The third word in the title seems to always belong with the first two.
Climate: Northern NJ, but expecting global warming to change things. Assume mixed climate.
Two vapor barrier questions:
(1) On brink of bathroom remodel, which insulation to use? Worried about moisture from bathroom making insulation ineffective (FG) or leading to rot (cellulose). (And yes already have bathroom fan etc.) With vapor barrier on inside, worried about condensation in summer. Advised to use polyeurethane foam (board) for insulation with green board internal wall. Problem: I hear in some jurisdictions green board cannot be put against a vapor barrier? Reason: moisture getting trapped between green board and vapor barrier. Assume two options: make sure no vapor leakage through bathroom wall (yeah right, recessed radiator among other things) or use gypsum wallboard that will allow moisture to permeate back. But there will be wonderboard around tub/shower, with insulation on the other side. Am I doomed to have moisture condensing on foam board and causing problems?
(2) Have acoustic tile ceilings in bedrooms covering (assumed bad condition) plaster and lathe. Planning to tear down tiles and plaster, sheetrock over laths, again with green board. Eventually R-38 cellulose (some dense-packed) in attic overhead. But with CAC will I now have problems with condensation against green board rotting laths or soaking cellulose? Alternative is I suppose gypsum wallboard, then I get same effect in winter as moisture permeates through ceilings.
Vapor barrier issues have been sent to challenge us….
Replies
1. No vapor barrier in the bathroom. Moisture will travel thru the green board but will travel out again and keep insulation dry. Regular fiberglass insulation will do. I wouldn't use styrene because it can be a vapor barrier too.
2. No vapor barrier in ceiling. Let moisture in house travel wick its way into vented attic.
You get out of life what you put into it......minus taxes.
Marv
"No vapor barrier in ceiling. Let moisture in house travel wick its way into vented attic."
Deliberately allowing moisture into the attic is nuts. That's a good way to get condensation problems in your attic.The brain is a wonderful organ; it starts working the moment you get up in the morning, and does not stop until you get to work.
Taylor,
Moisture condenses on cold surfaces, not warm surfaces. If the bathroom walls were to have green board over poly, why would the moisture condense on the poly and be "trapped" between the poly and the green board? Do you expect your poly to be cold in winter?
Why do you expect to have problems with "moisture condensing on the foam board"? Do you have any reason at all to believe that the interior surface of any foam board would be cold?
Your questions show that you may not understand the basics of diffusion, consdensation, and air movement. You have raised too many issues to address quickly, but, as a start, I'll say: vapor diffusion is not the same as vapor that is transported with air movement; all walls should include an air barrier; vapor diffusion is nowhere near as important in wall design as is the necessity of an air barrier; in your climate, walls should be able to dry to the interior as well as the exterior; it is essential that you prevent the migration of interior air into your wall cavities.
No challenge here at all -- a bathroom is the area in MOST need of a vapor barrier.
Use a poly vapor barrier on both outside walls and ceiling. Unless you get condensation (and lots of it) on the outside of your windows in the summer (due to having the AC set around 65), there will be no summer condensation on the poly.
The moisture won't get "trapped" between poly and your wallcoverings. The wallcoverings need to be sufficient to shed liquid moisture (where appropriate), so only vapor will pass through, and that vapor can just as easily pass back.
Keep in mind that the permeability of greenboard and regular wallboard are about the same. The greenboard is simply more resistant to water damage.
What you need in NJ is to prevent moisture from condensing in the stud cavities and attic in the winter, when the temp outside is, say, 20. The moisture will freeze in the insulation, degrading it, and thus causing even more moisture to freeze. Then when it melts, it will pool in areas and cause rot and mildew.
Note that you need a poly barrier even with foam sheets for insulation, to keep moisture from condensing in the area adjacent to the studs. Foamed-in insulation would be the only insulation that didn't require a vapor barrier.
I appear to be confused on moisture being trapped between (impermeable) foam insulation and green board. I'm getting this from Taunton's Remodeling a Bathroom book, which reports that some codes do not allow green board over vapor barrier. The advice on always having vapor barrier (my preference) appears to contradict this, unless as is very likely I am confused.
As far as insulation being cold: yeah in winter we may not heat the place up as much as others would, to save energy.
I appreciate the difference between preventing vapor diffusion and vapor via air infiltration. I just don't know how to reliably achieve the latter, to the extent that I will never get moisture in the cavities. I'm actually not putting electrical fixtures in outer wall just for this reason. Sure foam sealant around anything that goes in outer wall, but can the edges of the inner walls be reliably sealed? Perhaps I am mouthing the heresy of those that argue against vapor barriers, without meaning to I hasten to add.
In the winter, the air outside is dry, and won't cause condensation problems in the walls.
In the summer, the outside air is moist, but to have condensation on the vapor barrier, the temperature at the barrier has to be below the outside dew point. Given that the temperature in the wall is apt to be 10 degrees warmer than the inside air temp, and that a dewpoint of around 70 is about the worst you'll ever see, the likelihood of significant summer condensation is negligible.
The "greenboard over vapor barrier" restriction would be if you were expecting the greenboard to actually get wet (eg, when greenboard is used behind tile in a shower). In that case, some air circulation behind is advised. But greenboard shouldn't be allowed to get wet on a regular basis, and should never be used to line a shower.
Taylor
How did you ever solve your bathroom insulation / vapor barrier issue..
I am sitting here trying to figure what I should do .. currently I have nice open wall cavieties..
Thanks for any help..