Timber portal frame building wrapped with straw bales

Hi,

I haven't seen this done before, but I'm planning a timber portal frame building wrapped externally with straw bales.
This would result in the timber frame being fully visible on the inside (plastered sections in between portal frames).
Seems like a simple way to me, as it avoids notching and cutting bales as much as possible.
All bracing must be achieved by the portal frame or by adding shear walls.
External face would be completely plastered with no timber showing.

Would go something like this:

Poor slab - wide footings to allow for walls to be stacked outside posts.
Stand the portal frames (sized for full bales)
Roof
Fix welded wire mesh around the entire external face of building (this will form the internal face of the walls.
Fix window and door frames to the outside face of portal frames and wire mesh (can cut out wire mesh later).
Stack bales (minimal notching and cutting if windows/door frames are sized correctly).
Compress bales - Maybe with a box beam fixed to the top external face of portal frames and strapped to the toe?
welded wire mesh again to form the external face of the wall.

The part I'm most unsure about is bracing and getting the bales compressed.
Has anyone got experience/pictures using this method or is there a reason I can't find any examples haha..?

Thanks!

I seems I have seen this somewhere like maybe homestead rescue.

Here are some videos:







https://permies.com/t/94336/Questions-straw-clay-timber-frame#773974

I hope these help.

Sam, your idea will work.
As for the technical bits there are good publications which will give you details too hard to explain here.

The part I'm most unsure about is bracing and getting the bales compressed.
Has anyone got experience/pictures using this method or is there a reason I can't find any examples haha..?

In some cases compression is not even done, but it does cause issues with the wall plastering.
If you have trouble finding the books, get back and I will list some.

John C Daley wrote:Sam, your idea will work.
As for the technical bits there are good publications which will give you details too hard to explain here.

The part I'm most unsure about is bracing and getting the bales compressed.
Has anyone got experience/pictures using this method or is there a reason I can't find any examples haha..?

In some cases compression is not even done, but it does cause issues with the wall plastering.
If you have trouble finding the books, get back and I will list some.

Cheers for the reply, I've found these 2.

Straw Bale Building Details CASPA
Building_with_Straw_Bales_A_Practical_Guide_for_the_UK_the_Ireland_by_Barbara_Jones

Any others that specifically cover the compression for this method?

Hi Sam,

Thanks for the referral to Straw Bale Building Details: An Illustrated Guide for Design and Construction by the California Straw Building Association. As detailed as this book is, it’s not quite granular enough to answer your question.

It’s not uncommon to pair timber framed structures with straw bale wall wraps, with the timber frames either interior or exterior of the straw bales, but this coupling complicates the structural design.

Modern timber framed structures tend to use lighter-weight insulated wall assemblies like either foam-filled structural insulated panels (SIPS) or framed, plywood skinned walls with blown-in cellulose insulation.  

You’ll want a structural engineer to have a look at your plans because a plastered straw bale wall is considerably heavier than either SIPS or blown-in-cellulose wall assemblies. There’s an in-depth discussion on pages 69 – 74 in the CASBA book’s chapter on structural design. In a nutshell, the weight per square foot of wall area (PSF) for a plastered straw bale wall can be several times heavier than more conventional modern wall systems, i.e. 40 – 60 PSF vs only 11 – 15 PSF!

The wall weight needs to be factored into the calculations on loads and forces the timber frame resists, which will depend on where you’re building (high snow loads? high winds?  seismic activity?).

If I were building in Nebraska high winds would be the governing factor.  Where I live and worked in S. Oregon high seismic activity is a bigger concern.  A few years ago I worked on a timber framed (not a timber portal, which is a little different) straw bale wrap project.  The timber frame came from a one hundred-fifty-year-old barn that had been dismantled in Wisconsin and transported to the Oregon building site.  

The frame itself was in outstanding condition—clearly able to resist the snow loads and wind forces it experienced for over a century on the Wisconsin prairie.  However, when wrapped with straw bales on a building site where earthquakes are common, supplemental bracing was needed. In this case the engineer specified steel strap tension only “X” bracing at several places around the building perimeter, carefully anchored at the sill and top plates, to handle in-plane forces (like wind or earthquakes applied parallel to the walls).  There are at least nine other ways to address in-plane forces with straw bale walls, all detailed in the book.

To handle out-of-plane forces (wind or earthquakes applied perpendicular to the walls) the engineer specified 17-gauge galvanized stucco mesh tied-through to the same mesh on the opposite side at 2’ horizontal centers at every bale course. This effectively creates a “basket” so the bales work together--the wall can wiggle, but it won't buckle.

Other ways to address the out-of-plane forces (all of which are specified in the IRC (International Residential Code) Appendix AS Strawbale Construction, Section AS 106 Strawbale Walls—Structural, include the 2x2 14 gauge welded wire mesh you mentioned, polypropylene mesh (deer fencing, and often used with clay plasters), and also paired battens (bamboo or 1 x 1 fir poles, etc.) extending from sill to top plates and similarly tied through on 2’ centers at every bale course (think splinting a broken leg).  Although it has fallen out of use in the United States, interior pinning of bale courses with rebar, bamboo or wood poles can effectively resist out-of-plane forces, too.

Because straw bales have a rough surface that supplies plenty of tooth (lath) for plaster, mesh isn’t strictly necessary unless it performs a structural role. Many builders prefer to work without mesh if they can because it reduces both material and labor costs.

RE compressing the bales. If you purchase dense bales they won’t compress much, at least not in single story wall heights. Both CASBA's book and the building code IRC Appendix AS describe a method for determining a straw bale's dry density, which needs to be over 6.5 lbs. per cubic foot. I was able to find bales that averaged between 7 and 9 lbs. per cubic foot.

Bales placed on-edge compress less than bales laid flat, and bales filling the top several feet of the wall can be oriented upright (on end) and over-tightened (using compression straps) then released once in place. This functions like a “spring” to take up any space caused by settling.

The only other method I'm familiar with requires more planning and can be cumbersome; placing strapping (heavy-duty fiberglass pallet strapping) placed under the sill plates at 2' centers when the sill plates are first installed). If the strapping is long enough to run over the top plate on both sides-it needs to be coiled and tucked out-of-the way during most of the bale stack, or cut with just enough strapping exposed to attach buckles that secure longer pieces that run over the top bales before being cinched tight. When the second to last bale course is complete the straps go up and over that course and are sinched down, compressing the bales in the wall. With any luck, the top bale will snugly fit into the remaining space. I didn't favor this method for a couple of reasons: if we weren't doing the framing we needed to coordinate with another crew for which this was an unfamiliar step, we needed to keep the straps from getting tangled in pre-bale-stack construction tasks like running electrical wires and plumbing.

RE a box beam.  When I have wrapped a timber frame with bales we affixed a box beam to the timber frame and used door and window posts set in the exterior bale surface to support the box beam’s outside edge to prevent sagging.  

Finally, if your building performance goals include energy efficiency I wouldn’t attach mesh  to the outside of the timber frame (inside surface of the bale walls) and stack bales against the mesh for two reasons.  

First, it’s impossible to stuff gaps through the mesh from the inside, which will probably result in wall insulation that is uneven, with convective loops forming between the plaster skins that readily move heat between the wall surfaces, lowering overall R-value.  

Second, gaps will emerge where the plaster and timber frames meet—the plaster may shrink, and the timbers will swell and contract with seasonal humidity changes.  I have seen and attempted repairs on many such gaps in timber framed straw bale construction.

A better solution, assuming the structural design doesn’t call for mesh attachment to the timbers, is to secure (glue, screw, nail) thin (1/2’) plywood “fins” to the outside edges of the timber frame (inside surface of the bale wall), extending around 1” – 2” onto what will be the bale surface once the bales are stacked.  Interior mesh, if used, can be stapled to these fins which are then plastered over. As the gap between the plaster and timber frame opens and closes the fins block air movement.  I take an additional precaution and place a folded layer of building paper here, stapled to both the fin and the timber, not so much to separate the timber frame from plaster (code requires a separation between exterior lime plaster and wood framing due to lime’s tendency to wick moisture inwards), but to further air-seal that joint.