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Community and Q&A

Wall assembly

LeoPaul | Posted in Green Building Techniques on

I’m a bean counter so please forgive me if I dont get terminology correct. I have a question about my total wall assembly.

Zone 4 outside Philadelphia. Wall assembly from inside out.

Option A: dry wall, 2×2 furring strip, 2″ inches of roxul, vapor barrier, 2×4 framing w/ roxul, osb, tyvek, 2″ roxul, rain screen, verticle furring strip, vinyl siding

Option B: dry wall, 2×2 furring strip, 2″ inches of roxul, vapor barrier, 2×4 framing w/ roxul, zip system R-sheating insulate, seams taped, vinyl siding

Couple of things:

I don’t want to go back and plan on 2×6. Im hoping the 2×4 with 2×2 furring strips will allow me a comparitive rvalue as a 2×6.

I would prefer to have the moisture dry outward but I want to make sure that osb doesn’t warp to make a 48′ long wall siding look warped.

2 questions:

1. Is my wall assembly sound? Whats better option A or B?
2. Would i be able to achieve a better RValue (approx 25.5 – 18.5 inside and 7 outside) for similar dollars?
3. Is the Total cost (material + quality contractor costs) for option a or option b cheaper?

I know that there quite a few passionate folks here and I’m looking for honest advice. Also I’m trying to balance eco consciousness with monthly payments.

Thank you!

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  1. user-2310254 | | #1

    There are some other things about your assemblies that are questionable. Have you read this article:

    You can certainly use 2X4 and create an energy-efficient wall, but I don't think either of your stackups are the way to go.

  2. LeoPaul | | #2

    Yes I read that article and then came across Greg La Vardera Better and Best Swedish Wall (sorry the link won't pass the spam filter). However since my contractor provided a valid payback argument, I thought I could modify Greg La Vardera Better and Best Swedish 2x6 wall with a 2x4 recommended by my contractor.

  3. user-2310254 | | #3

    I found the post on La Vardera's site. Scott Gibson also has a post here about hanging vinyl siding on furring strips. Apparently you can,make it work but some manufacturers are more open to the idea than other. That said, I would have concerns about keeping the strips and siding coplanner. For that reason, I would be more inclined to use Option B. The Zip R without furring would be simpler since vinyl siding seems to dry fine even without a gap.

  4. GBA Editor
    Martin Holladay | | #4

    L. Philip,
    The three most glaring issues concerning your stack-up proposals are the following:

    1. Most vinyl siding manufacturers do not allow their siding to be installed over furring strips. For more information on this issue, see Can Vinyl Siding be Applied Over Furring Strips?

    2. You seem to want to include a vapor barrier, even though a vapor barrier is neither required by code nor recommended by building scientists. You don't tell us what type of material you want to use for the vapor barrier, but I am guessing that you are thinking of using polyethylene, which is probably not a good idea.

    The building code requires an interior vapor retarder, not a vapor barrier. (A vapor retarder is a less stringent layer than a vapor barrier.) Vapor retarder paint on your drywall is all you need to fulfill this requirement.

    For more on this issue, see Do I Need a Vapor Retarder?

    While your wall does not need a vapor barrier, it certainly needs one or more air barriers. You need to develop an air barrier plan.

    3. The third issue is a warning, not a problem: Installing furring strips on top of exterior mineral wool can be tricky. For that reason, it's a building method that is rarely used. It's not impossible, but you should go down that road with open eyes.

  5. LeoPaul | | #5

    Steve thanks for the input. I like Option B but I hate OSB. I have a 100 year old house and I/m hoping to closely align quality but trying to stay cost competitive. The zip system Insulated board does look appealing though.

    Martin: Thank you for the extremely detailed fact based answer.

    1. I got the idea from reading way too much. You know everything they say on the we "is true". Also, since don't have single builder's bone in my body, I could have mis construed the information.

    2. Yes, I meant a vapor retarder. I want the walls to breathe. And yes I want to it to be as airtight as possible.

    3. I think the exterior mineral wool is much more expensive than reclaimed rigid board. I think I may have come across a comment by you somewhere that recycled rigid foam is a good option. It's about $20 a board @ 4". That seems to be a good price.

    So maybe this is how it should go: Dry wall, 2x2 furring strip, 2" empty cavity for wiring and pluming, vapor retarder, 2x4 framing w/ R15 roxul, plywood/osb, taped seams, air barrier, 2" reclaimed rigid foam, rain screen, certainteed cedar impressions or lap siding.

  6. GBA Editor
    Martin Holladay | | #6

    L. Philip,
    Your latest wall design makes more sense. It should work.

  7. LeoPaul | | #7

    Thanks Martin. I feel like I've graduated 1st grade. Much appreciated.

  8. LeoPaul | | #8

    Martin: in another Q&A you mentioned that if one were to use rigid foam on the outside, then you should be careful in used inside insulation, did I misunderstand? Would the use of Roxul on the inside without a vapor retarder but an air barrier be a problem in my wall design? I'm in Zone 4 outside of Philadelphia.

  9. Expert Member

    L. Philip,
    Unless you are intending to insulate the 2"x2" service cavity, is it really worth including? It represents a fair amount of additional labour, and if you aren't using the vapour retarder as an air-barrier protecting it isn't that Important, nor are any penetrations that the wiring and plumbing would create if they were in the main stud space.

    It might be worth looking at what the service space will actually contain. What plumbing fixtures are on exterior walls? An 1 1/2" cavity isn't big enough for any drains, so really what you end up with is a couple of water supplies that could be routed through the floor and a few wires.

    2"x2" service cavities make sense with Mooney walls insulated with cellulose, where they reduce thermal bridging. I don't see much benefit otherwise.

  10. GBA Editor
    Martin Holladay | | #10

    L. Philip,
    A wall with an adequately thick layer of exterior rigid foam does not require an interior vapor retarder (because the wall sheathing will never get cold enough for moisture accumulation or condensation).

    It's perfectly OK to use mineral wool batts between studs when insulating a wall with exterior rigid foam.

  11. LeoPaul | | #11

    Thanks Martin! You guys are the best!

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