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Foamboard as sheathing?

dsummers | Posted in Energy Efficiency and Durability on

I am replacing my fiberboard lap siding with James Hardie siding and I am trying to improve my insulation at the same time (on a budget)   Thank you to those who replied to my other questions.

Given the sky high price of OSB sheathing right now, my siding contractor has recommended skipping it completely and just sheathing the house in 1.5″ XPS foam.

The house is in the Denver area.  2 story.  1600 sq ft of living space, about 2500 sq ft of wall siding area.

My current house (built in 1992)  has the following wall construction (from outside to inside):
 
1. Paperboard lapboard siding
2. 1/8″ thermaply sheathing
3. 2X4 studs with R13 fiberglass insulation
4. poly vapor barrier
5. 1/2″ drywall
 
 
 
Our new wall assembly would be:
 
1. James Hardie lap board siding
2. James Hardie house wrap
3. 1.5″  DOW XPS unfaced foam board (R7.5)
4. 2X4 walls with R15 fiberglass batts
5. poly vapor barrier
6. 1/2″ drywall
It is unclear whether or not the bid I have includes a rain screen between the siding and the foam.

Based on my research, it seems that air sealing the foam over time might be an issue.  Shrinkage over time will pull the seams open and allow for air infiltration.   Are there ways around this?  Special tapes?

The OSB sheating (and associated labor) adds about $7K to the project, and that $7K is not going to bankrupt me.  It is about 10% of my total project budget for siding, windows, and insulation.     Would most people recommend just paying for the OSB?

Colorado is one of the US states that has mandated low global warming potential XPS foam.   Does that mean that it is considered OK to go with XPS as long as it is the new formula?  My local big box stores still have a mix of the old and new stuff.   Are the new formulas better or worse than ISO and EPS in terms of global warming potential?

Thanks again for all of your help,  I am learning a lot from you all.

-David

 

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    David,

    Something is necessary to provide shear strength for the exterior walls. Either sheathing, or diagonal bracing. The OSB also adds a lot more resistance to high winds, and is much easier to detail as an effective air-barrier than foam.

    Against that is that the walls will perform better from a moisture management perspective without the OSB to worry about.

    My preference would be to include sheathing.

  2. user-5946022 | | #2

    You need something for shear.
    If this contractor really suggested completely eliminating the structural sheathing as something that is ok, and is not installing a rainscreen behind the Hardie, you need a new contractor. This contractor does not know what he/she is doing. I would be concerned about what else they don't know and/or don't care about.

    1. Expert Member
      Akos | | #3

      Not necessarily. In some areas interior drywall is considered bracing. I would prefer something much more solid though even if not required.

    2. dsummers | | #5

      The argument is that my house never had true structural sheathing. All it has is thermo-ply which is just foil faced 1/8" paperboard. I do live in a high wind area, and the house does shake pretty badly in the wind storms which tells me that whatever bracing it has is not good enough.

      I think that you've all convinced me that I need the OSB.

      -David

  3. Expert Member
    Akos | | #4

    I think the hardest thing to get right on a wall without sheathing is critter proofing. It is possible at the siding level but much more reliable with sheathing.

    As a budget alternative to OSB you can look at is exterior gypsum sheathing. Provides bracing, easy to tape the seams for a good air seal and quick to hang. About the only issue with it is that it won't hold nails, which should not be an issue for an installer that was happy to install directly over foam.

    You definitely want a rain screen for cement board siding. Makes a big difference on the life of the finish.

  4. nynick | | #6

    My contractor built my cottage in Canada with diagonal metal bracing and foam boards....no sheathing. Work fine and the house was well insulated.

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