Air Barrier on Cathedral Ceiling

[ellbaker]

While researching installing tongue and groove pine, I found a reference or two on the importance of air barriers on cathedral ceilings. I also have read that it is not recommended to install a vapor barrier in humid environments because of mold issues.  Most of the installations on this site appear to not have an air barrier and are nailed directly to the rafters. I have fiberglass batt with a ridge vent on a shingled roof.  Is it best to install sheet rock first (air barrier) and then cover with T&G pine?  I am not so much worried about heat loss as I am concerned about moisture and mold.

[Danfish]

The terms air barrier, vapor barrier and vapor retarder are ofen used to describe an attempt to slow down the passage of moisture ladden air.  The idea is to keep moisture from getting into wall and ceiling cavities.  Moisture moves from areas of high temperature and relative humidity (higher vapor pressure) to areas of lower temperature and humidity.  The greater the pressure difference, the greater the push.

In cold climates, the vapor pressure is higher indoors during winter and the moisture wants to move toward the outside wall.  In hot, humid climates the reverse is true (hence the trend to omit interior barriers), especial if the interior is air conditioned.  In climates with both heating and cooling, vapor movent is normaly strongest during colder weather...moving toward outside.

Research has shown the real problem is that air leakage moves the most moisture.  So if you are concerned about really keeping moisture out of your ceiling in an area of colder winters, you should install either a continuous, airtight membrane (Polythylene) or at the minimum, a detailed air tight drywall ceiling (lots of caulking, foam sealent, and gaskets).

[rwanders]

An alternative would be to use closed cell spray foam directly against the roof sheathing. You will not need a separate vapor barrier or be concerned about air leakage if the spray is correctly installed.

[Alan Gage]

While researching installing tongue and groove pine, I found a reference or two on the importance of air barriers on cathedral ceilings. I also have read that it is not recommended to install a vapor barrier in humid environments because of mold issues.  Most of the installations on this site appear to not have an air barrier and are nailed directly to the rafters. I have fiberglass batt with a ridge vent on a shingled roof.  Is it best to install sheet rock first (air barrier) and then cover with T&G pine?  I am not so much worried about heat loss as I am concerned about moisture and mold.

Depending on how your ceiling is vented you could have more problems without a vapor barrier. Vapor barriers in walls seem to be falling out of favor in all but the coldest climates (Canada and Alaska) but are still a good idea in ceilings.

If your attic (or whatever you want to call what's above the ceiling of an cathedral ceiling) has plenty of ventilation (good ridge vent and soffit vent inlets with a 1-2" air space below the sheathing and above the insulation) you might get by with a leaking ceiling since the moisture that will travel with the air will be pulled out the ridge vent. If your attic space is not well vented then all that moisture that moves up into the attic along with the warm air that's leaving your living space has nowhere to go. That's when you start getting mold and rot.

Like Danfish said, either plastic sheeting or ADA (Airtight Drywall Approach) on the ceiling. Or fill the space between the rafters with foam so there's no way for air/moisture to get up there in the first place.

As usual Building Science has some good reading on the subject:

http://www.buildingscience.com/documents/published-articles/pa-crash-course-in-roof-venting

http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation/?searchterm=roof%20venting

Not sure where you're building but since you mentioned humid climates:

http://www.buildingscience.com/documents/reports/rr-0302-roof-and-attic-ventilation-issues-in-hot-humid-climates/view?searchterm=roof%20venting

http://www.buildingscience.com/documents/reports/rr-0306-unvented-roofs-hot-humid-climates-and-asphalt-roofing-shingles/view?searchterm=roof%20venting

Alan