I invited my beloved to write a guest post reflecting months of research he has invested trying to find the best possible choice for the exterior and interior of our home. Please welcome Jason, and enjoy the writing he’s prepared for you today.
Straw bale houses come in all shapes and sizes and tend to reflect the diverse and creative group of people that look at baled grain husk and think, “I could build a house with that.” I’ve toured grand mansion/cabins and ratty, wavy-walled shacks all made from bales. The only thing that basically all straw bale houses have in common is plaster. Whether it’s old world lime plaster, cement stucco, clay-straw cob, or a dung-and-horse-hair pack, almost all straw bale houses use plaster.
Plaster is attractive for several reasons but notably because straw bales are inherently uneven. They vary in dimension, density, and don’t have nice, sharp corners and sides to stack. They look like nice little blocks but trust me, they’re not. Plaster easily rolls with that variation. Plus, bale houses lack the regular spaced studding that is necessary for cladding like siding, drywall, and plywood. But none of that is what makes plaster so crucial to the longevity of a bale house.
The reason is simple yet deceptive. Straw bales have a lot going for them – they are locally available, cheap, structurally strong, and easy to form and shape. But they have three big enemies – and all of them are water; liquid water, humidity (water in the air), and condensation (water falling out of the air). And a carefully designed plaster wall deals the best with that triple threat.
When most people think about protecting bales from water they think of water shielding. They think of tarps, shelters, and coverings. That is why many uninformed straw bale builders gravitate to high cement stucco plaster. It’s locally available, tradesmen are comfortable with it, and it is the most waterproof. But when you factor in the complete moisture picture it turns out to be functionally the worst plaster option.
The obvious and visible forms of water are rain, snow, dripping roofs, and the like. And obviously, any straw house must deal with that stuff too. But for most houses liquid water isn’t the biggest enemy. Roofs are excellent at redirecting it. But air has water in it – lots of water. If you condensed all the water out of the air in your house right now you would have literally hundreds of gallons of it. But at a constant temperature most of it stays locked in the air. The problem starts with temperature changes. Just the way water condenses on a cold bathroom mirror when it’s touched by the warm air from the shower; warm house air condenses when it meets outside air. If your house is +20 and the outside is -30 there is a 50-degree temperature difference. That’s why your house gets “foggy” when you open the front on a cold winter’s day. And this is unavoidable. As the warm air from your house moves “through” the walls towards the outside cold air, the moisture in it will condense out. Somewhere. Inside your walls. It is inevitable and however much you work to minimize it you must also work to deal with it and redirect it.
Modern construction tries to deal with this fact (with varying degrees of success) with plastic. Miles of plastic. You plastic the outside of the house, the inside of the house, you tape the window seams, you poly the interior of the roof, many roofers now poly the outside of the roof, and you tape, glue, tar, and staple everything. The problem is that it is quite literally impossible to make a house airtight. The reality is that water WILL get into your walls. The key is to keep the amount low and to know how to deal with it once it gets in. The lack of knowledge about this in the construction and building code world is why so many newer houses mold, leak, rot, and shift as much as they do. I have seen one-year-old houses with mold around the windows, water stains below the plug covers, sagging drywall, and more. The problem was that they literally can’t be airtight enough to keep warm, moist air completely out but they were too airtight to get rid of it once it was in. Like washing dishes with a rubber glove that has a single hole; the builders made a house that let a little water in and then trapped it.
Straw bale houses work on a different principle – they strive to keep liquid water out but they admit that humidity and condensation water will get in, then focuses on what to do to get rid of it. Bale walls are hygroscopic. That’s a fancy word to say that they “breathe”. Straw takes in and gives off moisture. It can “hold” some moisture problem-free. A lot of moisture, actually. Straw is typically baled at 12%-16% moisture. To get lower than that you’d need to kiln bake it. But straw doesn’t start rotting or molding until around 20%. By weight. And bales weight a lot. That means that my house, with the bales installed at around 14%, can take in over a ton of water without even flinching.
In a conventional house, 2000lbs of water in the walls would spell disaster. Wood holds little water, insulation less, and plastic holds none. The whole ton of moisture would pool at the bottom and wreck everything. But in a hygroscopic bale house that moisture is nearly evenly distributed among the bales. It comes out to less than 6 cups of water per bale. Hardly anything. So rather than building completely airtight (or worse, almost airtight) we built “liquid water proof” and then plastered the wall so that it could still breathe. True, this lets some moisture into the walls but it also makes sure that dry air either inside or outside will take moisture out.
This explains why siding, brick, stone, and drywall are bad partners of straw – they don’t breathe well and any moisture becomes trapped moisture. It may take years or decades but eventually your bales will rot. Plaster wins every time. But choosing a specific plaster involves balancing a lot of factors. Gypsum plaster lets out the most water; it “breathes” the best. But gypsum also rots when it gets rained on. So, it’s a good interior plaster but fails outdoor. Cement stands up to rain and sun the best but it breathes the worst and it is the most rigid; as your bales settle it tends to crack and then the cracks let in even more water. So, pure cement is better than siding but still not ideal. Straw-clay cob, a mixture of clay, sand, straw, and sometimes additives like fiberglass or hair is great. It stands up to rain better than you might imagine, resists the sun’s UV, is cheap (almost free), and breathes very well. But it is VERY labor-intensive to dig, screen, sieve, and slake clay, and it does erode over time. Lime plaster, the most common wall coating of western history until 50 years ago, is fantastic. But it is mostly available through specialty stores that focus on historical buildings. And it has to be shipped from California.
For us the best choice came down to high-lime cement stucco. It is a compromise between durability, ease of application, breathability, and longevity. Plus, it had the added bonus of having been time-tested in Manitoba weather conditions for several decades. We basically took the standard stucco recipe and nearly tripled the lime content. This made it stickier and caused it to set fairly fast but otherwise was easy to buy, make, and apply. When you use lime-only plaster you must use type-N lime (It’s prepared specially and hard to buy and expensive to ship from California) whereas high-lime stucco can use type-S lime which is readily available, cheap, and very proven. The mixture is not as waterproof as some, it is not as breathable as some, and it is not as flexible as some, but it is a good compromise.
It is important to apply the plaster in several layers. Typically, two interior and three exterior layers. The idea is to start with the hardest and most brittle layer at the base and then to apply each new layer with a higher sand ratio. This confuses a lot of people. I mean, wouldn’t you want your hardest layer on the surface where the weather gets at it? In fact, this is what too many bale builders have done, and they have paid a price for it. You always want your hardest layer (called the scratch layer or parge coat) on the bottom because it is the best at binding to the bales, benefits the most from the stucco wire reinforcement, will absorb the least water, and crack the most. It’s hard and brittle so it cracks. Its benefits are most realized at the base and the cracking is the least problematic once it is covered.
The next coat in a three-coat system is called the brown coat. It has more sand and less cement in the mix. In our case the ratio will also have a higher lime:cement ratio, too. This layer is applied much thinner than the scratch coat and is also sponge floated as it is curing. The reason is that the cement and lime aren’t the strength of the stucco. It’s the sand. The cement and lime are a binder, like glue, and hold everything together. The sand is the structure. Sand is an incredibly hard quartz mineral that is massively weather resistant. Plaster sand is rough, uneven chunks of quartz sand, never beach sand that has been worn smooth. This ensures that the sand can key together like a jigsaw puzzle and allows for a scaffold-like strength in the plaster. The sponge floating in the curing process mixes the sand around and makes sure that the smaller and bigger particles key together tightly. This makes the brown coat very strong even while the lower binder content makes it more flexible. If the process is done right and the curing is handled well the brown coat should contain no microcracks at all.
The final coat is the color coat. It is much higher in sand, much lower in cement, and largely uses lime as a binder. It is offering virtually no strength to the system but colors the wall beautifully. We sponge floated our color coat as the second layer on the inside where weather will be minimal.
We very much wished to experiment with straw-clay cob and measure how it compares to stucco for water resistance and hygroscopic breathing but we decided not to on our house. We might do a section on the garage and I will for sure try it on a chicken coop in the future so that with a moisture meter I can compare the strengths and weaknesses.
One last note of warning – whatever plaster you use for a bale house DO NOT use modern acrylic plasters. They are amazing, hearty, and nearly waterproof. But they were designed for modern watertight houses. They don’t breathe. They are essentially a fancy plastic coating. On a bale house, they will clog the breathability of the bales and ensure a musty, rotting house.
So far we have been very happy with our bale house. It doesn’t smell like a barn, it looks beautiful, the curves around doors and windows worked very well, and the plaster is completely crack-free. It also has the feeling of a hefty house. The walls are thick and filled with heavy bales. That makes a difference. We don’t hear or feel a wind howling outside. We don’t fluctuate temperature or humidity quickly because the bales act like a heat and moisture sink, and the house has a feeling of permeance. They’re not for everyone. I have built conventional houses before and there is at least 25% more labor in this house. If I had hired the labor out I would be broke. But we are happy with the results and the look is exactly what we had hoped for and a large part of that look and feel comes down to a well-executed, well-applied plaster.