Oil primer acts as quazi vapor barrier?

D107

I have just spent weeks removing old wallpaper from the bedroom wall then taking off the glue. Walls have been plastered/repaired. Most hardware people recommend an oil primer on an old plaster wall. (Not sure what kind of paint is there now.) Somewhere I read that some heating/venting specialists consider a coat of oil paint to be some kind of vapor barrier, which I'm thinking may complicate things. (This is 30 miles north of NYC.)



This is an old balloon framed 1924 colonial. from inside to out we have the plaster wall with a few coats of paint, wood lathe, 4-inch void filled with blown cellulose, 3/4" wood sheathing, black roof paper, clapboard, half-inch styrofoam insulation, vinyl clapboard.



I believe the black paper was the original vapor barrier. Before installing the half-inch styrofoam board i verified their sufficient level of permeability. A very knowledgeable Wallie once commented that in order to be effective and to prevent unwanted condensation, insulation on the outside of the vapor barrier should have twice the R-value as that on the inside of it.



The tech who advised me said he recommended keeping the walls free of wallpaper--or vapor barrier paint-- to allow the wall to breathe. So far in over three years since the styrofoam and vinyl were installed we haven't seen any problems with our setup, but should i try to use latex paint instead of oil?

Unknown

David,

I believe oil-paint does act (a little anyways), as a vapor barrier.



As concrete or brick is prone to absorb moisture, tar-paper was the only "material of choice" available to contractors at that time, to alleviate moisture permeation to the wood framing.

Typically the "voids" between the wall framing & finished side(plaster) were not insulated,, if you used "blown-in cellulose" (and found those voids), all the more power to you.

Any kind of vapor-barrier will help as now you have "sandwiched-in" the heat from the cold,,,, and once insulation gets-wet, the R Value is gone, and the moisture will permeate inside causing mold

Typically vertical-walls are not a problem,,, but un-vented  attics are.



BTW- Did you know Styrofoam SM(blue or red), should have a vapor-barrier?



 

D107

Thanks, the consultant I hired

held the opinion that given the existing black paper--which is still there--adding another vapor barrier would not be helpful. i think if it was new construction, yes they would put tyvek--under or over i forget which--the blue board.



Problem in the ny climate zone is as I've had it explained to me--that you have a cold climate mixed with a hot/humid climate, so designers want the humidity to be able to travel differently in these different conditions.



As i understand it, my cellulose insulation would have moved the dewpoint further outward. if humidity condenses outside that cellulose--maybe on the outside of the sheathing or wood clapboards, the vinyl is on loose enough to allow evaporation. A vapor barrier of oil paint might prevent humidity --in summer?--from exiting through the wall. Although of course the wallpaper that was there for thirty years probably did that too.



I've also read some great buildingscience.com materials to help guide me.



Thanks.

Unknown

You are welcome David,,,,

it sounds like you`re in good hands! ;-)



Always a pleasure to read your posts!

Mike Kusiak_2

Existing paint?

You mentioned that the rest of the house had existing multiple coats of oil based paint over the plaster. I would think that this would be a pretty effective vapor barrier preventing humidity from exiting the living space. I have a feeling that the oil paint you already have may be more of a vapor barrier than the tar paper.





 I am not sure how really effective tar paper is as a vapor barrier, compared to something like Tyvek. When I was replacing my roof several years ago, I read that the tar roofing paper is specifically designed to be somewhat permeable to water vapor, allowing it to dissipate moisture under the roof decking.  I think it would act similarly over the sheathing.



If you ever saw how much the tar paper expands and absorbs water when damp, you would question how effective a vapor barrier it really is.

D107

from BuildingScience.com

Thanks Mike, you may be right about the existing paint, not much I can do about that except to try to use only latex paint from hereon out, hoping that in the humid summer, at least some humidity will be able to vent to the interior.



Here are some interesting quotes from some BuildingScience.com documents, freely available on their website 'information' section. The 2nd quote suggests that insulating cavities INSIDE the sheathing can sometimes cause problems depending on other conditions:



From BSD-013: ..."In mixed and hot-humid climates, (e.g., Houston, Atlanta, Knoxville) a vapor barrier on the interior should not be used, and only latex paint on drywall will allow sufficient vapor to pass through to the interior. With few exceptions, any material with low permeance (e.g., oil paint, aluminum foil, vinyl wallpaper, epoxy) must be avoided on the interior..." (John Straube)



From BSI-028: ..."Let’s extend this discussion to cavity insulation in general. The same thing happens to the cavity side of sheathing when we insulate cavities. It gets worse when the cavities also have no air flow or convection. (Air flow—air change and convection—increases energy exchange therefore

increases drying potential. My Canadian friends heads are now going to

explode because they have been taught that air leakage leads to

“wetting” from interior sources. Yes, in extreme climates with high

interior moisture loads. Think humidified buildings in Canada. Not

everywhere else. Come visit the United States, the dollar exchange now

works for you and air leakage improves drying potentials.) Sheathings get colder. If the sheathing is hygroscopic, its equilibrium moisture content also goes up. Why take the risk? Insulate on the exterior of the sheathing. This makes the problem go away. The risk is so high in cold climates that folks are no longer insulating the cavities in steel stud walls in places like Ottawa and Montreal—all of the insulation goes on the outside..." (Joe Lstiburek)

Mike Kusiak_2

Permeability

Actually, I think your old paint vapor barrier is a good thing. I believe you said that you live in Westchester, which would really create your main vapor condensation problem in winter. Having the main vapor barrier  on the inside surface of the room wall prevents the humidity from diffusing into the insulated stud space and the cellulose insulation. In effect the water vapor is contained in the heated area where it cannot condense.



Unless you heavily air-condition the interior in summer, I would not think humidity infiltration from the outside would be a problem. If it was my house, I would rather use the oil based primer and avoid the wintertime transmission of inside water vapor outward into the stud space.



Here is an interesting paper on infiltration and miosture control: