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chigundo 01-12-2013 09:33 PM

Basement / removing vaper barrier
I just put up 1" xps insulation and want to put r-13 fiberglass batts in front of that. They only sell rolls of faced batts. I don't want faced. Can I just slice the Kraft paper to void the barrier or do I have to remove it completely? I tried removing and it sucked.

pro handyman 01-12-2013 10:32 PM

why would you want to remove the paper?

Gary in WA 01-12-2013 11:19 PM

Slicing a vapor barrier is ineffective;

"Furthermore, the amount of vapor which diffuses through a building
component is a direct function of area. That is, if 90 percent of the
building enclosure surface area is covered with a vapor retarder, then
that vapor retarder is 90 percent effective. In other words, continuity of
the vapor retarder is not as significant as the continuity of the air barrier.
For instance, polyethylene film which may have tears and numerous
punctures present will act as an effective vapor barrier, whereas at
the same time it is a poor air barrier. Similarly, the kraft-facing on fiberglass
batts installed in exterior walls acts as an effective vapor retarder,
in spite of the numerous gaps and joints in the kraft-facing.
It is possible and often practical to use one material as the air barrier
and a different material as the vapor retarder. However, the air barrier
must be continuous and free from holes, whereas the vapor retarder
need not be." From:

While you have that link open... your location Dec., Jan, Feb. low temperatures average 21*; with only 1" foam board in the basement- be sure to control the relative humidity as the cavity side of f.b. will get condensation at/above 27%RH at 70* room temp. very similar to Chicago, IL in that link- Fig. 10; notice the temp. scale on the left, and line and 40*---- yours is 35* temp. f.b. for 5 months- draw a line just under the "Potential for" (in purple color) at 35 degrees on the left side. Adding another layer of 1" XPS would raise the dew point temp. to 42* or 37%RH. Even 1/2" more XPS added = 39*-- 32%RH at 70* This would be wall portions above grade and close to the "frost line"= 42";


chigundo 01-13-2013 07:06 AM

Gary, are you saying that I would have to remove ALL the paper or is it ineffective in that it doesn't even matter?

And the reason I want unfaced is because the xps is a vaper retarder and I don't want another one (per everything I've heard)

Gary in WA 01-13-2013 02:18 PM

Any material left in the wall is effective as a vapor retarder. Remove it all. Your 1" f.b. is "Case 4" on pp.29;

Notice the reduced risk of condensation with 2" of f.b.--- Case 8, pp.35, compare that graph with yours on pp. 30....

PS. You could help limit moisture at the drywall surface from getting to the cavity with a vapor barrier paint, as the article said.

mikegp 01-17-2013 07:43 AM

Is Roxul an option for you?

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