Insulating exterior walls that already have existing kraft faced batts
I have a split-level home built in 1967 that has existing kraft-faced batts with the barrier facing towards the interior of the house. The batts are somewhat deteriorated and have very little insulation value at this point, it seems, as the house has been cold (especially the lower level) this winter. I was planning on having insulation dense packed into the cavities, but am reticent to do so from the the inside as this would suspend the kraft face between two insulation layers even though the batts are barely 1.5″ thick. Additionally, dense packing from the interior would force that layer towards the exterior. I live in the Pacific Northwest so I have concerns about moisture in the wall. Insulating from the exterior is not really an option at this point, because it would likely damage the aluminum siding, which I am not yet ready to replace. Should I even be worried about this? If so, are there other options for insulation I could employ to avoid the risk of moisture in the wall?
Please note: The attic has been insulated and properly vented already, so no discussion there is necessary.
Many Thanks!
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Replies
>" I was planning on having insulation dense packed into the cavities, but am reticent to do so from the the inside as this would suspend the kraft face between two insulation layers even though the batts are barely 1.5″ thick. Additionally, dense packing from the interior would force that layer towards the exterior."
An ASPHALTED kraft facer is NOT a vapor barrier, and will not create a moisture trap. As the moisture content goes up, the facer becomes vapor open. When the moisture level is low, it's a Class-II vapor retarder- fairly vapor tight. So it's always going to be sufficiently vapor open when it NEED to be.
>"I live in the Pacific Northwest so I have concerns about moisture in the wall."
An ALUMINIZED kraft facer can be a true vapor barrier (or close to it) but it doesn't really matter in your climate zone. Even if that 1.5" batt get's squished fully in half to 3/4" it's R/inch will run about R3 is more that sufficient R value for dew point control on 2.75" of dense packed cellulose (R10), since more than 20% of the total R is on the exterior of the vapor barrier. The code prescriptive for dew point control on a 2x4 wall in zone 4C is for a minimum of R2.5 on the exterior of a presumptive R13 cavity, or about 16%. You have plenty of margin here.
https://up.codes/viewer/washington/irc-2015/chapter/7/wall-covering#R702.7.1
Also, aluminum siding is inherently back-ventilated, meeting the "Vented cladding over wood structural panels" exception that allows use Class-III vapor retarders (standard interior latex is good enough) on the interior side of the assembly, if it happens that the kraft facer has crumbled away completely in places.