ah yes... the old vapor barrier question

Started by hnash53, May 31, 2006, 03:14:31 PM

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hnash53

Does the kraft face on fiberglass insulation act as a vapor barrier?

If I recall, it does.

Thanks.

Hal

bayviewps

   I helped insulate new homes around 1980-82 on a part-time basis for a small home builder.  He said to use it as a vapor barrier, one would have to "fold back" the top and bottom so one could staple it.  I guess it was to much work.

  Any way, we found that the kraft paper was good for hanging and stapling to the studs.  We would then use a razor blade to cut slits in the paper.  (To allow moisture through)  Then covered it with a poly vapor barrier.  

  Has anyone seen the pink stuff completely enveloped in poly?  It was cut to fit.  Supposedly you would not come in contact with the itchy stuff.  I always wondered if that worked.  Would you not be sealing in moisture?  I haven't seen it for sale for a while.  Maybe it didn't work well.

  We are considering the Icynene insulation when we build.  Besides insulating it also acts as a vapor barrier.

http://www.michaelholigan.com/departments/tvshow/seg_tscript.asp?ts%5Fid=7007&text%5Ftype=T&text%5Fpage=1&mscssid=25G54KPMA2T79NMMXNLR1DBWMCAT4FH1


jraabe

Kraft faced insulation is considered a vapor retarder. Poly is much closer to a vapor barrier. Kraft facing is fine for mild climates but you need a better barrier when you have real cold driving moisture through the wall.

Icynene is an excellent (if expensive) insulation - as much for its air tightness as the R-vaule and vapor barrier characteristics.

hnash53

And, of course, the vapor barrier is placed to the warm side.

So, John, you are saying in a cold climate like Wyoming, after putting up kraft faced fiberglass insulation, putting up plastic would be good as a vapor barrier?

Thanks.

jraabe

Hal:

I would do it for two reasons. One is the better vapor barrier and two is because it will be stapled over all the studs it will cut down air infiltration.

Most of the real moisture problems are carried into the wall on air currents - not vapor migration through the vapor barrier.

You may also want to do other simple air infiltration controls such as outlet gaskets and caulking at the sills.


hnash53

John,  when you say that most water vapor is carried on air currents, you mean air currents from the inside of the house from cooking, showering, etc, right?

Here in Wyoming, we are VERY arid, very little precip... 8.75" per year and mostly in the form of snow which here is very very dry with not much moisture in it.  So moisture problems here come from inside the house...not outside.

I am debating with a engineer about the vapor barrier issue.  With tarpaper on the outside, kraft faced insulation to the inside, with plastic, that should take care of any moisture problems.  As I understand it, what you want to prevent is condensation within the wall, on wood and within insulation.

I get it.  He doesn't seem to.

Thanks.

Hal

jraabe

That's exactly right Hal.

Vapor pressure is from the humid to the dry side (inside to outside) and as moisture laden air cools, it condenses - inside the wall, usually on the inside of the wall sheathing. Air infiltration makes the problem worse - air leaks from a bathroom without a good vent fan would be the classic problem.

Dberry

What about painted drywall?  Does that act as a vapor barrier?

jraabe

#8
Two coats of PVA drywall primer is a less than 1 perm vapor retarder. This is all you need in most climates if you have controlled the air leakage. Using airtight drywall techniques and the primer is usually the lowest cost solution for a house with a drywall interior. This is very cost effective in most climates and has a quick (and lifetime) payback in terms of reduced heating and cooling costs.

Here's a two page overview of the techniques: http://www.southface.org/web/resources&services/publications/factsheets/24ada_drywal.pdf

You need to combine this airsealing with fresh air ventilation - either automatic or manual.


Amanda_931

Yes on the fresh air.

My AC is out just now--got the solution just not hooked up.  So temporarily I'm bringing (often) cooler air in.  It's made a big difference with allergies.