Wood siding over straw bale exterior walls

I built a cape cod style straw bale home in 2012.  The interior walls are completely finished with a gypsum plaster and clay paint.  The exterior however has been in the first coat adobe phase for nearly 9 years!  It’s sad, I know and I feel shame every time I pull up and see it. I also have no desire to do anymore plastering, another sad fact.  Fortunately I have not had any issues with rodents or insects, I am in the SE USA.  The main level of the house has the sb exterior walls, the second level gable ends are conventional construction and have a hardi board exterior.  I am considering doing a board and bat vertical siding over the outdoor bale walls.  I feel leaving a 4” air gap would keep the bales happy and maybe even improve the insulative properties of the wall.  I also like the durability factor as it will better protect the walls from sunlight, wind, and physical hits by furniture and such.  The house has a complete wrap around porch so moisture has never been an issue. So I was just wondering if there is anything I am missing that would make this a not so great idea.  I am planning on using rough sawn and planing it myself to halp reduce the cost.

Howdy,

The only thing I would look for would be areas where adobe coating might be thin or places where you can see straw that is exposed. I've only worked on one straw bale house. I live in an all wood home, inside and out, going on 50 yrs.milled most of it myself, and the wood is tinder dry so the exterior siding is only going to get drier with time. After the fires we've had out here the last few yrs., I'm thinking more and more about residing with metal or cement siding.

Jeffery, Can you find somebody to finish plastering the walls ?
I have seen timber cladding done successfully.
Randal, fire protection is a special art.
Have you thought of sprinklers?

I don’t think I could afford to pay somebody to finish the exterior plastering, probably looking at hundreds of man hours.  
With lumber prices and fire hazards increasing, I am going to investigate cement board.  I already have it on the gable ends so architecturally it would come together quite nicely.

Howdy,

"Randal, fire protection is a special art.
Have you thought of sprinklers?"

Yes. On the top ridge of my house, is a sprinkler system, a 300' circle around the house (150 feet each side). I was surprised in sept.,2019 when I had to evacuate and would come up here during the day and it would still be going after 9+hrs (gravity feed,1000' 2" pipe,350' elev.drop/head). I am lucky with  a yr. round creek that has low summer flows bit has never quit. We were lucky that year and had a heavy rain in Oct. after 110 days of no rain and hot, over 100 degree temps. In the 1990's I was a wildland fire crew boss.

I do follow the fire seasons you've been having in Australia.

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