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Community and Q&A

A good foam sandwich?

rocket190 | Posted in Green Building Techniques on

Hi All,

I’m In the very early planning stages of a new home construction near Green Bay Wi (climate zone 6). The house will have a full basement. 

I would like this house to be extremely well insulated, but all fairly conventional to build. I want this house to be retirement friendly, so looking to use exterior cladding that will require minimal future maintenance. One problem with using brick or stone siding is the brick ledge requires a thick, and expensive foundation wall. This is compounded when using thick exterior foam, and also makes brick the attachment more difficult. I thought of a wall concept that I haven’t seen before, which seems very easy to build. 

wall assembly from outside in.

1.  Brick or stone exterior
2.  WRB
3. 2×4 structural wood stud wall 16” or 24” oc framing insulated with rock wool batts (no exterior sheathing)
4.  1/2” plywood “interior sheathing” with Zip taped seams detailed as primary air barrier
5. 4” of salvaged EPS foam taped or spray foam at seams (complete thermal break foam sandwich) (secondary air barrier)
6. 2×4 non structural wood stud wall framed 16” oc insulated with BIBs or wet blown cellulose
7. 1/2” gypsum board

Total wall thickness 12”.

Apparent benefits:
-easy to build
-east to air seal
-complete thermal break
-quick to dry in, can air seal
-bulk water leaks in WRB could drain through rock wool
-easy to integrate with brick ledge and easy to attach masonry

Apparent disadvantages:
-bugs or rodents could make it into rock wool?
-fairly thick to the interior, which would eat up some finished space
-shear strength not as strong?
-more lumber

Thoughts on overall wall? Sheathing wouldn’t be as warm as an external foam wall, but would the rock wool help?

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  1. Expert Member
    Akos | | #1

    Plywood should go on the outside, much easier to detail it as your main air barrier there since the floor joists are not poking through it.

    That is about an R43 assembly, if you skip the EPS and dense pack the double stud, you end up at R42. Not sure if it is worth your while to complicate the assembly if you are already dense packing. Something like this makes sense if you are DIY and want to use only batts.

    Otherwise, the assembly should work great and provide you with a durable assembly.

    As for rodents, they will happy live in any fluffy or rigid insulation, so you must keep them out of your wall. This means you need to detail your rodent barrier as well or even better than your air barrier. There can't be any holes or discontinuity in it.

  2. rocket190 | | #2

    Akos, thanks for your response. Can you explain what you mean by floor joists poking through the wall?

    My reason for not going to double stud wall is my concern over cold sheathing and moisture build up. Do you think 3.5” of rock wool would help keep the sheathing warmer in winter months?

    1. Expert Member
      Akos | | #3

      With the air barrier on the inner face of the stud, it needs to go under the bottom plate, down across the rim joist, back across the top plate to continue down the wall. Lot of transitions. Transitions are where air barriers leak.

      Cold sheathing is not a problem provided you have vented cladding. There are a lot of standard double stud wall houses that have this. The sheathing moisture content does increase in the winter but it quickly dries by early spring.

      Plywood is already a better sheathing material as it is more permeable and can handle moisture better. Make sure to follow proper brick cladding details, have proper vents on the top and bottom, minimal mortar droppings and good flashing details.

      Any insulation will make your sheathing colder, so the only way to get it warmer is less insulation, which we don't want. What can definitely help is cellulose. Unlike any other insulation, cellulose can buffer and redistribute moisture in the walls plus it is also the greenest insulation out there.

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