interior vapor barrior or not?

[ITE]

what is the thought on interior vapor barriors such as 4mil plastic over the interior of a wall in which the wall will be sandwiched between the tyvec and plastic put up on the inside? I have talked to inspectors who told me to put it in and in every book I have read they have suggested putting it in. however, several builders have told me not to do this. they claim that the paper vapor barrior on fiberglass is the only thing that is needed on the inside and if you put plastic in the natural moisture in the wood will be trapped in the wall essentially and eventually rotting the studs in the wall and causing mold to grow in the wall. I have also had a builder say that fha will not allow you to do this. what are your thoughts?

[bkrahmer]

I won't claim that is is for sure necessary, but I dare somebody to back their rediculous claim that 'moisture in the wood will be trapped in the wall'. Tyvek allows water vapor out.

The paper on fiberglass insulation is a vapor retarder. Plastic, when done correctly, can be a vapor _barrier_ as well as an air barrier.

Search this forum better. This topic has been discussed several times.

[Sweep]

The importance of a vapor barrier is determined by your location. It is needed in cold climates, especially in rooms that produce moisture, to prevent condensation on the cold side of fiberglass insulation.

The foil or paper facing on insulation, if installed correctly, does exsctly the same thing as a polyethylene sheet. The choice between the two is a matter of installation preference.

Wood will not rot if its moisture content is below 20%. You should not be using wood with a higher moisture content regardless of the vapor barrier design.

In a hot, humid climate, in an air-conditioned space, it can be argued that the exterior of a wall should be less permeable than the inside to allow moisture movement and drying to the inside. In such a case no vapor barrier would be used on the inside and 30 lb felt would be better than Tyvek on the outside.

The use of Tyvek in a cold climate is acceptable because of its air-barrier capabilities but proper installation to achieve such a barrier is so difficult and so misunderstood that I prefer 30 lb. building felt.

What is your location, sheathing and exterior finish?

[ITE]

Im in Indiana. sheathing is 7/16" plywood with vinyl siding.
there is a lot more to this than I originally thought.

[danh]

I won't claim that is is for sure necessary, but I dare somebody to back their rediculous claim that 'moisture in the wood will be trapped in the wall'. Tyvek allows water vapor out.

Even in the summer?

Further, Idaho is ICC Climate Zone 5B & 6B - dry. Your summers are too cold, dry, and short to see the kind of moisture accumulations that happen in Indiana (Zones 4A & 5A - moist). Unfortunately that means that your field of experience does not necessary apply to ITE's.

they claim that the paper vapor barrior on fiberglass is the only thing that is needed on the inside and if you put plastic in the natural moisture in the wood will be trapped in the wall essentially and eventually rotting the studs in the wall and causing mold to grow in the wall. I have also had a builder say that fha will not allow you to do this. what are your thoughts?
Newsflash - wood ain't the moisture source in these wall failures. It's air leakage & inward drying of reservoir cladding systems.

Just build your house really really poorly - if you get enough air leakage, you can restore the type of drying through the poly that the average house relies on.



My goodness, I'm getting fairly cantankerous in my old age.

[rbisys]

Greetings,

From a radiant barrier guy who has been thru this for the past 30 yrs.

Tyvex will allow VAPOR to pass thru. Once the vapor has been condensated in the insulation, it is WATER. No passing.

Paper VB is not only a joke it is also a disaster. Any one pulled old walls with paper backing? How about the smell and all those silver fishes.

A foil backed FG batt with foil facing in IS a vapor barrier. More importantly if the surface of the foil is depressed inward to form an air space, the foil airspave has more insulation value than the FG. Of course the mfgr doesn't mention this on the lable.

I have never seen a FG wall that didn't show signs of condensation. Condenstaion increases the energy flow up to 50% or more. True for Cel too.

I've wriiten a paper on this subject for contractors and other interested persons. E-m me your address and Ill send it to you. Also, has a simplified heat calc chart.

After 30 yrs + of the mfgrs bs, and others too, I'm getting a little cantankerous too.