La Casita

La Casita, the inlaw shack.
If you build it they will come.
uh-oh. might be a mistake!
We live in a rural area. You won't talk to anyone unless you meet em along the fenceline, down at the mailboxes on the main road, or the falafel shop down in town.
This spring we began working on this post and beam straw bale home.  My wife did the drafting and design work. 20'x32' outside dimensions with 10' and 12' covered area on the east and west sides respectively. Orientation and eve depth were studied for optimum seasonal performance.  16" x 16" concrete grade beam on gravel trench. Have all the windows (used and free) and bales stacked up in the greenhouse.
Fixin' to be dried in soon. Then I'll move on to whatever pick-up framing needs doing...Slow going being the only employee.
Once I get into bales and mud I'll be in uncharted territory for myself and could use all the armchair quarterbacking you can throw at me.
Critiques, comments, suggestions, welcome. I've never done this before.

Hi Ralph,

A couple of observations.  

It appears that you're using internal rebar "pins" to resist some of the lateral forces the walls might be subjected to?  Whether you need to pull a building permit or not, or had this structure engineered, there's a building code that covers this aspect of straw bale construction.  See IRC Appendix AS Strawbale Construction, Section AS 105.4.2 Pins.  You can download a free copy of this building code from the California Straw Building Association website, www.strawbuilding.org.  Even if your state hasn't adopted this code and you're not required to pull a permit, it offers useful guidelines on building code-level straw bale structures.

Paragraph (3) of this code section covers internal pins, and specifies spacing (24" OC), laps (a full bale depth), and connection to both sill and the top plate/roof assembly.  The commentary to this code states ..."the practice of internal pinning has fallen into disuse by most practitioners of straw bale construction due to its greater difficulty and cost compared with other means of achieving the same or better wall stability."

Even though it's considered archaic in N. America, plenty of straw bale buildings built prior to the early 2000s used rebar for internal pinning--it's just no longer common, due primarily to the difficulty of accurately impaling a 60 lb. to 85 lb. straw bale on the rebar so the bale surface stays in plane with those above and below it.  There are a couple of work-arounds--pre-drilling a pilot hole using a hole saw with bit extension, or using vertical 2x guides to keep the bale aligned as you push it through the rebar.  Note that internal pinning may not be sufficient by itself to resist lateral forces--you might need some other kind of bracing--if you haven't already planned on this.

Also, most straw bales in N. America used in straw bale construction are either 2-string or 3-string.  3-string bales are reliably 15" tall, 23" wide, and between 46" and 52" long.  There's greater variation among 2-string bales, but they are most often 16" tall, 18" wide, and between 40" and 46" long.  The only dimension among these common bales sizes that fits on a 16" wide footing is a 2-string bale placed on-edge.  I worked on one project in N. California where we used 2-string bales that were 16" tall x 16" wide, so bales could be placed either on-edge or laid-flat, but it's not a common bales size.

You might check out the California Straw Building Association's Straw Bale Building Details: An Illustrated Guide for Design and Construction,  also available through the CASBA website or on-line retailers.  It has been in print  for around five years (since summer of 2019), and is still the most current book on straw bale design and construction as practiced in N. America.  It references the straw bale construction code (available for adoption in U.S. code jurisdictions, and advisory elsewhere) throughout, and describes best practices for everything from insulating between sill plates to handling electrical and plumbing in straw bale walls, attaching partition walls and ledgers for cabinets, handling and stacking bales, plaster preparation, plastering, and more!

Jim
Many Hands Builders

Hi Jim,
Thanks for your well informed reply. I appreciate it.

I have been thinking about exactly this issue since it became apparent to me. How do I notch for timbers, keep the bales tight to each other and precisely stab bales onto the rebar?

The frame will be traditionally wood braced to resist shear forces. For out of plane forces, the plan had been the rebar, but meow....
I've got a few books on straw bale building, including the CBSBA book.  I should probably read them more thoroughly and not just glance at the pictures.  The detail concerning pins clearly states exactly what you mention.

The grade beam is actually 18" wide (as are the bales), not 16".  Had a micro-panic attack and had to go check. Once I'm done with a number my brain tends to purge it.

You've got me thinking about other options now. I may, as you suggest use 2x material as a guide for stacking, and then let in battens on either side, fasten them to the sills and top plate and sew them together.

We have no building code other than state electrical and plumbing.

I don't reserve much bandwidth for issues I'm not currently up against, but will be here soon.  Once the lid is on in the next week or so I'll be on to the pick-up framing, and finishing out plumbing rough in etc....  I doubt I will be stacking any bales until March or April.

Thanks again.

Ralph

Hi Ralph,

Do take a look at Chapter 3, Structural Design Considerations in Straw Bale Building Details: An Illustrated Guide for Design and Construction. The posts and beams should resist the roof weight and any snow loads you can expect. If an engineer didn’t have input into post size and spacing, and beam size, along with roof rafter or truss design and spacing, the next best thing is input from someone with lots of local experience building structures that have have stood the test of time.  

When deciding on a lateral force resistance system you need to know what you’re up against. Are the “governing forces” going to be wind or earthquakes, and if one or the other, what is the likely maximum velocity or magnitude?  Regions with very little wind and no seismic activity require less lateral force resistance designed into the wall assembly.

As for handling the other part of shear—"in-plane forces,” you might need to rethink your plan to use “traditional wood bracing.”  That can mean different things depending on the practices of where you build. In stick-frame construction it once meant letting in diagonal 1x or 2x braces at a 45 degree angle at all corners.  This was before plywood (and then OSB) sheathing became common as shear wall elements. It could also mean “diagonal metal strapping,” often called “X” bracing. And if this were a timber framed structure, it might mean “knee bracing.” Unfortunately, all of these bracing methods were developed for relatively light framing systems, i.e., wall weights of between 10 lbs. and 15 lbs. per square foot of wall (see explanation on page 72 of CASBA’s book).   A plastered straw bale wall weighs between 45 lbs. and 55 lbs. per square foot—much heavier!

Also, building codes (all buildings, not just straw bale buildings) require a fire barrier between walls and attics or roof spaces, which a box beam accomplishes. Your building appears to have a 4x top plate (?) to separate just a portion of the bale from roof cavity insulation. One option is to  attach ½ or 5/8” plywood plates the width (18") of the straw bale wall to the bottom edge of your 4x top plate, then add a 2x to the inside edge of that (...and you’re about ½ way to a box beam!).  The 2x gives you something to attach interior mesh to for the interior plasters (if you use mesh), and also gives you something along the ceiling edge to better secure drywall or T&G.  This “shelf” makes it easier to stack the top course bales as they don’t need to be notched around the 4x top plate, but it’s cantilevered and will sag unless supported along the interior edge either from the roof assembly framing or thoughtfully spaced 2x “posts” at windows and doors.

Jim
Many Hands Builders

Hey Ralph, this looks great! What will you be using as roof insulation?

Thanks. It’s coming along. Slowly but surely. The insulation for the ceiling is still to be decided. If I had my druthers, I would be using wool. Still hoping to find a rancher that might be able to accommodate. Even the meat sheep get shorn.  Probably will end up using recycled denim. In the great arm wrestling match of time, energy, performance, and price, babylon tends to win, but we will put up a struggle. We’ve already compromised the original design ethos with the 1” of rigid foam over the roof deck, not to mention the OSB or concrete grade beam replete with PT plates..
I finally got the metal on and am all buttoned up on top. A few details and it’s time to start stacking.