Natural plasters go hand-in-hand with straw walls, protecting them from moisture, fire, and pest intrusion, highlighting subtle wall shapes, and providing a distributed thermal mass that makes straw walls energy efficient and comfortable. Natural plasters also have a very low embodied energy cost compared to manufactured wall surfaces like cement stuccos or drywall. Clay plasters can also be applied over sheet rock surfaces to create a hand-crafted surface that absorbs and releases moisture, helping to moderate indoor humidity.
Why does a straw bale wall need to be plastered?
The bale surface is rough, with straw poking out and creating edges, folds, and voids. Air can move through this area, it’s a surface prone to flash burning, and it can become habitat for insects and mice. Plastering the bale surface seals it from air movement, pests, and makes it fire retardant. Even if the exterior finish surface will be siding, or the interior will be drywall, the bale surface needs to be sealed with a plaster (interior) or stucco (exterior).
Does earth or lime plaster have different thermal properties than drywall?
Earth plasters are usually between 1” and 1.5” thick, and lime/cement plasters are about 1" thick. Drywall is usually only ½” thick. Per unit thickness, earth or lime plasters store much more energy than drywall. It’s like the difference between a steel wood burning stove and a masonry wood burning stove—they both burn wood, but one cools off as soon as the fire goes out while the other stays warm for a long time. The distributed thermal mass of thick interior plasters on a straw bale wall slowly absorbs and gradually releases heat-- moderating temperature swings due to daytime warming and night time cooling. This reduces the need for fuel or electricity to regulate interior temperatures.
Is it OK to use cement plaster on a straw bale wall?
Yes and no. Cement is much stronger than lime, and can be used in the structural design of the building. However, it has a very high embodied energy cost and fairly low vapor permeability, and many believe it traps moisture inside the wall. Very old cob buildings in Europe and adobe buildings in the American Southwest were originally covered with lime plasters, and stood for centuries. They experienced serious degradation only after cement plasters were used in place of lime plasters. The problem?—moisture trapped inside the wall. The solution?—replace the cement plaster with lime plaster. When plastering over a straw bale wall, replacing just 25% of the cement with lime greatly improves the vapor permeability without compromising strength. Lime, often used in both exterior and interior plasters on straw walls, has an embodied energy cost 80% lower than cement, so it’s a better environmental choice.
Why do plasters and stuccos have a scratch, brown, and finish coat?
We need multiple layers of cementitious plasters and stuccos because they’re relatively brittle and crack. The first coat—the “crack” coat—is covered by the second coat, which has fewer cracks, and that is covered by the third coat, which has fewer still. Unless there are issues caused by settling or other structural stresses, finish coat cracks are superficial and generally don’t go through all three coats, so water intrusion isn't a problem. Lime plasters cure more reliably when applied in thin layers, too. Earth plasters are often applied in a three-coat regime because it’s familiar—and handy to build up flatter and more uniformly thick surfaces--although this isn't strictly necessary.
How to plasters and stuccos “cure”?
There are two kinds of lime plasters—hydraulic limes, and hydrated limes. Hydraulic limes like TransMineral’s NHL have an initial set caused by contact with water, then a secondary set created by the reabsorption of C02 from the atmosphere. Hydraulic limes tend to set a little faster than hydrated limes, like Type S or slaked lime. Even though they may appear to be dry, Type S lime, often called “bag lime,” has already been partially re-hydrated, so the hydrated limes set by reabsorbing C02 from the atmosphere. Both hydrated and hydraulic lime should not be applied when temperatures are expected to drop below 45 degrees while they are curing as they can absorb CO2 only above this temperature. Earth (clay) plasters cure by moisture evaporation. When mixing earth plasters it’s wise to use the driest mix that is still workable, because really wet plasters tend to crack more—as the volume of water leaves the plaster it creates voids…or cracks.
Can plasters be applied over drywall?
Yes, and this treatment adds a texture and luster not possible with paint! First, the drywall needs to be prepared as carefully as if it were to be painted—joints taped and smoothed so they’re not obvious. Depending on the plaster treatment, you may need to “prime” the drywall with a sanded wheat paste to give the plaster more grip. Then a very thin—1/16” to 1/8” plaster is troweled onto the wall. Hand applied clay plasters evoke an old-world life and radiance, reflecting light differently than a completely flat, painted drywall surface. Also, clay plasters absorb moisture and slowly release it, moderating room humidity, unlike painted drywall where moisture tends to collect on the surface, bead up, and run in humid places like bathrooms and kitchens.
What’s the difference between plaster and stucco?
It’s nomenclature unique to North America. Stuccos and plasters can be the same material and mix, but stuccos are applied to exterior walls, and plasters go on interior walls. In Europe, they use the word “render” to describe exterior plasters.
Are interior earth plasters durable? Are they easy to repair?
Earth plasters can be very durable, and they can be easy to repair. They can also be fragile and challenging to repair. Because they are much thicker than paint it can take some doing to chip or scratch a clay plaster. Anticipating the need for occasional repair, most plasterers leave the homeowner with “cookies” made from the dried left-over clay plaster. If a wall gets scratched or chipped, just reach for the right colored cookie—add a little water to form a paste, and apply it to the damaged area.
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