Replies

1. GBA Editor
   Martin Holladay|Feb 07, 2015 03:22pm|#1

   Alan,
   The vapor permeance of plaster and lath ranges from 11 perms to 15 perms. That is quite permeable.

   The old paint changes things, however. Two layers of oil-based paint on plaster and lath brings the permeance down to 1.5 to 3 perms.

1. 
   Ben Brewer|Mar 13, 2019 11:45pm|#3

   Hey, Martin.

   Do you know the vapor permeance of blueboard and plaster veneer? Wondering how replacing plaster and lath with blueboard and veneer could impact moisture management.

   Ben

1. GBA Editor
   Martin Holladay|Mar 14, 2019 04:44am|#4

   Ben,
   I imagine that veneer plaster base ("blueboard" lath) and plaster are both vapor-permeable, although I'm unable to find a technical source that lists the vapor permeance of veneer plaster base.

   If you are using these products, I would assume that they are vapor-permeable, like drywall. If there is a reason that you need a vapor retarder at this layer, you could include a vapor-retarder primer (paint) or a smart vapor retarder like MemBrain.

1. 
   Ben Brewer|Mar 14, 2019 08:27am|#5

   Thanks

2. 
   Alan B|Feb 07, 2015 11:16pm|#2

   Excellent, thanks :)

3. 
   Tom May|Mar 14, 2019 03:40pm|#6

   Yeah old, dry plaster and lath is quite permeable. Just throw some water on it and watch it suck it up. What condition are your walls and ceilings in? If they are in good shape, a good coat of primer/sealer and paint should suffice as others have suggested. I occasionally go over old plaster using diluted dry wall mud, skimming completely with three or more coats for a new smooth finish with minimal sanding.
   New veneer plaster is pretty much water proof, especially after painting. One can put blueboard directly over old plaster without all the hassle of removing the old stuff if the situation allows, especially, and a little easier with ceilings.

1. 
   kjmass1|Mar 15, 2019 08:50am|#7

   Interesting you say blueboard with a skimcoat is waterproof. I redid the walls in my 3 season room with blueboard w/ skimcoat 3 years ago and just recently I noticed one wall the skimcoat is completely coming off. This is an unconditioned room so it sees a wide temp range in MA. This wall shares against a conditioned space and has little insulation underneath in the exposed area. Somehow that area was ripe for failure, where the other 3 walls are fine. Just my 2 cents.

4. 
   Tom May|Mar 15, 2019 12:57pm|#8

   Is that wall or room subject to freezing temperatures? If it is on an uninsulated wall, perhaps moisture got in behind or into the blueboard and froze. I've also seen batches of plaster that had to be recalled over the years due to a bad mix at the factory., this also happens with blueboard occasionally. If old blueboard was used on that particular wall bonding may not occur if the bb is too dried out.
   Sometimes even when purchasing plaster, especially at the larger retailers, they get bad bags or have bags that got wet and they sell them anyway. These bags will usually have large chunks of plaster within indicating that they once were wet and the plaster may fail since the chemical reaction has already been started and will not cure correctly, usually setting up to fast. Sometimes if the room is to hot, the plaster dries out too fast after applying and becomes prone to cracking.

5. 
   Fred Williams|Oct 26, 2020 06:40pm|#9

   Martin,

   I too live in an older house (built in the 1860's) in zone 5 with lath and plaster finish (and old paint) on the interior side of exterior walls constructed of 2 x 4 (actual dimension) studs and clapboard siding (no sheathing, no insulation, and no building paper or air/moisture barrier). I am planning to upgrade the insulation and air sealing but would prefer to work from the exterior if possible, leaving the interior plaster and finishes intact. Should I consider only assemblies that dry to the exterior (e.g. Rockwool//m.etiketa4.com/article/wrapping-an-older-house-with-rock-wool-insulation) or will the painted plaster permit enough interior drying of the assembly to avoid problems (assuming a proposed whole wall R-value between R-20 and R-40)? I notice that Betsy Pettit added XPS exterior insulation to her own 1860's Greek Revival house and foil faced polyiso to her 1915 four square (Remodeling for Energy Efficiency
   //m.etiketa4.com/app/uploads/sites/default/files/Remodeling%20for%20Energy%20Efficiency_FHB194.pdf). I understand both of these types of foam prevent exterior drying. I wonder if she encountered this same question.