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So fiberglass batts 12" thickness. The goal is to not let the insulation become compacted or have gaps between unconditioned and the conditioned space.

When a drywall nailer is fastened to the top plate of a wall that means if projects into the attic space. The fiberglass forms an arch over the protrusion.

Is the most preferred method to slit the kraft paper and hollow out fiberglass so it sets flush with the future ceiling plane? Is there a gain doing this...assuming the insulation would otherwise just have a crown in the attic meaning still the same thickness?

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no.no.no. don't slit the paper. its there as a vapour barrier. having your insulation humped up over the backing is no issue, as long as your vapour barrier and insulation are complete and with full integrity. just make sure that there are no gaps between the adjacent insulation and your humped piece of insulation (ie. where cold air can get down against the cold side of the drywall. here in Ontario, nobody has used paper backed insulation for years. we have the requirement of a seamless plastic barrier, taped, pocketed and sealed at all penetrations to make a giant air balloon out of every heated space. the logic is the same for any place on earth, even if you are working with old materials or new materials that use old technologies.

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  • True on the insulation humping over a partition block or other drywall nailers, no need to cut the paper over something that changes the plane 1 1/2". Just a mention, what may be required in Ontario will not necessarily be in other areas. The OP did not say his location, but did say the insulation is kraft faced. With the inherent gaps where the paper is stapled to the joists, an extra cut, where appropriate, is not the end of the world. The batts or rolls in some cases have to be butted together where the next piece is added to complete a run in a joist system.
    – Jack
    Commented Nov 28, 2015 at 1:29
  • The vapor barrier is only the paper facing...and it is interrupted by every seam. Colder climates enforce a barrier on the walls And ceilings. My main question is the concern of time taken to 'notch' the insulation around blocking instead of having it be lumped over the blocking.
    – CCCBuilder
    Commented Nov 28, 2015 at 5:56
  • here in Ontario, we used to have waxed paper vapour barriers back in the sixties, but even then it was standard practice to tape every joint, and code was that the paper had to lap 3in over onto the adjacent paper. batts actually came with a 3" overhang on the paper to make sure that it was done that way right out of the box. it sounds like that's not your standard there, but with 12" fiberglass, you are trying to get r30 or higher on your OIV in the ceiling there. out of curiosity, where are you located? and when was your code standard last amended for insulation and VB requirements?
    – personal privacy advocate
    Commented Nov 28, 2015 at 18:26
  • @personalprivacyadvocate zone 5. Not sure what current code the city uses but I believe it is 2005 or 2008. IRC2012 says the r-38 in attic is required for zone 5 if I remember right.
    – CCCBuilder
    Commented Dec 3, 2015 at 1:09
  • what city? state? USA or Canada?
    – personal privacy advocate
    Commented Dec 3, 2015 at 2:37

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