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So I am going to use 12" R-38 kraft (paper) faced fiberglass insulation in my attic. I plan on face stapling the paper to the bottom chord of each truss. I imagine most of the seams sideways where stapled will overlap.

My question is since I am working with batts 48" long...should I tape the seams where each one meets? The paper facing is to serve as my ceiling VB.

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  • Vapor barriers should be installed to the conditioned side of the space. Putting it on the other side doesn't prevent the condensation that occurs when warm, moist air inside contacts cold air outside (somewhere in the insulation).
    – isherwood
    Commented Mar 20, 2016 at 13:36

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Yes, you tape any insulation's lateral seams, especially any fluffy stuff...aluminum HVAC tape lasts the longest. The fluffy stuff really only works best in dead air space. Which also means the un-faced side (where you don't tape anything). If there's a good swap of air or you can feel a breeze when it's windy out, you'll want to seal that out.

1/4" Rigid Foam boards are good for this as they'll seal out most air movement, but still allow humidity to escape into the venting. I think they still have them, the accordion style works very well if you have a number of larger open areas within the truss structure. Otherwise, get the same size as your batts & just put the foam on top.

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I have never taped or seen it taped, butt the batts tightly together, If it makes you happy tape, most masking tape really wont hold for long on kraft paper because of the temp swings the glue migrates into the paper.

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  • Ah, I agree you don't see it taped...normally, if ever. But you also normally see attic insulation rolled up in longer lengths than 48" - this results in 6 to 7 seams per truss cavity. Multiplied by 40 to 50 truss cavities results in a lot of seams that I do not know if is normal. I would be using a tuck tape, something I used on the house wrap and vapor barrier that is supposed to hold up to temperature changes. The temperature of the tape at the seam however should only be a delta of 5 to 10 degrees from my interior temperature.
    – CCCBuilder
    Commented Nov 20, 2015 at 1:45
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You don't need to. In fact, you don't need kraft-faced batts at all. In fact, you'd be better off blowing cellulose into the attic. Buy a bunch of bags of the stuff and you get the blower machine for free. It's higher-performance.

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