What happens to inward drying assemblies after a dozen coats of paint?
rocket190
| Posted in Energy Efficiency and Durability on
I’m just curious what happens to the assemblies after a house has been lived in, repainted, sold repainted, turned into a rental (lots of re painting), etc. you get the idea. I’ve only read about permeability after a few coats of paint and want to know expert opinion about if this can lead to future disaster.
Also, has anyone measured the permeability of GWB with a covering primer like Kilz that is shellac based?
Finally, how does the permeability of thin veneer plaster compare to level four drywall or skip troweled drywall mud?
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Or has vinyl based wall paper put up? Or behind acrylic showers? Tiled walls? It has never seemed like a very resilient drying strategy to me.
Rick,
The short answer is that each coat of paint reduces the vapor permeance of the painted layer, and that vinyl wallpaper, acrylic showers, and glued-on mirrors are vapor barriers.
Does this matter much? As with all building science questions, the answer is, "It depends."
Vinyl wallpaper and glued-on mirrors can definitely cause mold problems in air-conditioned homes in hot, humid climates. In such climates, don't install vinyl wallpaper, and think twice before installing a glued-on mirror.
Drywall with two coats of latex paint has a vapor permeance of about 5 perms. That's fairly permeable.
Plaster painted with two coats of oil-based paint has a vapor permeance of 1.6 to 3 perms.
Another source reports that two coats of enamel paint on smooth plaster has a vapor permeance of 0.5 to 1.5 perm.
One coat of vapor-retarder primer is supposed to have a vapor permeance of 0.6 to 0.95 perms.
In short, multiple coats of paint lower the vapor permeance of the plaster or drywall, but not so much as to cause problems.
One key to making a wall assembly more resilient: make the interior finish layer as airtight as possible. This limits moisture entry into the wall assembly. If the wall assembly never gets very wet, it really doesn't have to dry very much.
Martin wrote:
"One key to making a wall assembly more resilient: make the interior finish layer as airtight as possible. This limits moisture entry into the wall assembly. If the wall assembly never gets very wet, it really doesn't have to dry very much. "
That's very solid advice.I do think though that, a major attribute of resiliency in building assemblies is the ability to withstand errors in execution, or change in conditions over time. Drying to the inside just seems to pre-suppose that homeowners have an understanding of building envelopes I don't ever see outside enthusiasts here.
Shellac is fairly vapor permeable- wouldn't worry too much about the Kilz. Old school leaded alkyd paints can be very low permeance, and can be an issue on a number of fronts.