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Building Wall

3dHoTBycF3 | Posted in Green Building Techniques on

I received quite a bit of information on my last question and I thank all of you for that. Another question I have concerns putting a 1/2″ EPS or XPS rigid sheet on the inside of the wall, then 1/2″ drywall. It may be overkill, and that is why I’m asking. This would be in addition to 7″ of cellulose in the 16″ OC bays, 1/2″ CDX, 2″ XPS, 3/4″ furring strips 16″OC and then cedar clapboard. Crazy thought?

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  1. wjrobinson | | #1

    GM, add horizontal interior strapping if you like or strips of foam on the studs. Do not add layers of foam that are separated. It's just a bad idea period. It could be fine or it could be a disaster.

    Why? Here's the example I used to use. Take a Ziplock bag, close it with some insulation in it. poke a few tiny holes in it. That is your wall. Now sit the bag on a plate and add a bit of water to the situation. What you'll get if your test is like mine, is water in the bag and wet insulation. Next take the bag off the plate and see how long it takes for the water to dry out of the inside of the bag. Forever! Not good. Structures always leak. Build structures that have only one tough plane to penetrate. Then whatever gets in moisture-wise can have half a chance at exiting.

  2. GBA Editor
    Martin Holladay | | #2

    In case AJ's answer wasn't clear -- I got a little lost there -- you should include only one layer of foam in a wall. You don't want to sandwich wooden components between two layers of foam.

    I'm pretty sure that's what I answered the last time you asked, and my answer remains the same.

  3. sunstone | | #3

    Why then do some builders put an air barrier at the sheathing and another at the internal vapour barrier?

  4. 3dHoTBycF3 | | #4

    Actually, you did mention it but I guess it did not sink in...sorry. And I believe the consensus here is that adding the XPS is best on the outside of the wall...correct? Anything wrong with using say a 1" XPS on the inside (covered with drywall), dense-packed cellulose in the bays and just CDX, house wrap and siding on the outside? My guess is that it will eliminate thermal bridging, save on the XPS costs (1" vs 2") and save considerable money on the windows and doors that will need alterations with the exterior 2" XPS. Just trying to understand the best solution for efficiency and costs...

  5. wjrobinson | | #5

    GM, it is done all the time.

  6. GBA Editor
    Martin Holladay | | #6

    Q. "Why then do some builders put an air barrier at the sheathing and another at the internal vapour barrier?"

    A. There's nothing wrong with two air barriers -- just not two vapor barriers.

  7. GBA Editor
    Martin Holladay | | #7

    Either interior foam or exterior foam can work. The advantage of exterior foam is that it addresses thermal bridges at partition intersections and rim joists; interior foam doesn't do that.

  8. 3dHoTBycF3 | | #8

    I'm not sure I understand. Is my inside scenario as good or not as good as the outside? If it is not, then why? Sorry if I am missing the boat, but I really don't want to make any mistakes with this my age, I believe it will be the last house I live in.

  9. 3dHoTBycF3 | | #9

    Sorry Martin...I was typing and did not see your response. Thank You...

  10. GBA Editor
    Martin Holladay | | #10

    It's not as good, for two reasons:
    1. You are choosing interior foam with only half the thickness (and therefore half the R-value) as the exterior foam that you intended to use.

    2. As I wrote in my last post, exterior foam addresses thermal bridges at partition intersections and rim joists; interior foam doesn't do that.

  11. wjrobinson | | #11

    Just to really add confusion, when the best exterior foam insulation pros do their work, they use foil faced foam in two layers with each layer carefully fitted and taped at the seams with a quality tape. Some even caulk the first foam layer as it sits on the sheathing to stop any air movement. Some will space the sheets instead and then sprayfoam the gap attempting to "glue" the sheets together.

    Foam sheathing is no good if any air can move around it. A tiny space can make a huge section of the insulation meaningless. I am friends with two contractors that have used insulation like this for decades and they really give it their all to detail the install as perfect as possible. They even buy one foam company over another just because the sheets come more exact in size so as they can fit them as tight together as humanly possible.

  12. 3dHoTBycF3 | | #12

    Thank you both for your answers...they have helped me make a decision on which way I will go.

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