breathe or not? waterproof or not?

[brogar]

Hello -- My exterior walls, in cross-section, going from inside to out, will be like this:
1. drywall (sheetrock),
2. 6-mil plastic vapour barrier (Code needs this in Ontario, Canada)
3. 5-1/2" fibreglas batts insulation between 2x6 studs
4. 7/16" OSB sheathing
5. here's the question -- I want to use 15-lb. "roofing felt" (sometimes called "builder's paper") here, instead of housewrap. Why? Cheaper.

I have read and heard, at different times, that roofing felt IS moisture-proof, and that it BREATHES. Should be one or the other, shouldn't it? Which?

Have also read and heard, at different times, that housewrap IS moisture-proof, and that it BREATHES. Should be one or the other, shouldn't it? Which?

My question is, can I use roofing felt at step 5. above? If it really doesn't breathe, then, with the vapour barrier at step 2, I'm going to get condensation in that sandwich, true?

In a nutshell -- do you think it's safe to go ahead and use roofing felt there?

Thanks in advance for any advice.

[Don_P]

Yes, it's fine to use tarpaper as your weather resistive barrier.

Moisture occurs in three phases in the natural world, as a solid, a liquid, and a vapor. One goal is to exclude liquid water while allowing water vapor to pass out of the wall. Tarpaper is made of wood pulp saturated with asphalt. When wetted the wood fibers expand and being saturated with tar they block passage of just about everything. The wood fiber also just absorbed and stored some of the water temporarily. In a light leak it stored the water away from more sensitive materials and blocked passage of bulk water. As it dries it shrinks, draws taught, opens small gaps between the fibers and allows tha assembly to dry. Pretty high tech.

[bighammer]

Personally anothr benefit of the housewrap is the height of the roll, unlike felt paper we can cover the enitre 8',9',or10' high wall in 1 coverage and not needing to worry about seams.

[brogar]

Don -- I gotta say that's amazing. Pretty high tech for sure. That's the first time I've come across the explanation for what that stuff is actually doing. Thanks very much.

Bighammer -- That I didn't know, either. That is an advantage, definitely. A roll that wide kinda makes you finish the wall right now, once you start it. In my case that's a big plus. Thanks very much.

I assume housewrap works something the same as tarpaper does. Seem to remember reading something in these forums a couple years ago where somebody pointed out that we know what tarpaper looks like, and how it stood up to the job, after 30 or 50 years, because we've seen it in there when we tear apart old walls -- it looks the same as the day it was put in. And for housewrap, well we just don't know yet how it stands up over that much time.

Anyway, I guess I gotta compare prices closely.

Thanks again. -- brogar

[RTF]

Both products work fine. Felt is half the cost of housewrap, but housewrap goes up faster. I prefer the housewrap.

[brogar]

Thanks, RTF. -- Brogar

[bill mystere]

The main purpose of typar/tyvek in new construction is breathability.

[Don_P]

The main purpose of the weather reistive barrier is to...resist any weather trying to drive in. But yes, it does need to breathe to allow drying.