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Interior trim for outie windows in thick wall construction

whitenack | Posted in Green Building Techniques on

Hi all,

I think I have made a mistake and need some strategies on how to fix it.

I have a new home construction with exterior rigid foam. I am planning “outie” windows. When I ordered the windows, I didn’t think about the interior trim strategy and the windows came with the standard interior trim pieces (not sure the terminology) attached. In other words, the window is set at the exterior, the window trim extends back into the house 3″ or so, and then I have another 3″ or so of no trim until I get to where the interior wall will be.

How do I make this look right? I guess I could just butt some trim boards against the edge of the window trim, but there would be a seam where the two materials meet, and there would be texture and possibly color differences. There isn’t enough space around the window to cover the window trim with a single piece of trim board. Can I remove the window trim in the field before they are set?

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  1. GBA Editor
    Martin Holladay | | #1

    The pieces you are calling "trim" are the interior jambs. Builders deal with this issue all the time, by building jamb extensions.

    If you want to remove the factory-installed interior jambs, go ahead and experiment. Be careful not to damage the windows -- see if the jambs are easy to remove. You might want to call the window manufacturer first to get some advice.

    If you are able to remove the factory-installed jambs, and you want to do so, go ahead.

    Jamb extensions can be made of ordinary pine lumber or drywall. If you use pine lumber, it makes sense to leave a 1/4 inch reveal where the jamb extensions butt into the factory-installed jambs. The 1/4-inch reveal makes the joint look deliberate rather than making it look like a mistake.

    The stool is a different question. You want the stool to be one piece -- ideally, a piece with ears that extend under the interior casing.


  2. GBA Editor
    Martin Holladay | | #2

    Below is a photo of a window stool with ears -- the right way to do it.


  3. Irishjake | | #3


    Everything Martin mentioned is 100% correct.

    A couple of things:
    It is wise to shim in several spots the additional jamb extension, and especially under the sill. You can pre-apply thicker shim material prior to putting the jambs and sills on too, so you aren't fighting with smaller shims in a larger space.

    I personally like removing the factory applied jamb extension and creating a jamb extension that goes all the way from the window out to the wall. Depending on the manufacturer, and the pre-applied jamb comes off easily. If the window is installed - I like to pre-drill and screw the three jamb extensions and sill together and then put it in as one unit. The Kreg jigs work nicely for this.

    If you haven't installed all you windows, and can remove the factory applied trim and apply the full depth jamb extensions to window. You can also just add them to the factory applied jamb extension too. Definitely hold back a 1/8-1/4" reveal. I almost always leave a reveal nearly anytime two wide pieces of stock are meeting on edge to break that joint up.

  4. whitenack | | #4

    Thanks guys. There are a couple of windows already installed, so I have a dilemma. I can just do them all the same and worry about the reveals on the jamb extensions at that time. I could take the jambs off the un-installed windows and leave the 3 windows already installed as is, or I could go to the trouble of taking the 3 windows out and taking off the jambs and putting them back in.

    There is aggravation with all 3 choices. The windows are marvin integrity, so I'm not sure if anyone has experience with removing their jambs. I spoke with the salesperson this morning and she referred me to the manufacturer tech support. Haven't talked with them yet.

    If the stool needs to be one piece, then my options are more limited. If I leave the factory jambs on, I will need to build up a trim piece over the existing jamb. There is room, but it will make the space under the window handle a little tighter.

  5. Irishjake | | #5


    How many windows do you have to do still? I think I would lean towards leaving the factory jambs on, and making them all the same. Like Martin said - it is a common occurrence that every trim carpenter deals with.

    Here are several ways I've attacked the problem when keeping the factory jambs on.
    Every solution requires you to shim around the window, between the extension jambs and the window buck or RO lumber.
    1.) Use a small biscuit or dowels to lock the two jambs together. On smaller jambs I've used trim head screws to lock the two together. On deep jambs, this doesn't work so well - so go to option 2 if it won't work.
    2.) Rabbet out the new jamb to slip in behind the factory jamb. It gives you a good gluing surface, provides a strong joint, and you can nail through that lap into the window buck.

    Depending on the level of finish spec'd, sometimes taking all the factory jambs off is what is required to look more custom. Custom = time.

    A good compromise (what I recommend the most) is to leave the factory applied side jambs on, and just remove the factory applied sill extension and attach a full width single sill (this makes shimming the sill much easier too, as there are two levels to shim if you keep the factory sill extension in place.

    Once you figure one window out - You can pre-cut all the header, side jamb, and sill extension material. Then just make them as a box (this way you can screw it all together from the back with pocket screws or via end screws). This modulates the construction and it goes a lot faster.

    A couple added notes that might help:
    Make sure to put a sloped sill pan (I use cheap pine lap siding/clapboard, under the window with my flashing applied on top) .
    On top of the window buck, put a length of that same lap siding on top of the buck, so water runs away from the wall (cover it with flashing too). You want to put these on before the window goes in.
    Add some backer rod around the windows too (before you install the extension jambs). Push it back so that it is between the window frame and the window buck. You don't want it between the extension jamb and window buck.

    I have pulled hundreds of factory jambs off of windows...if done carefully it can be done without damage. If you do any prying, make sure to pry on the rough opening side of the jamb and not the finish side. The jambs are often attached with 1/2" wide staples. You can do this......

    Good luck!

  6. whitenack | | #6

    We still have 25 windows left to install. At lunch, I went out to the jobsite and laid a scrap piece of 5/8 OSB on top of the bottom window trim piece. There was still enough room to comfortably grab the handle to lift the sash. The problem with that, though, is I don't believe it is "architecturally correct" for the side jambs to go down inside the stool. I think it is proper for the side casing to lay on top, and I'm not sure it will look right if the side jambs go down inside of the stool. Maybe it will be back far enough into the window opening it won't be as noticeable, but I worry about it.

  7. Irishjake | | #7


    Like I said - pry off the bottom sill extension, and then you are good to go. I think you will be pleased with the look.

    Something else you might want to consider - if you using square stock, you can ignore this. If you are using a molding profile on the sill, I like to choose my own, and before cutting the sill to fit in the window opening I will route the edges on three sides, so that I don't have to cut returns on the end of the sill. It speeds things up a bit, and with a bit of sanding it turns out really nice. Just make sure to sand in between coats of paint, so the end grain that gets raised up with the paint doesn't show thru.

  8. Irishjake | | #8


    I'm using Integrity windows in my Barn/Garage. I still have some to install, I'd be happy to video prying off the jamb extension for you - if you're feeling apprehensive. I won't get to it till tomorrow afternoon though.

  9. whitenack | | #9

    Brad, that would be so awesome! I'd love it if you would be so kind! I have searched YouTube for videos but haven't found any.

  10. Irishjake | | #10


    I didn't see your response till just now, I will video it on Tuesday and get it up for you to see.

  11. whitenack | | #11

    Looks like it isn't going to be as popping the bottom jamb off the window. The bottom jamb doesn't run under both side jams. The right-hand side jamb runs all the way to the bottom. Arg.

  12. Irishjake | | #12


    I'm confused, are you saying you don't have extension jambs or just that the bottom jamb extension doesn't run under the side jamb extensions. If the latter is the case, that is an easy fix. That is, as long as we are talking about the jamb extensions and not the window frame......

    Can you post a pic by the way?

  13. user-1135248 | | #13

    My windows didn't come with any concept of jambs to begin with,
    and I wound up with 100% custom jamb boxes that the guys doing
    the reno built out of generic pine shelf stock. Worked out
    fine, and we all agreed it was about the simplest solution for
    trimming out all of the openings in a consistent way. All
    dozen of them, it wasn't a huge job all told.

    about halfway down. It was all just cosmetic; the flashing and
    air-sealing had been done around the actual frames at the
    exterior plane.


  14. whitenack | | #14

    Sorry for the confusion, I knew I needed pics but didn't have my phone with me. Yes, the window has extension jambs. The bottom extension doesn't run under both side jamb extensions. It runs under one side, but not the other.

  15. Irishjake | | #15

    Just use a multi-master tool to cut the jambs to the same size then apply the sill as discussed. If you send me a pic or two I can lead you along. It will look great!

  16. whitenack | | #16

    How are the jambs attached? Is it just with the little staples I see at the corners?

  17. Irishjake | | #17


    Can you post a pic????

  18. whitenack | | #18

    Yes. I'll post some this afternoon.

  19. whitenack | | #19

    See the attached pics.

  20. Irishjake | | #20


    You can definitely cut down the side jamb and remove the bottom sill/jamb. Depending on how long the side jambs are, you can drop the height of your bottom sill, to accommodate the opener a bit more.

    The jambs will be attached not just at the top, but along the side too, usually with staples also.
    I would use a multi-master and cut the side jambs to the height you want the top of your sill to be.

    When it's all said and done, it's up to you which route you go, but removing the jambs is not hard, whether it's just one or all of them.

  21. Expert Member
    MALCOLM TAYLOR | | #21

    If you don't have a multi-master and aren't too keen on spending a couple of hundred bucks, Harbour Freight sells some quite serviceable ones dirt cheap.

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