Replies

1. 
   LucyF | Jun 26, 2013 11:41pm | #1

   Tom,
   I don't know the answer, but it's a good question.

2. GBA Editor
   Martin Holladay | Jun 27, 2013 06:00am | #2

   Tom,
   The vapor permeance of painted plaster or painted drywall varies widely, depending on the type of paint used and the number of coats applied. Here are some permeance values from laboratory measurements (list assembled from various sources):

   2 coats aluminum paint: 0.30-0.50 perm
   1 coat latex VDR paint: 0.45 perm
   3 coats white lead/oil paint on wood siding: 0.30-0.99 perm
   3 coats white lead/zinc oxide/oil paint on wood: 0.88 perm
   2 coats oil-based paint on plaster: 1.58-2.99 perms
   2 coats of latex paint on gypsum drywall: 5 perms
   Unpainted gypsum drywall: 49.74 perms
   1 coat of vapor-retarder paint on drywall: 0.6-0.95 perm
   1 coat of primer-sealer on drywall: 6.28 perms
   1 coat of vinyl-acrylic primer on drywall: 8.62 perms
   1 coat of semi-gloss vinyl-acrylic enamel on drywall: 6.61 perms
   2 coats of enamel paint on smooth plaster: 0.5-1.5 perms
   1 coat of primer plus 1 coat flat oil paint on plaster: 1.6-3 perms

   In general, it's safe to say that painted plaster or drywall allows enough drying toward the interior that you don't have to worry that the paint will trap moisture.

3. 
   nvman | Jun 27, 2013 05:47pm | #3

   I too am curious about the answer.
   Our home started off with two coats of latex on drywall.
   Our teenage son and daughter each wanted a different colour after a couple of years.
   So two more coats of paint.
   In a few years, they moved out and my wife wanted the rooms painted so two more coats.
   In seven years, two rooms have had six coats of paint.
   Would that make any difference?

4. 
   cldlhd | Feb 15, 2023 07:31pm | #4

   I wouldn’t think it would trap moisture but in a cold climate during heating season assuming the wall cavity is colder than inside the room couldn’t you have condensation inside the wall cavity?

1. Expert Member
   DCcontrarian | Feb 15, 2023 07:48pm | #5

   At the risk of replying to a ten-year-old thread:

   You always need a vapor barrier. In a cold climate, if you're letting the wall dry to the interior you've got a wrong-side vapor barrier. With a wrong-side vapor barrier you always need enough impermeable insulation on the cold side so that any permeable insulation stays above the dew point.

1. Expert Member
   Michael Maines | Feb 16, 2023 09:29am | #6

   DC, I disagree--you always need vapor control, but having a vapor barrier in a wall is rarely a good idea. A vapor retarder, preferably one with variable permeance, is often a good choice. (You have your pet peeves regarding words; I have mine!)

1. Expert Member
   DCcontrarian | Feb 16, 2023 10:58am | #7

   You're absolutely correct, words matter, I should have said that you always need vapor control. The original question is vague, but if the issue is the only drying path for the wall is to the interior then presumably there is some sort of vapor retarder or barrier to the exterior.