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Community and Q&A

Outie windows with vinyl siding and furring strips

JonathanDaley | Posted in Green Building Techniques on

I’ve read every article on this site about rigid foam and outie windows, as well as lots of others and I thought I was all set, but now that I’m almost ready to flash the rough openings, I realize there is a detail that I haven’t seen covered.

I’m in climate zone 5, though only a mile or two from 6.

I have 3 inches of XPS installed over Typar (I’m treating the foam as the WRB, but the inspector wanted to see the wrap under the foam).

I have outie windows with a 1-by picture frame mounted flush with the outside of the rigid foam. Flashed with self-stick Siga products meant for foam, and that passed the backyard tape test reasonably well.

I was planning on putting PT plywood for furring strips under the vinyl siding, but I now realize that my flanged windows need to be farther out to account for the furring strips, so the vinyl can fit inside the flange.

It seems there are a couple options:

1. Build the picture frame flush with the furring strips.

2. Put furring strips around the window  (on top the flashing).

3. Skip the furring strips and just mount the vinyl on the form with long nails.

#1 seems the best, though that means I have to take out my picture frames and put in new ones. 

#2 seems good, though it isn’t totally intuitive to have the window mounted into the furring strips on top of the flashing, and there might be a problem with that, but incorporating the furring strips into the flashing/WRB seems worse.

#3 sounds even better, since some question the use of vinyl siding with furring anyway.

I realize many of these details have controversial answers, so the only question I’m interested in right now is the picture frame method with furring strips, as every example I’ve seen says to make the picture frame flush with the foam, and maybe that works for other types of cladding, but not vinyl?

Thanks for reading.

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  1. maine_tyler | | #1

    Do you have 1x ?4? lumber on the flat screwed through the foam into the structure behind? Or do you have more of a 'buck' with the 1x extending from the RO out to the foam face?

    I am assuming it's the former since you have flanged windows and use the term 'picture frame.' If this frame is flush with the foam, did it become so by rabbeting (recessing) the foam by 3/4" to accept the 1 by's?

  2. seabornman | | #2

    I drive by a house everyday where vinyl siding is being installed over furring strips The installation looks very poor, with waviness and an uneven appearance. Maybe cheap siding, but most siding manufacturers don't warrant installing over open furring. On the other hand, nailing through 3" of foam seems iffy too. I'd at least use screws. If you put furring on top of the nailing fin will the j-mold contact the window?

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Jon.

    The picture frame can be flush with the furring strips (proud of the foam) and be flashed to the rough opening and to the foam. Builders do it all the time. That said, not only can vinyl be fussy when installed over furring strips, but it is unnecessary since vinyl siding creates an air space between it and whatever it is installed over. Whether or not the manufacturer allows you to install it over 3 inches of foam, I do not know.

  4. JonathanDaley | | #4

    Tyler, I couldn't get 3" XPS here, so I have a first layer of 2" , and then put in the 1-by lumber, and then surrounded it with the second layer of 1". A nice side effect of the two layers is then I can alternate seams.

    Joel: yeah, that's the problem that I realized. The vinyl will be farther out than the window flange. The j-channel elsewhere, like on corners, etc. could be put on top of the furring easily enough.
    As for the wavyness, I don't know. Certainteed, my manufacturer, apparently gives different answers based on who you talk to on the phone. But I found enough people that said it was fine that I was going to go that route. I hadn't thought about screws. Do they make screws that have a small enough shank to go through the pre-notched holes in the vinyl? When I was getting screws for the picture frames, I could only find these 5/16" shanks, which seemed like overkill and certainly would be overkill for the vinyl (and probably wouldn't let the vinyl expand, etc)

    Brian: I just hadn't seen anyone talk about it when talking about these sorts of things. I do wonder about the self-stick flashing having to go out to the extended picture frame and then back into the foam, but maybe that will stick fine. Glad to hear that you've heard of people doing it regularly.
    I agree about the airspace being unneeded for vinyl.

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