Hi Charles,
Unless you find other builders in your area who have successfully added silicone to an exterior plaster of any kind, I would avoid it. If you do find someone who has done this, ask if you can see the job. I haven’t seen this practice described anywhere in
books on natural building and natural plastering published in the past twenty-five years. If you decide to try it anyway, just be prepared for a do-over in case it fails.
If your climate is very humid year-round, I'd rethink building with straw. But if it's seasonal, read on. Humidity can be an issue, but in my
experience here in the temperate Western U.S. where we have warm dry summers and cold wet winters, humidity is a challenge mostly when we're applying plasters.
A dry straw bale (under 20% moisture content, and preferably a lot lower!) doesn’t absorb a lot of moisture from the air even when it’s not plastered. Much like the way moisture content of dry
wood fluctuates comparatively little, the moisture content of densely packed straw bales may drift upwards during humid seasons, and then down when drier conditions return. I have monitored many un-plastered straw bale walls through the long, cold and damp winters of the Pacific North West and seen very little moisture content change. When bales are stacked dry (e.g., between 8% and 14% MC), prior to plastering they might absorb enough moisture from humid air to raise their MC a few percentage points, but I have never seen a reading above 20% MC caused by humidity alone.
You might check out
Serious Straw Bale by Paul Lacinski and Michael Bergeron. Although published in 2000 and perhaps somewhat dated in that many building practices have evolved in the past decades, it’s a great resource, and most of the many homes profiled in the book were built in Quebec, Ontario, New Hampshire, and other parts of South Eastern Canada and Nort Eastern United States. I'm guessing the climate is similar to yours.
Once the walls are plastered inside and out, and if you have done a good job of air-sealing all the gaps, cracks, joints, penetrations, electrical outlet box holes, etc., not much air is getting through the wall, and studies show that over 95% of the moisture found in walls rides on these air currents. A well-applied plaster is a fabulous air barrier, and if made of clay or lime it also has a vapor permeance of between 10 US Perms and 18 US Perms (per inch of plaster thickness). The bale wall itself—total width—is less permeable at 3 US Perms, so
water vapor migrating trough the wall driven by pressure differences (high pressure towards lower pressure) will take quite some time—perhaps weeks—to move that distance.
My concern about applying plaster to bales in high humidity conditions is that you need to think about how any moisture introduced to the bales incidental to plastering will get out. The liquid water in a plaster will initially exit the plaster skin in one of two directions. It’ll either soak into the surface of the straw bales it has been applied to, or it will evaporate. Moisture that soaks into the bale surface will eventually come back out, but that can take some time.
Here in Oregon I preferred to plaster exterior straw bale walls either in the spring so any moisture in the walls had the length of our warm, dry summers to exit, or no later than early fall so the moisture could come out before our wet, humid winter season began. Plastering interiors differs in that you can control conditions by increasing heat and running dehumidifiers. Outside you’re at the whims of nature, so best to do this work when conditions favor it.
Note that lime plasters need to be damp cured after each coat, so you’re looking at around three weeks (about one week per coat) of wanting the plaster to remain damp but not soaked, and then dry the walls as quickly as possible. Clay plasters don’t need to cure—they can just dry out. The more humid it is, the longer it takes.
My experience with light earth-straw (we call it light-straw-clay here) is different than Phil’s, and I’d hesitate to recommend it for humid climates unless the actual dry-time for that wall assembly was well understood. Conventional wisdom is that it takes about a week per inch of wall thickness, so a 12” wall (R-21, give-or-take) would require about three full months to dry enough for plastering. That may be true in arid climates like New Mexico and Arizona, but here in S. Oregon I think we need to wait much longer. A house I worked on a few years ago had 12” LSC walls. The building site was well exposed to wind and sunshine, and the LSC was placed in May of that year. Five months later when we planned to plaster the exterior we found that the walls were still far too damp inside, with MC readings into the high 20s and 30s in many places, so we postponed plastering the walls until the following spring. There's a chance our monitoring equipment wasn't suited for the task--we used moisture meters typically used for hay and straw. Lydia Doleman (author of E
ssential Light Straw Clay Construction, New Society Publishers) worked with me on that
project, and given that there aren't many other moisture reading devices out there, we decided to trust the meters.
I didn’t want to add a layer of plaster that would even further slow the dissipation of moisture from the wall. Instead, the homeowner heated the interior and ran a dehumidifier for much of the winter, and by early March the walls were plenty dry. All MC readings--and I recorded dozens per wall--were well below 20% MC.
I'm not saying “don’t build with LSC (or straw bales) in a humid climate." Just be prepared to hurry the drying process or postpone the plastering if the walls aren’t ready when you are.
Jim
Many Hands Builders
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