Kraft faced insulation
tjones1014
| Posted in General Questions on
CZ 3A/4A (N GA area)
2001 gut reno
Our insulation company wants to use kraft-faced R-38 batts on our attic floor (blown-in would be much more difficult because of accessibility issues). They would put the kraft side of the batt facing the downstairs ceiling/drywall side.
I know kraft can act as a vapor retarder. Is this a concern in our humid climate?
Additionally, the ceiling joists are 2x8s I believe, and the batts are about 12″ thick. The batts would stick up above the joists quite a bit, leaving small gaps between the edge of each batt. Is this a thermal bridging concern?
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Replies
Vapor drive usually isn't an issue with kraft paper, which is a sort of low-grade vapor retarder. Vapor BARRIERs like polyethylene can be a problem, but that's not the case with what you're considering here.
Yes, the gaps between the ceiling joists are an issue, because you have essentially no insulation there. Common bast practice when using batts in a situation like this is to put in batts of the correct depth to fill the space between the joists, then put a second layer of unfaced batts above the joists running perpindicular to the joists, using thick enough batts for the second layer to get the assembly up to your target final R value.
I'm amazed they think blown (loose fill) insulation would be harder to install here. Blown insulation is usually cheapest/easiest because it's the least labor to install. I don't know how the "accessibility issues" would make batts easier than loose fill -- it's almost always the other way around. Loose fill insulation also covers all the ceiling joists, so you get a better final result since you end up with a sort of homogeneous layer of insulation that fills in all the gaps. This is why blown in insulation tends to both be cheapest to install and the best performing way to insulate an attic floor.
Unfortunately, there are a lot of knee walls, so the space in the attic is tight. I think they're concerned about being able to get proper coverage with drywall up, especially in our devil's triangles. Honestly, I'm not 100% sure why they're leaning so heavily towards batts.
They mentioned we could have our drywallers install the ceilings only, then do blown-in FG, then have the drywallers come back again to do the walls. We were trying to avoid multiple drywall trips, but it sounds like blown-in is best for getting proper coverage and would just be better overall, so maybe that's worth the extra cost if we have to go that route for this.
Additionally, we have a hip roof and aren't sure how to maintain proper ventilation in those areas at the soffits (since traditional baffles don't fit well in those areas). Any suggestions for maintaining ventilation with a hip roof?