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A “Pretty Good” building envelope for a partial retrofit

koselig | Posted in Green Building Techniques on

Hi all!

We have been living in a “pretty good” house in Zone 7 (near Two Harbors, MN) for a few years. It was originally built for a couple, so the 1bd/1ba layout is very inefficient for our family of 6. We are making a modest addition which will result in an overall 4bd/3ba configuration.

The house is already orientated to the South with primary glazing on that elevation. We are following that with the addition.

My plan is to use naked T-Studs 24″ OC and either BIBS fiberglass or dense-packed cellulose. But I haven’t solved the puzzle of the overall wall system.

I realize these are both probably “wrong” but the two options I’ve drafted so far are:

1. drywall / 6mil poly / T-Stud+BIBS or cellulose / OSB / wrap / steel siding

2. drywall / 6mil poly / T-Stud+BIBS or cellulose / OSB?? / 2″ GPS foam or rockwool / wrap / rain screen / steel siding

The siding is EDCO board and batten steel.

Then we have the original structure, which is conventional 2×6 framed with high-density fiberglass batts in the walls from before my time. The thought I had was the foam to the exterior would help insulate the original structure to better match the T-Studs.

My big concerns are:
1. We live in the forest. Mice and carpenter ants are both worthy adversaries. I know I cannot “win” but I want a fighting chance. What system helps us get there?

2. Is it “overkill” to do the foam plus the T-studs, or should I focus on building tightness?

For what it’s worth, we are balancing home-schooling 4 kids and running a business, and obviously doing this on a budget, so while I am sure a team of consultants and architects could easily figure all of this out, I do not have that luxury. As an engineer I am eager to figure out the best plan for our family using the resources I can find, such as this awesome forum.

Thanks all!
Patrick

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Replies

  1. Expert Member
    Michael Maines | | #1

    Patrick, it's never a good idea to have two vapor barriers (aka ≥0.1 perms) in an assembly, unless they are directly in contact with each other. Steel siding qualifies, though some drying may occur at the ribs. 2" XPS is around 0.7 perms so it allows some outward drying but not much. I would recommend switching to a variable permeance membrane at the interior, such as Siga Majrex or Pro Clima Intello.

    The blowing agents in XPS are terrible global warming agents, so if you're buying new I would use EPS or polyiso.

    To guard against ants, I would look for R-control EPS, which has a borate additive. I would also use borate-treated cellulose insulation. Both may have some resistance to mice but you should provide other rodent control methods.

    I don't know how much T-studs cost; they may or may not be worth the extra money. My guess is that if you're using continuous exterior insulation it's probably not worth it.

    In any case, focus on building airtightness.

    1. koselig | | #2

      OK cool, yeah I was guessing we needed to switch out the poly, but wasn't sure to what. I will look at the Siga and the Intello.

      The foam I was thinking about using is GPS, which is just like EPS but made with graphite.

      Do you feel that borate-treated cellulose would be superior to BIBS?

      What are some techniques we should look at for airtightness? Guessing this speaks to window installation technique, taping against vapor-sealed electrical boxes, etc.?

      Thanks!

  2. Expert Member
    Dana Dorsett | | #3

    The GPS Platinum will be north of 1 perm at 2" (unless you go for the Type-IX version, which could squeak just under 1 perm, since it's specified max would be 1.25 perms @ 2".)

    https://www.insulfoam.com/wp-content/uploads/2016/08/15003-Platinum-Insulation-TDS-v.01-17.pdf

    Using copper for drip edge & flashing, & copper-clad sill gaskets creates a toxic environment for the gut-flora carpenter ants need to digest/tolerate eating wood. Like copper, borates also kills those gut flora, and yes cellulose that uses only borates for fire retardent also inhibits carpenter ants, and to some extent termites.

    There are also commercial borated spray-on wood treatment products such as Pena Shield (tm) or Bora Care (tm) (there are others) for treating wood framing & sheathing in-situ prior to air sealing and insulating the cavities. (It's pretty cheap stuff compared to copper flashing.) Scott Risinger did a little demonstration video on how to termite-proof rehabs & new construction on his YouTube channel a year or so ago (using Bora Care for the wood-treatment part.)

    https://www.youtube.com/watch?v=LIvPla-07fo

    1. koselig | | #4

      Dana - awesome - I have seen your responses around and was hoping to hear your thoughts.

      I have been told by some that doing the foam on the exterior is "overkill" with the T-Studs, but the idea is to help even out the insulation overall since the original building is conventional 2x6. What do you think?

      And, if you think we should do the foam, would you say GPS or mineral wool?

    2. koselig | | #5

      Dana, I am curious to get your thoughts on our mechanicals too. Once we get this design nailed I will be happy to do a video series on the build, modeling process, etc.
      https://www.greenbuildingadvisor.com/question/mech-design-for-pretty-good-addition-retrofit

  3. Jon R | | #6

    It's not clear to me how that steel siding will behave in terms of moisture movement. My guess is that it will have enough air flow to be considered > 1 perm and enough to act as a pressure balanced rainscreen (but not as a capillary break). If not, < .1 exterior perms, OSB and no exterior foam is a bad combination.

    I'd build about like your #2 option, but with a smart vapor retarder and plywood replacing OSB. Bora-Care with Mold-Care makes sense to me anywhere.

    1. koselig | | #7

      understood - so maybe we should plan on minimum 1" GPS just to have the capillary break? I am obviously concerned with the labor cost, but if it's the right thing to do, we're going to find the budget.

      I was picturing lath of some sort between the foam/wrap and the siding, either furring or the contractor suggested some type of metal product that unrolls to provide a rain screen.

  4. Expert Member
    Akos | | #8

    For critter proofing, you need to think of the critter barrier the same as the air barrier. It has to be continuous through any cross sections of the house. This generally means wrapping your transitions with wire mesh. Also protecting the spray foam around rough openings with either mesh or steel wool. If you are including a rain screen, the vents in it should be wrapped with perforated J trim. Wrap the edge of any exterior insulation with mesh.

    Generally most critters won't go directly through plywood but love insulation. As much as I'm a fan of exterior rigid, I would design with something else. For interior as others have mentioned cellulose with borate is the best.

    Budget high R value wall is 2x8 on 24 OC. With a smart VB on the inside and a rain screen gap behind the siding should perform quite well in Zone 7.

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