by Andrew Morrison
I know there are those in the world of straw bale construction who don’t like the idea of using welded wire mesh on their projects. I understand that for some, the inclusion of the mesh is considered counter to the concept of natural building as the production of steel is relatively resource heavy. I, on the other hand, believe that the use of mesh is vital to creating a strong, durable, and well built structure that will not only ultimately reduce the use of resources over time, but also speed and simplify the construction process. Consider that a home built with mesh is stronger and more durable than one built without mesh and that the added durability will mean less repairs, less chance of water intrusion and less reuse of resources such as plaster to make the otherwise needed repairs. I have built and/or consulted on hundreds of straw bale homes and I have used many different methods over the years. I can honestly say that the use of mesh has improved the overall results so much that I can’t imagine going back to building without it. To give some reference as to how I got to this place, I should mention that I have built projects without mesh of any kind, with jute netting, with chicken wire, with plastic mesh, with 16g and 14g welded wire mesh (1” x 2”, 2” x 4”, and 2” x 2”), and with several different types of either rebar, wood, or bamboo pinning (both internal and external). The best results from all of these approaches has been the use of 2” x 2” welded wire mesh (either 14g or 16g), bar none.
Following is a list of the benefits of building with welded wire mesh from a construction standpoint.[restrict userlevel=”subscriber”]
The use of welded wire mesh can effect the shear value of the structure in two main ways. The mesh can be used as the primary shear resistance for the structure or the secondary/redundant shear resistance. In the case of the primary shear, the mesh is the only requirement for protecting the building from racking under shear loads. Although the requirements of this system are rigorous, they can simplify the construction process and save an owner/builder money during construction. What is more common is to use the mesh as a secondary/redundant shear system. This is accomplished by using the mesh in conjunction with a primary shear system such as X-bracing, brace frames, or other similar systems.
Plaster is similar to concrete in the sense that it has very high compressive strength but relatively low tensile strength. Although lime and earthen plasters are more flexible than cement based plasters (which should not be used on straw bale structures at all in my opinion), they still benefit from reinforcement. The 2” x 2” welded wire mesh provides plenty of space for the plaster to move through it and key tightly into the straw. This process means that the mesh will end up fully embedded in the plaster, strengthening it, without isolating the plaster from the straw. The plaster to straw connection is vital for long term durability, so this size mesh is a perfect match.
The old method of hanging cabinets on wood backing affixed to the bales with dowels is simply not as good of an approach as the use of the mesh. I attach all of my cabinets by notching wood nailers into the bales such that they end up flush with the surface of the wall. I cover them with roofing felt and then temporarily tie them to the bales with baling twine. Once the mesh is installed on both sides of the wall, it is stapled to the nailers roughly every six inches and then sewn from one side of the wall to the other. The nailers are thus locked firmly in between the mesh layers and unable to dislocate as a result, no matter how much weight is applied to them.
I was never a fan of the old school vampire spikes used in the electrical installation process. I found that the boxes were never attached well enough to the wall to avoid shifting during the plastering process. I also found that leveling the boxes was difficult as they had a tendency to pull out of the wall, even when plaster lath was used to help anchor them to the bales. I now use plywood plates with “old work” electrical boxes installed in them to create very strong and permanent points of installation. The plywood plates/electrical boxes are anchored to the wall with the mesh, and cannot move up, down, side to side, or in and out of the wall. Plaster rings are then used to extend the boxes to the face of the finish plaster, meaning the depth of the box will always be exactly what it needs to be.
Simplifying stuffing and shaping
Adding stuffing to the walls won’t do much good unless you have a way to hold it in place. After all, loose stuffing will easily fall out of the wall if not anchored firmly behind either mesh, burlap, or some other fastening system. The 2×2 mesh does an excellent job of anchoring the stuffing while still allowing the plaster to penetrate and bond directly with the straw, something that burlap or other “solid” materials cannot do.
The mesh adds excellent out of plane resistance to the walls. This means that when a force is applied perpendicular to the wall, the bales will not fall over or move “out of plane” when the mesh is properly installed. The strength that is thus added to the wall improves the overall quality and durability of the entire wall system. It also improves the plaster penetration and bond because the walls are able to manage more force applied to them when the plaster is installed. The harder you can push the plaster into the walls, the better.
In my opinion, if you want to build a durable and high quality structure, then mesh must be an integral part of your design and construction process. Making sure that the mesh is installed properly, with adequate tension and fastening is paramount as well, so don’t skimp on the installation. Be sure to overlap the mesh by at least 6” when neighboring pieces abut each other. Fasten the mesh to all exposed wood at least every 6” with 7/16” crown, 1 3/4” long, 16 gauge staples. If you plan to use the mesh as your primary shear system, then the fastening details are more stringent; however, for most applications, the above details will suffice. As with all aspects of construction, the quality of the materials you use and the installation practices you employ will translate directly to the quality of the structure you are building. I suggest that you do what it takes to build the structure right the first time, and build it to last for generations to come. Mesh is a strong step in that direction.[/restrict]