Say "No" to Straw Bale Roof Insulation
I must ask: WHY? Why use bales as roof insulation? The time, effort, cost, and embodied energy involved seem to heavily outweigh the benefits.
If two string bales yield an R-value of R-40 (debated, by the way) across the 18" direction, then one would only achieve an R-40 roof at best (perhaps a bit more with the inclusion of the other construction materials. A standard roof assembly provides R-38, so what is the point on Earth?
If you want a super-insulated roof, add a layer of rigid insulation on top of the roof assembly or an extra framing member to deepen the joist cavity and add more insulation.
There are environmentally sound insulations out there, and ultimately, the roof will have less embodied energy and be more environmentally friendly if done in a standard frame.
Consider engineered lumber, which uses smaller trees, and cotton insulation, which uses a renewable resource.Now compare that with ferro cement, poles big enough to handle the weight of the bales and the cement, 1x4 nailers, a heavy moisture barrier, and chicken wire. All this is experimental and may fail.
To me this is another example of trying to place bales in a section of a building where they do not belong. My two cents.
-Andrew Morrison