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Wall with exterior foam

Brian Osburn | Posted in General Questions on

Hi, I am currently breaking ground on my first house. I am wanting it to be as air right and energy efficient as I can reasonably afford. I want to make sure I am setting up the wall properly. I live in climate zone 3, but I am within 50 miles of zone 2

2×4 wall with full cavity open cell spray foam
7/16 osb sheathing – all seams taped all nail holes filled with liquid flash
Benjamin Obdyke hydrogap house wrap
1” ACH foam control plus+ exterior foam sheathing – seams taped
Board and batten vinyl siding.

Does this look like a good set up?

Has anyone used or have any background with the ACH foam control plus+ insulation?

Any help with this would be greatly appreciated!

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Brian,

    In zone 3, you should try to hit R-20, however you may feel that you are in the warmer region of zone 3 and are willing to lower that R-value a bit and focus on air sealing. Do you know what R-value your building inspector will want you to hit?

    Your definitely off to a good start with air sealing.

    We'll see if anyone stops by that has used this product, however, I think it is pretty common EPS.

    1. Brian Osburn | | #2

      Honestly, where I live, there isn’t a real inspector. He may or may not show up to the job site, and if he does it is just to chat with the builders and leave. Most builders where I live have never used rigid foam on the exterior, and many have never heard of zip system if that tells you anything.

      Thanks for the info, any tips would be much appreciated, as this is new to me.

      Do you have any recommendations on installing windows with the 1” of foam? Should I just have them frame out the window buck or install the window over the foam? I’ve tried to find best practices for this, but there is so much info that it is hard to tell which one is best.

  2. GBA Editor
    Brian Pontolilo | | #3


    Below is an article on the two ways to install windows in foam sheathed walls. In your case, with 2x4 walls and only 1 inch of exterior foam and no rain screen detail, innie windows would probably be simplest. They'll be easy to integrate with the WRB and will fit the 2x4 wall on the inside. You'll need to develop some type of exterior extension jamb, but that's a fun trim carpentry job.

    By the way, you don't need drainable housewrap. It's okay and will have some benefit for drying your walls, but it will also reduce the performance of the exterior foam some. Most builders use flat housewrap in this situation.

    Here's the article:

  3. Jon R | | #4

    See here for arguments in favor of drainable WRB behind foam. Better water control and insignificant thermal effect.

    1. Brian Osburn | | #6

      Thanks for the link. I think I am going to stick with the hydrogap.

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