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Hardie Board > WRB > plank boards > closed cell spray foam?

smgb | Posted in General Questions on

My house was originally built in the 1960s here in Massachusetts.  I found newspaper articles about JFK’s campaign in some of the walls, but that’s besides the point.  

The house has been resided with Hardie cement horizontal lap boards and here’s the current layout, from exterior to interior:

2×4 construction, new windows recently installed

Hardie Board > Hardie WRB > wood plank boards with gaps > old insulation + paper > maybe a vapor barrier > drywall 

I’d like to make the house less drafty and more energy efficient.  I got a quote to rip out the drywall and old insulation, spray closed cell foam to a value of R-21, and install new drywall.  

1- is an air gap needed between spray foam and the back of the plank boards?
2- will the spray foam get in between the plank boards, expand, and cause problems?  Should I have felt paper installed in the bays first?
3- is a vapor barrier needed before installing new drywall?
4- what type of spray foam is least harmful health-wise?
5- any other options you’d suggest?

Appreciate your advice here.  

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  1. woobagoobaa | | #1

    If it is not too late (sounds like it is), I would use a self adhered WRB over the gap-ed board sheathing (providing a very good water and air control layer). IIRC the Hardie WRB is mechanically fastened and probably leaks more air.

    1. smgb | | #2

      You’re right, the Hardie board / plank is already installed. Only access is via the interior now. My research on this forum seems to indicate closed cell foam will be an air sealer and provide more insulation R value than whatever is in the walls now. Plus I’d try to have them spray the rim joist and the intersection between the first and second floors, if they’re able to access it. I want to confirm this is the best route to go before signing the contract. Thanks.

  2. Expert Member
    Michael Maines | | #3

    Closed cell spray foam ages to about R-5.5/in, and in a 2x4 wall the average depth would be about 3", so you would have about R-16.5, not R-21. Factor in the studs and the wall would be about R-15.

    All closed-cell spray foam is a potential health risk, especially if it doesn't cure properly, which fortunately is a fairly rare problem but it happens often enough that we have all heard horror stories.

    As long as the foam remains adhered to the studs, it will be an effective air seal, though it's common for the foam to pull away from the studs and then it's no longer an air barrier.

    It would be an effective vapor retarder, even if it pulls away from the studs.

    If the gaps between sheathing boards are large, the foam could cause problems. If the gaps are 1/4" or less, it's probably not a concern.

    Due to the many risks and marginal benefit of spray foam, I would use a safer, more benign product, even with a slight loss in R-value. You could fill the bays with dense-packed cellulose, fiberglass batts or mineral wool batts for about R-13 and add a separate membrane such as Siga Majrex for air and vapor control.

    1. smgb | | #4

      I did and still do have reservations about spray foam due to health and environmental impacts. Since there are additional risks you raise…What about using rigid foam on the interior? I hear you on cellulose, but if I’m going to rip down the existing drywall, I’d like to get as high a performing wall as the budget will allow (~$5k).

      I’m in Massachusetts, 2x4 framing, lap board sheathing.

      So from current:
      Hardie Board > Hardie WRB > wood plank boards with gaps > old insulation + paper > maybe a vapor barrier exists? > drywall

      To option #2:
      Hardie Board > Hardie WRB > wood plank boards with gaps > 3.5” of mineral wool > 1.5” to 2” of EPS, taped at the seams > horizontal strapping > drywall.

      That’ll provide a R value of ~20 before thermal bridging etc? I don’t have a good handle on how to calculate that.

      Four windows and one door will need jamb extensions, and the electrical boxes will need extensions as well.


      1. Expert Member
        Michael Maines | | #5

        That would work; including the studs and 1.5" EPS, the wall would perform at about R-19.

        You would get the same R-value using this approach, using less foam: EPS and mineral wool have similar levels of carbon emissions so this would only be a benefit if you were using fiberglass, or ideally cellulose or wood fiber insulation.

        1. smgb | | #6

          I don’t have access to that FHB article unfortunately

        2. smgb | | #7

          And I should ask: the plank board sheathing has gaps between them. Even with the WRB, will moisture be able to dry to the outside? This is assuming I move forward with EPS on the interior.

  3. smgb | | #8

    I’ve finally figured out how to attach a picture - see attached picture of the sheathing boards with gaps.

    I have decided not to use spray foam.

    Would this option #3 allow us to be “comfortable”, meaning not feel the draftiness that currently exists and be a fairly decent performing wall, assuming the details are done correctly?

    And can peel ‘n stick Henry Blue skin be applied to the back of the sheathing boards (inside the stud bays) to act as an air sealer?

    Option #3:

    Hardie Board > Hardie WRB > wood plank boards with gaps > Henry Blue skin on the back of the board sheathing and spray-can-foam to air seal the top and bottom plates as required > 3.5” of R-15 mineral wool > Vapor Barrier > air tight drywall method > primer and paint

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