No vapor barrier - what to do?

About 10 years ago I insulated the open beam ceiling of a bedroom in my house (no attic). Unfortunately I did not put up any plastic (vapor barrier). Should I take down the drywall and put up a vapor barrier? I'm in Southern California near the beach where it only gets down to between 40 and 50 degree at night during the coldest months. Is it possible that the 4 X 6 rafters are getting moisture damaged up in there? What could realistically go wrong?

 

Moisture forms when the air reaches the dew point. If inside relative humidity is moderate, 40º to 50º outside temps will not cause condensation, and that is without any insulation. Make sure your bath fans and dryer are vented all the way to the outside.

One easy way to judge the moisture potential is to watch your windows. If condensation is not forming on them, it will doubtful be a problem on your ceiling.

The one critical condition would be air leaks. If inside air can leak directly through to a cool/cold surface, then that would be like your windows. If the windows are not showing condensation, you are all set. If they are, air seal and in the worst case, apply a good coat of paint, as a simple layer or two of paint can act as a VB.

Bud

 

I read somewhere that a solvent based paint makes a good vapor barrier for walls/ceilings that don't have them. The molecules [or whatever] in solvent based paint are closer together than the ones in latex based coatings. While I wouldn't recommend applying a solvent based finish paint [like oil base enamel] applying a coat of an oil base primer might have some benefit. Oil based primers can be top coated with latex paint.

 

Many thanks for the feedback!

 

To answer your questions ("Is it possible that the 4 X 6 rafters are getting moisture damaged up in there? What could realistically go wrong?"),
1. No.
2. Not much.

You didn't say where in SoCal, but if you're in or below LA county, that's defined as a dry climate in the 2009 Energy Conservation Code (Figure and Table 301.1)

Barring a roof leak during a rainstorm, you don't live in a climate zone that is likely to promote accumulation of water in the attic space, which I define here as the area between the ceiling and the roof. Regardless of that, long before moisture damage to the beams could occur, you would have water stains appearing on the ceiling.

That's my $0.02.

Best regards and rest easy(er),
Doug

 

I'm in Pacific Palisades, 2 minutes from the beach. Never seen condensation on the Windows before. When it warms up I think I'll apply some oil based exterior primer, then repaint.