How NOT to Plaster a Straw Bale House Wall: Avoid Cold Joints Whenever Possible
It's not unusual for people to get excited about plastering their straw bale house. After all, this is when a simple structure starts to look like a home. When the plaster goes on, suddenly, the building gets a new lease on life, and it really begins to feel like completion is near.
This is also a very difficult aspect of the construction process and one that will be very visible for the life of your home.
When people come to visit, the plaster is likely the first thing they will notice about the straw bale house. It will either look great and wow them, or it won't. Of course, you're likely hoping for the wow factor!
Okay, back to my point. Take a look at the photo above. What problems do you see? I see one major problem that happens far too often.
Several different people have applied the plaster and there are a series of wet edges all drying at once. Here's the biggest problem: this picture was taken during a lunch break, so the "wet edges" were left to dry for a while.
These dried edges will ultimately result in cold joints, areas where plaster of different wetness is in contact.
The fresh, wet plaster will not fully bond to the drying plaster shown here, resulting in a cold joint on your straw bale house wall. That's not a huge deal in this case because we're looking at a scratch coat, and cold joints are common throughout the field on scratch coats.
That said, it is always best to limit the cold joints in your scratch coat as best you can. Here’s how you can avoid cold joints on your straw bale house walls:
People plastering a straw bale house
As you can see in the picture above, working with lots of plastering hands is easy to accomplish while keeping a wet edge. Each person simply sets up near each other and works from the bottom up (or top down if you prefer) so that they overlap on the edges.
Section by section, the strawbale wall is completed with a constant wet edge in contact with a wet edge. Should you break for lunch or some other reason, it's best to stop in a straight line that will be located differently in each coat.
In other words, don't always stop 1' from the ceiling when you take a break, as the cold joints will then travel through all of the coats in which you stop in that location. You can imagine, though, that if this crew stopped for lunch now, their cold joint would be much more manageable than the one shown in the first picture.
-Andrew Morrison