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Exteror wall construction / insulation — new construction

Ani Brown | Posted in General Questions on

Hi, I live in Zone 5a Northern IL and am building a new home.
I have questions on exterior wall insulation/construction.
I’m confused on interior vs. exterior drying/condensation/dew point and insulation.

My architect suggests the following standard construction method. 2×4 studs 16″ On center
Masonry |House wrap Tyvek| OSB| wood studs with fiberglass batts in between the studs |drywall

Is there a better construction method?
Will the studs act as thermal bridges? Should I be worried about this? What would be best insulation method outside vs inside?? In regards to drying/condensation??

I mentioned to the architect about rigid foam insulation on the outside but he said something about not liking it because it did not provide any structural support. How is this addressed?

What would be the optimum construction method? Something like this?
Masonry | Air Gap | Tyvek | Rigid Foam |wood studs with spray foam in between | drywall ??

Thanks!
Ani

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Ani,
    I'm not sure what type of masonry exterior you are planning. Will this house have brick veneer?

    Q. "Is there a better construction method?"

    A. Yes. The wall assembly suggested by your architect is probably the worst possible wall assembly that it is legal to build.

    Q. "Will the studs act as thermal bridges? Should I be worried about this?"

    A. Yes and yes.

    Q. "What would be best insulation method outside vs inside?"

    A. There is no single answer to your question. However, many cold-climate builders are aiming to build an R-40 wall. After taking into account the thermal bridging through your studs, the wall assembly proposed by your architect has an R-value of about R-9 or R-10.

    There are lots of ways to improve the wall: you can use 2x6s instead of 2x4s; you can include rigid foam insulation on the exterior (many builders install 4 inches of exterior rigid foam); you can build a double-stud wall that is 12 inches thick and insulate with dense-packed cellulose.

    If you are planning to install brick veneer, I think it's a good idea to include a layer of rigid foam. The foam will improve the R-value of the walls, address thermal bridging through the studs, and stop inward solar vapor drive, which can be a problem with brick veneer walls.

    Q. "I mentioned to the architect about rigid foam insulation on the outside but he said something about not liking it because it did not provide any structural support. How is this addressed?"

    A. Here is a link to an article that will answer your question:
    Four Options for Shear Bracing Foam-Sheathed Walls
    .

  2. Ani Brown | | #2

    Thanks Martin.

    The house will be in the Tudor style and will have stone on the lower half and stucco on the upper half.
    I understand for stucco it gets even more complex, but for the sake of discussion let’s just look at the lower half.

    I have read the article you have provided.
    So a layer of OSB or Plywood below the rigid insulation would provide structural support?
    Which material should I use OSB or Plywood?
    The OSB sounds better structurally, but the plywood sounds better for moisture control/drying?

    What are the pros and cons of this wall assembly?
    Stone | 2” Air Gap | Tyvek | 2” or 4” Rigid Foam | ½” Plywood | 2x4 wood studs @16” o.c. with spray foam insulation in between | drywall | Paint

    How does this handle drying?
    Is there a preference in materials for the house wrap Tyvek vs Typar vs R-Wrap vs..?
    For the Rigid Foam, I’m thinking XPS, but should it be foil faced and if so which side?
    For the spray foam, I’m thinking open cell? Is this correct or should it be closed cell?

    Ani

  3. GBA Editor
  4. Ani Brown | | #4

    Thanks Martin for the links.

  5. Ani Brown | | #5

    Martin thanks again for the links, very good reading.

    Based on the articles I have modified my wall assembly thinking.

    What are the pros and cons of this:

    3 coats of stucco/stone masonry veneer
    Mortar screen-wire lath (galvanized or fiberglass)
    1 layer of asphalt felt or Grade D paper
    Delta-Dry Rainscreen
    Tyvek or Typar
    2” Rigid Foam (un-faced XPS)
    OSB
    Wood Studs with Spray Foam (Open Cell) in-between
    Drywall
    Paint

    Or am I over thinking this??

    Thanks!
    Ani

  6. Expert Member
    Dana Dorsett | | #6

    2x4 with batts-only doesn't even meet code in northern IL.

    Either 2x6 with R20 (not R19) or 2x4/R13 + R5 continuous insulation (an inch of polyiso or XPS) are what code requires.

    Masonry & stucco contractors start whining about cu$tom masonry tie$ whenever the foam is more than 1" thick. If you went with 2x6 / R20 AND R7.5 (code min for not requiring an interior vapor retarder) you would have literally twice the wall performance as the sub-code 2x4 with batts assembly. If they piss & moan about not being able to do it with 1.5" of foam and insist that it can't be done , cut back to 1" of foil-faced polyiso (not XPS), and use R23 rock wool (if batts) or damp sprayed cellulose (preferred), and use a "smart" vapor retarder such as Certainteed MemBrain or Intello Plus on the interior (but under no circumstances is poly sheeting a suitable substitute or you'll have a severe moisture trap.)

    In general polyiso is the "greener" than XPS due to the high global warming impact of the blowing agents used for XPS. And in the event of a fire polyiso won't melt- it chars in place, and has a higher ignition temp to boot. Wherever you can (and insulating sheathing is one of those places), substituting polyiso is generally the better way to go, and it even costs slightly less per unit-R in most markets. Derated properly for temperature in a sheathing application in your climate figure an inch of polyiso is R5.5 ~0F outdoor temps, but R6 or more when outdoor temps are above 25F (which is more than half the binned hours of the 3 coldest months in most N.IL locations.)

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