So poly is bad, and in to out vapour diffusion is a myth : what about wet rooms?
If read enough articles about the dreaded old school polyethylene vapour barrier that it is likely not a good idea for most walls of my building (zone 5.5, 2X6 wall with 2″ xps). If I go dense pack cellulose, will likely have to use membrain to appease the inspector (or convince him the special paint is ok).
Even with all of this bad press, does poly still have a place in my zone? I routinely see it specced on the attic sealing (just above the drywall)by well informed people, is that still the norm? Why is the ceiling different?
Also should walls/ceilings be treated differently in frequently used bathrooms with showers? I have seen my share of moldy corners on the drywal ceiling of showers, so i know there can be a moisture issue, even if ventilated to/above code. Is poly or a vapour retarder/barrier a good idea for bathrooms? Are there any other tricks (eg paint) to keep the moisture from penetrating the drywall?
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Q. "Does poly still have a place in my zone?"
A. That depends -- where do you live? I have never heard of Climate Zone 5.5. Certainly, polyethylene has many uses. It should almost always be installed under a concrete slab.
Q. "I routinely see it spec'd on the attic sealing (just above the drywall) by well informed people. Is that still the norm? Why is the ceiling different?"
A. Polyethylene above ceiling drywall usually causes no problems in Climate Zone 5 or Climate Zone 6, but it is unnecessary. Most moisture problems in attics are caused by air leakage, not vapor diffusion, so the important thing is to make sure that you have a ceiling air barrier.
Q. "Also should walls/ceilings be treated differently in frequently used bathrooms with showers? I have seen my share of moldy corners on the drywall ceiling of showers, so I know there can be a moisture issue, even if ventilated to/above code."
A. There are two reasons for the type of mold you describe. Neither reason has anything to do with polyethylene. (1) This type of mold is often due to high indoor humidity, which is often a result of insufficient bathroom ventilation (due to a missing exhaust fan, failure to use the exhaust fan, or an exhaust fan that performs poorly). (2) A second possible cause for this type of mold, especially in a cold climate, is missing or thin ceiling insulation. If the drywall is cold, it can become a condensing surface, leading to mold.
Q. "Is poly or a vapor retarder/barrier a good idea for bathrooms?"
A. The answer depends on your geographical location. To learn about code requirements, see these articles:
Vapor Retarders and Vapor Barriers
Do I Need a Vapor Retarder?
It should also be pointed out that tub areas and shower areas often need a moisture barrier to prevent problems related to splashing. If you plan to install tile around your tub or shower, you need to follow specific requirements to avoid problems related to water from the showerhead.
Thanks Martin. Zone 5 is probably the best estimate.
Maybe it's common to install poly on ceilings because it gives the illuision of an easy instant air barrier?
I am aware of the tile requirements. A few manufacturers even make "systems" to solve the water problem in showers (Schluter-Kerdi comes to mind). They are quite pricey from my research to date.
There are number of anachronistic building practices that persist because of the way the construction process is typically structured. In Canada using poly as an air and vapour barrier falls into that category.
The main advantage of using it as an air-barrier is that it can be inspected and tested as a discrete activity prior to the wall and ceiling finishes being installed, and it can be installed by a separate trade responsible for its integrity. It is also easy to detail openings and penetrations.
So while it may not have any use, or even be detrimental, as an interior vapour-barrier, I don't think it's fair to characterize it as "giving the illusion of an easy instant air barrier". After all, the tightest house ever tested used poly for that purpose.