Answering Your Straw Bale Building Questions – Part 2
Question
I feel that meshing up the walls with metal-mesh is like sitting in a metal box, especially if there is a thunderstorm approaching. What can I use instead?
Andrew’s Response
Unfortunately most building codes that apply to straw bale construction require you to use the 14 gauge 2″x2″ welded wire mesh because it provides protection from shear forces. This is why it is the preferred system of most straw bale builders and the shear system I describe in the comprehensive DVD.
There is research using a plastic mesh in place of the wire, but it is not even close in strength. If you really want to avoid using the mesh, you could use a 16 gauge metal strapping from Simpson Strongtie and place it at 45 degrees throughout the building. You need to wrap the strapping around the two toe ups before you frame and keep the strap coiled up at the exterior toe up. This provides the required nailing surface for the strap. After your walls are up, bend the strap so that it runs at a 45 degree angle and nail it to the top beam. This takes some pre planning so you avoid windows and doors and get a good angle on the strap. You may need to run a wood block at the wall half height and run two straps in narrow areas. The straps need to run in both directions so there is always a strap in tension as they do not function in compression. Then you can use the plastic mesh (orange contractor fencing with a tight diamond grid is best) to shape the walls.
Question
Does thickness of plaster help moisture problems?
Andrew’s Response
Sort of. If the plaster is at least 1¼” thick, it will help keep direct moisture off the bales. Be sure to use a plaster that breathes or the problem will be made worse. I like the Natural Hydraulic Lime (NHL) shown in our Comprehensive DVD. It does a great job as a breathable material and reduces most if not all moisture problems.
Question
I have heard that metal in bales can cause dampness due to condensation does this not apply to the two inch welded grid (welded wire mesh)?
Andrew’s Response
The grid is considered thin enough to not introduce a lot of condensation. The idea is that the condensation forms where there is a drastic temperature difference. The grid does not fit that concept as it is so small it does not contain a temperature gradient large enough to introduce condensation.
Question
I am researching steel frame construction for possible hurricane/tornado resistant type home, can I utilize straw as the insulating medium for the outside walls, sandwiching the straw between an outer and inner steel wall?
Andrew’s Response
Straw can be used with steel framing although you need to isolate the steel from the straw with roofing felt so condensation does not enter the bales.
Question
Since I live in sunny England there is of course an issue with rainfall and splash back onto the plaster wall. Do you have any suggestions that can help me with this problem?
Andrew’s Response
There are two main ways to protect against rain splash. The first is to use a house wrap or building paper around the bottom three courses of bales. Simply staple the wrap to the bottom toe up under the bales and then use landscape pins to hold the wrap in place along the top edge. This works fairly well. The other option is to build pony walls that actually raise the bales away from the floor by about 2 feet or so. Two feet is a good number because you can use one piece of either drywall on the interior or plywood on the exterior ripped in half to cover twice the distance in sheathing. This also provides a chase for all the wiring and plumbing in the walls. You can use the space below the pony walls (short framed walls) to run all of that stuff while avoiding the bales entirely.
Question
Can straw be used in states where the humidity is high in summer months?
Andrew’s Response:
In terms of humid states, it depends on how humid and if the area gets a chance to dry out. If the relative humidity is so high that the bales would reach and sustain moisture contents of 20% or higher, then you cannot build with bales. I do not know the comparison of bales with foam. Bales are generally rated as R-40. Perhaps that will help.
Question
How do I go about plastering the ceiling?
Andrew’s Response:
Plastering the ceiling is very much the same as plastering the walls. The biggest difference is you need goggles and a big hat! Okay, that’s not entirely true. You will likely drop a lot of plaster in the process, so glasses and a hat are a good idea. The ceiling will only be a finish coat of plaster as it will be drywalled, not baled. If you use the Natural Hydraulic Lime (NHL) for the finish coat, increase the amount of lime in the mix to make it a bit stickier. You don’t have to do a lot. If you removed two shovels full of sand per bag of NHL, that would do it. If you are using the premixed finish NHL, you may want to run a bit more water in it to make it more workable. In other words make spread further. If you are using a different plaster, like earth or gypsum, the key is experimentation. You want the mix to be sticky, that is for sure. Be sure to use some type of joint reinforcement at the ceiling edges. This could be paper or fiber drywall tape embedded in the wall plaster and mudded to the ceiling. I would suggest you do the ceiling before the final coat on the walls but after the scratch and brown on the walls.