Wall Vapor Barrier ???

[shinnlinger]

Hi,

I have some conventionally framed 2x4walls sheathed with zipwall on the outside.  I put fiberglass in the studs as you would normally, but I then stuck sheets of 2" polyiso on the inside of the frame and strapped the poly to the studs with long screws and I will hang drywall on the strapping.

I was always going to tack up some plastic sheathing for a vapor barrier on the strapping before I hung the drywall, but I am now concerned that the wall won't be able to breath if I put up plastic as I doubt the ability of the zipwall to breath as well as JM HUber says it will.  I realize it is not ideal to have the outside of a wall more impervious to vapor than the inside, but what would you folks suggest???  The polyiso is pretty impervious, and I did foam and or caulk between the sheets, but there are some seams here and there.

Thanks.

Dave

[jamesamd]

Too tight,is too much.IE: no good for Your Family.
Water vapor needs to leave the occupied space,from inside out.
If you have a wide open,ridge vent in the attic,with vents that are properly sized.
to remove the moisture You exude.You can find a balance.
If You are using wood stoves to provide heat,the equation grows.

Adjustable vents,would be the way to go.
Computer controlled,depending on the actual moisture content,in the entire home.
Moisture must leave,fresh air must enter.
As long as You have a good exchange of moist air and fresh air,You are good.
Idealy,the incoming fresh air will pass through a heat exchanger,that is linkedf to one of Your
wood burnering stoves

JMO,Jim

[Ironwood]

Moisture movement thru a wall in cold climes, combined w/ a heat gradient that will cause (in theory) internal condensation. Practical would say vapor barrier on the inside and breathable (Tyvek style) membrane on the outside. Some insulations will kill internal wall convection, not fiberglass batts though. We did everything well except fiberglass batts. We also recycled 2" thick rigid fiberglass ceiling tiles and face and firring stripped the outside before Tyvek. Cozy house and CHEAP to heat. 









Ironwood

[PlicketyCat]

If you're cold climate, I'd definitely put at least 6 mil poly sheeting on the inside as a vapor barrier.  The Zipwall might not be perfectly breathable to the outside, but if you stop the vapor from getting into the wall from the interior in the first place, then you won't have much/any moisture in the wall to worry about "breathing".  Just make sure your barrier is continuous and that you acoutistic caulk and tape seal the seams and any passthroughs really well in both the Zipwall and the vapor barrier... one little hole allowing for infiltration and transmission will concentrate moisture in that area.

[ljmathias]

Seems like there isn't just a simple answer to this one; recent issue of Fine Homebuilding has more:


"Fine Homebuilding" <FineHomebuilding@e.taunton.com>

[frwinks]

and even more from Joe Lstiburek
http://www.buildingscience.com/documents/digests/bsd-106-understanding-vapor-barriers/files/bsd-106_understanding-vapor-barriers.pdf

IMO, not sure if you want two class I/II retarders with a space between them.

here's one method of building a "superwall" worth checking out :
http://www.cchrc.org/docs/best_practices/REMOTE_Manual.pdf

[PlicketyCat]

A lot really depends on the materials you use in the wall assembly, your climate, and the seasonal variances you may encounter. 

Living up here in Interior Alaska, any "hollow" wall assembly really should have a vapor barrier (<0.1 perm) on the interior wall surface because we have many more heating days than cooling days. Our relative humidity is also very low, so chances are excellent that, during the winter, your interior air will be much warmer and much more humid than the exterior air and the warm, moist air will attempt to migrate out... get trapped in the wall at dew point, return to bulk form, freeze, and then rot out your assembly when it thaws if it can't dry to the outside when things warm up in the summer. We also rarely use air-conditioning in the summers since there are very few days where it's actually warm or humid enough to require them. So, in our case, 99% of the time the warm/moist air is on the inside trying to get out. In order to facilitate drying to the outside, we decided to use 15# felt as the weather barrier on our sheathing and use uncoated sheathing... both of these dry out better than some of the newer-fangled systems or heavier felt. We don't (normally) get a lot of rain and our snow is also fairly dry, so we don't need super waterproof weather barriers on the outside.

Now, if I were building in a completely different climate, I'd have to change things up or eliminate vapor barriers entirely. My sister's house in central TX is a good example. The winters are colder and wet, the warmer air inside passes through the wall but the humidity is the same on both sides and the wall usually isn't cold enough to hit dew-point inside. In spring, the temp and humidity is usually the same on both sides of the wall, with a lot of bulk water (rain) on the outside. In the summer, it's hot and humid outside so the air-conditioned interior is cooler and drier and the warm moist air outside tries to move inside. The latter part of summer into autumn, the humidity inside and outside is the same (low) but the warm air outside still attempts to migrate inside. Because of the wide variance of seasonal temperature and moisture, she doesn't have any vapor barrier on the inside of her walls and she used an exterior house wrap (weather barrier) that blocks bulk water from entering the wall from the outside but promotes vapor transmission out from the inside. That way, she can keep the rain out, but the wall can dry both to the inside and outside depending on which season it is.

If you live somewhere where the humidity inside your house is the same as the humidity outside your house during each season, regardless of the air temperature differences, then installing an internal vapor barrier is not useful. 

If you live somewhere that the external air is warmer and moister than the interior air for most of the year, then you do not want to put in an internal vapor barrier because you need that moisture to dry IN. It won't dry out since the outside air is too humid. In this case, the only way for the wall to dry out is if there is a long enough period where the outside air is both warmer and drier than the wall and the interior.

Warm air always migrates towards colder air, but what happens to the water vapor in the air depends on whether it gets cold enough to condense and whether or not the colder air can take any of the water -- if it's cold and humid, that water isn't going anywhere :(

[frwinks]

hi PlicketyCat,
have a read through the CCHRC's publishings.  They are in your neck of the woods and have designed building systems to deal with your harsh climate.  The key is to keep that "hollow" wall assembly as warm as possible and dry to the inside

http://www.cchrc.org/docs/northern/Northern_Fundamentals_REMOTE.pdf

and many more @

http://www.cchrc.org

[meddins]

This study out of UMass Building and Consrtuction Technology dept. compared different type of housewraps. Pretty interesting conclusion - plain old 15 lb. felt seems to be as good a choice as any of the newer synthetic barriers.

http://bct.eco.umass.edu/index.php/publications/by-title/housewraps-felt-paper-and-weather-penetration-barriers/

An interesting finding was that water vapor permeability of felt varied with the humidity of the air:

"Felt paper absorbs water and ranges from a low of around 5 perms when it's dry to over 60 perms when it's exposed to relative humidity above 95%."

In the past, on new construction we've always put up Tyvek or similar on walls and then felt on roofs.

I think on my house I'm going with felt on the walls too. And on the subfloor.

M.E.

[PlicketyCat]

Yes, I've read a great deal from the CCHRC and have been to tour their test facilities.  Right now, there are still two schools of thought... dry out and dry in, both get complicated and both have their pros and cons.  Still no definitive solutions that work consistently, a lot still depends on the individual micro-climate and the individual thermodynamics of the building.  On the rainy coast, drying in makes more sense; but in the dry interior drying out makes more sense. And they still can't agree which side of the wall the insulation and barriers should go on.  One thing they've been trying recently is to build a double stud wall with a gap between and placing vapor barriers on both sides of that gap (the "inside" of the outer wall and the "outside" of the inner wall). The outer wall gets insulation inside the sheathing and foam-board outside the sheathing, wrapped in weather barrier and cladding with an offset drainage plane. The inner wall is loosely insulated or left hollow with all your cables and pipes.  Seems like an awful lot of layers and complex designs increase the risk of failures.

That's one of the reasons we're planning on straw-bale walls with earthen plaster and wide overhangs on the "big house" -- no vapor barriers required, just let the house breathe from both sides cause it's what clay/lime plaster does best