posted 3 years ago
Hi Jeffrey,
Especially in areas with wind-driven rain, it's common to see straw bale buildings with rainscreen siding installed after a scratch coat has been applied to the exterior surface of the bales. It sounds like you have scratch coat in place; if well done (no holes or cracks or gaps at joints), this keeps critters out and helps air-seal the wall.
In conventional construction, horizontal siding is usually attached through sheathing into stud walls, on the same centers as the studs. If your building is a post-and-beam (also called non-load bearing) structure with the straw bales serving as insulation infill between wood framing members, you already have some of your siding attachments in the wall. Vertical siding is similarly attached, although the supports run horizontally and have holes drilled in them or are scalloped to allow for air flow and drainage.
For most post-and-beam framing systems used in straw bale construction, vertical members are usually at corners, and surround windows and doors. Horizontal framing is usually only at the sill plate and top plate/box beam, and above and below windows. Vertical framing may also be placed at other locations on the exterior wall depending on post size (2 x 4, 4 x 4, 4 x 6), top plate or box beam design, and expected roof loads. Sometimes posts are on 2' or 4' centers, sometimes on 8' and 10' centers. Just depends on site conditions and roof or second floor loads.
Most rainscreen siding systems here in S. Oregon where I work leave just a 3/8" to 1" gap between the exterior sheathing and the backside of the siding--that's really all you need.
If I were presented with your project, I'd first identify where the existing vertical framing is, then decide how frequently the siding needs to be attached. 24" centers is pretty common, and most commercial siding is designed for this--it flexes if attachment is less frequent. You may be able to offset that by using thicker siding boards. Still, you'll probably need to let in some vertical support members so the horizontal supports aren't bouncy. Not difficult to do, but can be messy. Clay-straw-sand plasters can be chipped or cut with hand and power tools--cut a vertical channel for a 2 x 4 let into the plaster surface, and secure it to the top plate and sill plate with nails. Once you have enough vertical supports on centers you're comfortable with, and they're and their all more-or-less in plane, you can attach your horizontal 1 x or 2 x supports for the vertical siding. Be sure to use metal insect screen at the top and bottom of the wall to keep critters out, but still allows air to flow and rain that makes it past the siding to drain.
You'll also need to address how this new wall surface meets windows and doors, which are now going to be recessed the depth of the horizontal stand-offs + air gap + vertical siding thickness.
There's a detail in the California Straw Building Association's book Straw Bale Building Details: An Illustrated Guide for Design and Construction, 2-24, page 43, "Rainscreen over straw bale wall." It shows horizontal siding, but you can easily imagine how vertical siding would attach.
Jim
Many Hands Builders
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