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Community and Q&A

Siding replacement and exterior wall upgrade

TJO1488 | Posted in Energy Efficiency and Durability on

My house is in central Ohio so in zone 5A. It was built in 1987 and has the following wall characteristics:

– 2×4 frame with kraft faced batts
̶- plywood corners for shear (pretty sure its 1/2 inch)
̶- Foil Faced polyiso panels for rest of sheathing field (not taped at seams, pretty sure 1/2 inch)
̶- no WRB
̶- vinyl siding

When you look down a wall there are definite waves where the studs are located.

I am looking to replace the vinyl and need to address the waves along with r-values and WRB.

I’m not sure on how I should go about doing this. My thoughts have been:

1st Option:
– Replace polyiso with plywood or OSB to correct the waves
̶- Add XPS with taped seams for thermal break
̶- possibly add furring strips depending on XPS thickness
̶- LP Smartside or Hardie

2nd option
– Leave polyiso on
̶- add ZIP-r sheathing (thinking 1 or 1 1/2 for thermal break) over plywood corners and polyiso field with taped seams. My thought on this is to utilize the polyiso that is in place and add to the R-value of it with ZIP-r
̶- mesh rainscreen
̶- LP or Hardie

Neither of these options are ideal around windows and doors as I would have to pull and reset most likely due to being inset and having a flange, but my thought is to do it right the first time

I am open to any other options and any advice on how I should proceed would be greatly appreciated


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  1. GBA Editor
    Martin Holladay | | #1

    Either of your options will probably work, although you might want to run Option #2 by an engineer. (Fastening Zip R-sheathing over polyiso messes up the engineering assumptions for bracing -- but in your case, your walls may already have enough bracing with the plywood corners and -- who knows? -- possible diagonal steel strapping.)

    If you want to install more rigid foam, I advise you to avoid XPS, which is manufactured with a blowing agent that has a high global warming potential. Either EPS or polyiso is a greener choice. For more information, see Choosing Rigid Foam.

  2. Expert Member
    Dana Dorsett | | #2

    In that location adding an inch of polyiso over the existing shear panels & thin polyiso would bring the performance up slightly ahead of IRC 2015 code-minimum performance. Check the fastener spacing & types on the shear panels to make sure they're up to snuff before covering them.

    Plywood wouldn't necessarily correct the waves any better than adding 1x4 furring over a layer of foam and using the more rigid siding. Vinyl is pretty floppy stuff, and if it's tight to the foam any waves in the foam will appear. With 1x4 furring at every stud there'll be 3/4" of space to accommodate any waves, and fiber cement siding is pretty rigid & flat stuff.

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