did for my trim guy and I took a picture of the finished sill. The mock-up was done with Boral, but I switched to Azek for the actual install. My windows are wood and three of the exposures have aluminum cladding that is thermally broken from the frame. The wood frame allowed us to nail the Azek buck in place until we could firmly attach it to the furring strips. I used a split back tape and attached the 5/8" strip to the face of the wood windows. Coravent comes in 3/8" and 3/4" thickness. Since we needed to get the Coravent flush with the 2x4 furring above and below the window, we placed a 3/4" piece of Azek on the Roxul and then the Coravent on top to get the proper depth. The trim is caulked at every joint and to the window, except where the window meets the sill. The aluminum cladding has weep holes and so we added a back dam to allow any water that gets behind the cladding to drain away from the window.

        Tyler's comment about the metal sill is a sore point for me. The windows came with extruded aluminum sills, high quality stuff. But I ordered the wrong color and we increased the Roxul depth so the sill would no longer fit. These sills had side dams, attached to the window frame so acted as back dams, and a j-channel on the front lip for a capillary break, that I unfortunately could not use. I won't make that mistake again. If you do go with European windows, they should be able to provide you with sills.

  8. maine_tyler | | #20

    In my opinion the answer about the interface has been covered within the responses and the resources linked. That said, below are a few more resources that may be of help.
    It's true that few to none are explicit on whether to caulk, but to me it's just a given that you would add a bit of caulk (sealant actually). The historic wooden picture windows in my 1910's bungalow are inset a bit and are done this way (wood returns onto the wood frame, and then some sealant and paint). Either way, in our modern approach, it's not the primary water control layer so perhaps that's why the use of sealant is not mentioned explicitly. You could always design the detail and then decide later whether you want to add sealant.
    Also, from the BSI doc you linked above: "Photograph 1: Beautiful “Innies”– The trim is returned to the flange or face of the inset window."

    —Zola president talking about 'over-insulating' the frames and how far to extend the aluminum cladding (note that the trim itself would cover the aluminum):
    [that video ^ may get at your point the most, though it still doesn't mention use of sealant]

    —A great overview from ProTradeCraft on boral extension jambs (essentially what Jonathan and I both did):

    —Another Hammer & Hand video with ext. roxul:
    and that video also sits within another one of eric's blog:

    —See how manufacturers of vinyl are doing it:

    —Matt Risinger, overview of options:

    If the bigger issue is the sill, ultimately I think you can do with thicker material what you do with metal. You just need to ensure clearance so that the material (and it's slope) is accommodated. The specifics of the aluminum cladding on that detail there is a bit beyond me. Maybe Zola has specific recommendations?

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