vapor barrier and stucco wall

[tom31415926]

Greetings,

I have a 1959 house in California's central valley (low humidity). I'm remodeling the interior of one room, and decided to pull off the drywall on the interior side of an exterior wall. Looking through the interior wall at the exterior side, I can see tar paper that applied over horizontal wires, and where the tar paper is broken I can see the underside of the stucco.

The tar paper is moldy in some places, and in those places the wires behind the tar paper are rusted through. My guess is that the tar paper on the outside of the wall acted as a vapor barrier, and during the winter, humid air from inside the house penetrates the wall and the moisture condensed on the cold tar paper.

There was blown-in insulation which I removed. I'm planning to replace it with R-15.

I will try to clean up the mold (brush and vacuum) and maybe apply diluted bleach to kill it. The wood studs to not appear to have rot, I will thoroughly test them (poke with screwdriver) to be sure.

what should I do to repair/replace the tar paper that has fallen apart?
I'd like a vapor barrier on the inside of the wall under the drywall (just to keep me separated from the mold), but then the wall would have two vapor barriers.

What is the best thing I can do for this wall while it is opened up?

Thanks,

Tom

[pushkins]

You should not install another vapor barrier under the drywall, this as you mentioned would give you a double vapor barrier and this is most definitely is a no no. If you did this it would trap moisture between the layers and this would lead to many problems.
You could cut pieces of tar paper and stick it over the holes apply a bead of roofing caulk to bond the two pieces together. As long as you can seal the holes you should be good to go.
Remember when buying you new insulation that it needs to be unfaced as you already have a vapor barrier.

[tom31415926]

thanks!

I'll repair the tar paper where it's got holes and I'll seal it in places where it would otherwise drip inside the wall.

Does this method of construction have a built-in flaw? That is, a vapor barrier on the cold side?

Tom

[pushkins]

The debate regarding vapor barriers is a long, well worn one.
The "older" thought process was that moisture is on the exterior of the property in the form of rain, snow, humidity etc...etc...(humidity controlled internally by AC) and internally excessive moisture isn't/shouldn't be there.
So we use a moisture barrier on the exterior side of the wall to "deflect" (want of a better word) moisture away from studs, insulation and the like.
Now, this is where it gets interesting:
The "new" thought process which has a lot of scientific proof and thought behind it and is in the process now of being reviewed by the IRC (God's of Code ) is, that the moisture barriers are really a Vapor Diffusion Retarder (VDR), that moisture is created during winter on the warm side (interior) and is conveyed to the exterior by the simple physics of cold and warm) and visa versa in summer. Think of the moisture in a home in winter heated to 77 degrees, while outside it's 35 degrees (think about the moisture that will build up on an non air proof window) this moisture is pulled through the drywall and into the wall cavity. Moisture travels in air and if the wall is not "AIR PROOF" then moisture goes where the air goes. Almost all standard vapor barriers are not "air proof" thus allowing moisture into wall cavities, therefore a vapor barrier doesn't effectively stop moisture (though it obviously goes a way towards helping).
Stucco finishes should be sealed either with a waterproof additive and/or sealing coats on the exterior to stop water penetration.

I've attached an article that explains it in depth, it makes for very interesting reading on the future of "moisture barriers"

Understanding Vapor Barriers - Building Science - EcoHome Magazine

[tom31415926]

Pushkins,

thanks for the information; I'll read the article.

Good suggestion on the unfaced insulation; I wouldn't have thought of that.

Tom