Greetings,
I’d like to hear opinions on using a 6 mil poly vapor barrier under drywall on and ICF wall with true stucco on the outside. Went to a seminar by an EIFS manufacturer yesterday and heard that the latest thinking is to use a vapor barrier in climates of 8000 heating degree days or colder. Here in west central Wisconsin we are almost at that.
The ICF manufacturer and local reps say vb not needed, just screw the drywall right to the inside of the blocks. I guess the rational for not using the vapor barrier is that the wall assembly can dry both ways if moisture gets through the double layer of grade d paper under the stucco. And that in the hot humid summer with indoor a/c cranked to cool you might get condensation of the inside of the poly from moisture driven into the wall by humidity outside. I’ve heard that the guy at EBBA ( Lstiburek ) is now saying no vapor barrier needed.
Any comments?
Rich Palmer
Replies
This is an old debate, RICH. Around here (Illinois) I have had problems with visqueen under drywall and I have heard of several others having problems with it. Also the former electric company energy consultant was against it's use as you mentioned.
Here's my theory: The visqueen provides a prime place for condensation to happen. With contributing conditions you get the water droplet fast on the visqueen. The Boy Scouts use a piece of it to demonstrate getting water out of the air for survival. If you lay a piece of the stuff out in the sun to dry, the top drys while the bottom gets wet from the ground below.
On the other hand if you use a hard foamboard (inside/ my preference) as insulation/vaporbarrier the heat/cool from each side is dispersed. It is hard for a droplet to form. Foamboard, good. Visqueen, bad.
Is the condensation on the drywall side of the Visqueen or on the stud / insulation side?
If mildew is showing up on the drywall side the moisture must be on that side.
fonzie.. no condensation can take place on either side of the vizqueen / poly unless it's surface temp. is below the dew point..
so it is effective as PART of a system... without a good insulating layer behind it , it is just a condensing surface...
if it is on the warm side, behind the drywall, and it's surface is above the dewpoint temp. no condensation can occur... so the moisture vapor just conitinues to circulate until it finds an exit... either a controlled exit ( like an exhause fan )... or an uncontrolled exit.. ( like a penetration to the the attic )..
bottom line... a vapor barrier CAN be an effective part of a moisture control systemMike Smith Rhode Island : Design / Build / Repair / Restore
Mike-
Will any small amount of moisture that does find it's way into the studbay with foamboard, eventually migrate it's way to the outside thru the sheathing, tyvex/felt, siding by pressure from the heating source transfer inside the house or eventually just thru the exterior walls being heated up in the summer months?Half of good living is staying out of bad situations.
The other...proper application of risk.
rez.. i'd just be guessing.. but what i 'd say is ... depends..
if the water source is high...
if the hole is big..
if the condensing surface is cold enough ..
moisture vapor will dissipate and move to the dryer side.. condensate will flow by gravity and cause either rot or mildew / mold..
the air diffusion strategies rely on controlling the moisture level.. and letting the moist air move thru the air diffusion layer ( the well -sealed sheetock ).. slow enough so it will dissipate and pass harmlessly to the outside....
in cold climates like ours.... the outside winter air gets so dry that it is easy for the moist air to pass to the outside and dissipate... the colder it is , the less moisture the air can carry...
so the wall structure has to be designed for moisture to pass thru the outer layer from inside to out..
where YOU live is very important in developing your strategy... that building science site of lstiburek's is great for help in determining your own planMike Smith Rhode Island : Design / Build / Repair / Restore
another thing... the summer months can reverse the drive.. forcing moisture from outside to inside..
the vapor pressure ( excluding the sun , which is a whole 'nother factor ).. is from wet to dry...
humid exterior vapor wants to move to dry interior surfaces ...
Mike Smith Rhode Island : Design / Build / Repair / Restore
Edited 2/2/2003 8:28:46 PM ET by Mike Smith
A rain vent seems to be making more and more sense.Half of good living is staying out of bad situations.
The other...proper application of risk.
Mike I'm not an expert on this and I appreciate your comments. I'm interested in learning about this fundamental building problem.
What about this situation: With visqueen as the vapor barrier behind the drywall what if... a hole somewhere, a fault, allows outside air to spike a cold spot on the visqueen.
My theory is that anywhere this happens with visqueen instant condensation would be encouraged.
If this happened with foamboard the cold spot would be dispersed and condensation would be discouraged and probably NOT happen.
What do you think?
what's on the other side of the foamboard ?
i think the answer is still ... depends..
but a hole is a hole. ... bad karma for any system..
but it does point up one reason a lot like the idea of gypsum as an air diffuser.... easy to inspect... so no holesMike Smith Rhode Island : Design / Build / Repair / Restore