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I am remodeling a one car garage in an old house. As part of the remodeling, I am taking all the insulation out (in the walls that have insulation). It is a combination of faced and unfaced fiberglass insulation and rigid insulation. The fiberglass insulation is all jambed into place (not sure if it is as an artifact of time or the installation was not done right). The wall shared with the house has no insulation!

I'm leaving intact the vapor barrier (tar paper) between the 2x4 studs and the exterior walls that have siding. The studs are a variety of distances ranging from 13"-18" apart. Most of the studs are a non-standard 18" apart center to center (but even those are not exact). The house is in Central Texas.

I am thinking of buying 24" faced fiberglass insulation (pink one) and cutting it to be 1/2" bigger than the opening (whatever the opening is in the bay I am working on). Installing the faced side towards the inside of the garage. Is this the right thing to do around all the exterior walls? So that I don't have too many wasted strips can I tape (what type of tape do I need?) two pieces together and keep the integrity of the vapor barrier and insulation?

Also, I see that Owens Corning has a FastBatt product that doesn't have flanges. This is supposed to be held between the studs by friction against the studs and not by staples through the flanges. Any opinions on this product? I would like to use this in the shared wall with the house. Given that my access to the wall is from the "outside" of the house, I won't have the ability to stable the flanges to the interior of the 2x4 studs. Hence, I was thinking of installing this type of insulation with the faced part towards the house sheetrock and let friction keep it in place. Then, install some type of vapor barrier on the side of the studs facing the garage. Then, install the fireproof 5/8" drywall.

Thoughts on both the exterior walls and the shared wall are appreciated.

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    Out of curiosity, why are you removing the old insulation? For design reasons, or is it performing poorly?
    – Joel Keene
    Commented Apr 25, 2015 at 17:58

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Vapor barrier (if any) goes on the warm side of the wall. That's the inside for those of us in the frozen North (on average, for the greater part of the year.) In central Texas I'm guessing that might be the outside, on average.

Best solution to messy irregular stud bays might be damp-spray (aka glue-spray) cellulose (not a DIY product, but really good at dealing with irregularities and no vapor barrier is needed.)

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The paper facing on insulation acts as a vapor barrier that should be only on the interior side of the wall, not the exterior. DO NOT put a vapor barrier on each side as it will cause condensation. Since you are working from the outside, you could install Tyvek or visqueen to the studs, wrapping them, so to speak, then a non-faced insulation held by friction, followed by your 5/8" drywall. The variations of stud centers would require purchasing 24" insulation. Good luck.

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