A “Pretty Good” building envelope for a partial retrofit
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.
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