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Community and Q&A

Double stud retrofit with T1-11 existing siding (perm ok?)

maine_tyler | Posted in Green Building Techniques on

(Zone 6a, Maine)

I’m going to be insulting a garage, which is currently 2×4’s open to the inside with t1-11 siding painted red on the exterior. Nothing else in the assembly currently.

My plan is to build an inner 2×4 wall to make it a double stud assembly, resembling the ‘Lstiburek’s ideal double stud wall’ but likely with intello plus in place of the sheathing on the ‘exterior of the interior wall.’ ala 475

For clarity, the plan is, from inside to exterior:

-tongue and groove (or shiplap) pine running horizontal (its a shop so I want to be able to hang stuff anywhere)
-2×4 non load bearing wall with batt (roxul?) insulation and acting as service cavity
-smart vapor retarder and air barrier (intello +?) attached to exterior of interior wall. must allow cellulose blow in.
-space between framing (2-4″) filled with dense pack cellulose
-2×4 wall (existing) with a continuation of the dense pack (one blow)
– t1-11 sheathing (existing)
– WRB?
– rain screen gap?
– cedar shingles?

I have a few questions.

1) Most generally, does this assembly seem like a good choice to retrofit a garage for heat given that I want a high performance wall at a low price point (labor will be done by me, i.e. free).

2) Will the T1-11, being painted (unknown type of paint), cause any issues with permanence/ drying potential? I am not 100% set on it, but I think I will add shingles to match house, in which case would a WRB installed over the T1-11 (with a rain screen) be the best route?)

3) I am debating the size of the wall gap. Do I need to be concerned with some ratio of where the air/vapor barrier is in the wall (I read something of a 30 to 70 ratio, 30 percent of wall inside to 70 percent outside of barrier). I don’t remember where I read that and can’t find detailed articles discussing it.

4) Any reason I should use DB+ vs intello+ vs membrain? (Can blow behind membrain?)

5) I will be installing foam on exterior of slab/stem walls. The t1-11 currently comes closer to the ground than I would like. Any reason not to cut away 6″ or so of the t1-11 and then extend the foam that much higher up the wall for better splash protection? I suppose I would loose the connection to the sill plate if I did that… Perhaps foam extending over the t1-11?

Lots of questions, I know. thanks for any help!

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  1. maine_tyler | | #1

    * vapor permeability, not permanence :)

  2. charlie_sullivan | | #2

    Overall seems like a reasonable plan.

    I'm not sure whether to be worried about the permeability of the paint. It does seem like a significant unknown. It might be a reason to add to the outside ("Larson truss" style) rather than the inside, so that the T1-11 becomes like the "smart vapor barrier".

    I wouldn't worry about ratios--that would really only be a consideration for hot, humid summers with A/C inside, but the smart membrane is designed to mitigate that problem anyway, even if the membrane is all the way on the inside.

    I don't think Membrain is strong enough to dense pack behind. I don't know about DB+ but Intello is definitely strong enough. You could also double up--use open "netting" to hold the dense pack pressure and Membrain over or under it. But I doubt that's worth the trouble especially since it's unproven, as far as I know.

  3. charlie_sullivan | | #3

    Continuing with my answer to your last question, 5.

    One concern with extending the foam up would be providing a path for carpenter ants who get into the lower foam to get up into the house. Mineral wool is probably less hospitable to carpenter ants.

  4. maine_tyler | | #4

    Thanks Charlie for the thoughts!

    I did consider the idea of building out externally, but it felt a bit more complicated due to having to deal with the roof overhangs and since one side of the garage is currently flush with the house (and so would become proud if built out further). I wouldn't mind keeping all the internal space however.

    I hadn't even thought of ants. I'm not sure they are a problem here, but of course I don't know that. Perhaps the Roxul board would be a safe bet...

  5. charlie_sullivan | | #5

    Maine certainly has carpenter ants. I suspect that they are more of a problem the closer the woods are to your house.

  6. KeithH | | #6

    Why not just do 2 layers of Roxul? Seems like blow in for half the wall is a complication?

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