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I live in Portland Oregon and I did a second-story addition that has 2 x 12 vaulted ceilings. I plan to put in high density R-38 and I have a continuous ridge vent at the peak. I will have 1" plastic baffles from the bird locks to the peaks and most ceiling rafters are 24 on center.

My question is related to what should I do about the vapor barrier options.

I could get unfaced batts, or Kraft faced batts.

If I get the Kraft faced batts, then all I have to do is put them up and I'm done with the vapor barrier basically.

If I put up the unfaced batts, then it seems like I have the option of putting a four-mil plastic vapor barrier or over the ceiling surface or I can do some kind of spray on vapor barrier.

I kind of like the idea of the Kraft faced batts because it seems like having a little bit of breathing around the edges might not be a bad idea, but all the installers seem like they'll do a plastic thing to save cost and time..

I've also toyed with the idea of going with unfaced batts and doing plastic on the lower part but putting tyvek at the peaks for a little bit of breathing for any moisture that gets stuck in the peaks..

Basically I'm not sure what the goal is and what the best way to achieve it would be..

I had a problem before in my vaulted ceiling where water would condense on the roof sheeting when it would freeze and then when it got above freezing the water would run down the ceiling and drip on the floor.

My father inlaw had a problem on his vaulted ceiling where water would condence on the ceiling and mold would form because there was no attic ventilation.. Seems like if I put in a solid vapor barrier I'd have the mold problem..


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The problem your fil had with condensation at the peak of his vaulted ceiling was simply cold drywall and humid air. Two things in play here, moist air is lighter than dry air, so along with warm air being lighter, the higher moisture is up there as well. Then, few vaulted ceilings have sufficient insulation. Remember, code level requirements are NOT the best, they are bare minimum. Add in that fiber insulation is very susceptible to air circulation and often performs below its sticker value.

We are still in an age where we are testing by trial and error on how best to insulate vaulted ceilings. IMO, I like a couple of inches of rigid foam board installed to form the vent channel with 1.5 to 2" of vent space. Then fill with Roxul and if code requires, a vapor barrier. Personally, I used plastic vapor barriers long before they became unpopular and they did fine, but I'm not in your climate zone. Check local requirements.

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