Plaster and Lathe = vapor barrier?


Old 12-28-03, 07:04 PM
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Plaster and Lathe = vapor barrier?

I am slowly reclaiming one room at a time from the winter.

Right now I'm working on a small room (7' by 9') with no ceiling insulation. It has a suspended ceiling 12 inches below an old plaster and lathe (and paper) ceiling. Above that are 2x8s and then the upper attic floor (easy to lift up and get underneath.)

My thought is - build a new sheetrock ceiling 12 1/2" below the lathe and staple up some faced R38. Then lay down unfaced R11 on top of the lathe, below the attic floor, to bring it up to R49 (for my area) and hopefully stop heating the attic...

Anybody have a guess as to whether the plaster will come back to haunt me? Will it act as an evil second vapor barrier? Will it do the opposite and hold in moisture between the layers and grow some exotic mold colony? Should I just knock the plaster and/or lathe down now?

It's a small room this time, but there are two old bedrooms (16' by 20') that I'll be doing in the future... and that plaster dust hangs in the air for days... (Not a good thing on the domestic front!)

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Old 12-29-03, 12:59 PM
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If the old ceiling has been painted with oil based paint, it will act as a vapor barrier.

Can't you just add enough blown insulation into the attic to get you to your desired R-value (by removing the flooring)? I think 15 inches of cellulose would get you to where you want to be, while keeping the nice plaster ceilings, and not loosing any ceiling height.

If the old plaster ceilings are starting the crack and fall out, then just go ahead and remove them. If you don't, they will eventually fall and crush any interstitial insulation rendering it ineffective.

Caveat: You can't insulate any area that has old-style knob & tube wiring. All this needs to be upgraded to modern 90* wiring.
Old 12-29-03, 08:01 PM
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Ahhhh... oil based paint. I'm sure there's a layer of it somewhere between the at least five layers of paper on the ceiling. (I hope that's not load-bearing paper...)

The 9' walls were redone with 8' of sheetrock, leaving 12" of old plaster on the walls (above the suspended ceiling.) I don't want to lower the ceiling (or raise the attic floor) so fiberglass on either side of the lathe seemed to be the best way to do it.

And good point on the eventual collapse of the plaster... I'll get the rubber mallet, dust mask, and duct tape (seal myself into the room!) out and knock it all down.

Old 12-31-03, 07:08 AM
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I am also in the process of dropping old plaster, lathe, and multiple layers of wall paper. In my case, plaster dust is combined with 90 years of dirt and coal soot as it falls.

To help limit the spread of the dust, I put a large fan in the window sucking the air from the room and blowing it outside.

All in all, it has helped all but the car parked under the window.
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