Strawbale flashing details question

Alright, anyone out there with Strawbale plaster experience, we have our bales on this 16” stone pony wall.   The stone is wall tied to a stud pony wall and sits an inch lower than the stud plate with mortar making up the last inch to the bales.  

Here’s the main question.  Should I/how would I flash this transition from bales to the stone/mortar?  I’ve seen many people suggest a plaster j channel or something similar where plaster meets foundation but that would obviously be much straighter than our stone wall.  

Any ideas or thoughts welcome!

What alternatives has research shown you that dont apply?

Sorry I should simplify.  The question I’ve had for the last couple weeks is “How do I flash this and make the transition look halfway decent?”

I think I fan probably find a stucco j channel to use but there’s not really anything solid for it to land on to support it.  I can pin it to the bales, but that still seems suspect to support the weight plaster will place on the channel.  
I’ve thought about doing the channel and filling any space underneath with mortar.  

Another option would be to just run steel roll flashing over the seam as low as I possibly can, since I can get it about 1” below the level of the bales, but there would be no kick out and I’m wondering if I’m asking for moisture to wick back and into the bottom of the bales?

Apologies if this is unclear.  Happy to try to clarify more.

Hi Eric,

Where are you building (country, climate)?  And just to confirm, this picture is of an exterior wall?   If you are in a relatively mild, dry climate that doesn't experience dramatic temperature swings (day to night, or seasonal), you might be OK with just running the exterior plaster down to the top of the stone wall so long as the walls are well protected from wind-driven rain and moisture from the ground doesn't splash/wick up the stone wall and into the bales.  

If you're building in a temperate climate or area with more extremes, or if the wall isn't well protected from moisture....

I have seen this challenge a few times when renovating older straw bale buildings.  Someone placed straw bales directly on a stone foundation and dropped the plaster down to the foundation, or even if the bales were on a sill plate the plaster still ran onto the foundation.  After a couple of years there were two problems:  (1) moisture wicked upwards into the bales causing them to begin decomposing, and (2) the thermal and moisture expansion characteristics between the plaster, foundation stones, and bales were different enough that the plaster at this joint began to crack, crumble, or delaminate,  

Over the nearly thirty years since the straw bale building revival started in the American Southwest we have learned a lot, and this is one of the lessons builders learned the hard way.  

That's why the building code available in the United States, IRC Strawbale Construction Appendix S addresses this specific concern in Section AS105.3 Sill Plates.  There you'll see that sill plates must be at least a nominal 2x4 (4x4s are also common) of a naturally durable or preservative treated wood, and the plates should be flashed to cover the joint between the exterior plate and the foundation (or stem wall) to prevent intrusion from water that might accumulate at the bottom of the wall.  You can obtain a copy of this building code from the California Straw Building Association's website, www.strawbuilding.org.

It sounds like you used mortar to cover the sill plate that you might have attached weep screed (a galvanized steel channel usually 3/4" to 1 1/2" wide, depending on plaster thickness, that has weep holes in the bottom?  If you're concerned about long term maintenance issues you might consider chipping away the mortar at the top of the wall, then adding the weep screed.  

In N. America the weep screed we have is easier to attach to something taller than a 2x4 laid flat.  You could let a 2x4 on edge into the bale wall in plane with the exterior surface; that would give you 5" of wood to attach a weep screed to.  I'd leave a narrow gap at the bottom so if water did drain from the screed it would drip free.  Because the screed will be somewhat above the joint between the stem wall and the sill plate, water dripping from the wall might actually be drawn into the joint.  You might consider flashing that joint with a peel-and-stick membrane.  The weep screed holds the bottom edge of the plaster above the stones, preventing contact with a material that may have different thermal and moisture characteristics, and it keeps water from wicking up into the plaster (and bales).

Jim
Many Hands Builders

Jim thanks for your response this is extremely helpful!

We are in the states, in TN to be exact, zone 4…last year we saw the full range from 90+F to single digits lows.  A good amount of rain but never more than a couple inches of snow.  

I’m embarrassed to say I have the straw bale appendix.  I’ve mostly worked off of it but also made some (wrong) assumptions learning as I go and this is the stage where I’m trying to get it right while I still can.  

I cant seem to attach another photo in the reply but this is what the wall detail involves.

Bales are up on a 14” pony wall.  
We’re looking at the exterior wall as you guessed and the exterior pony wall, I set in 3” and used stone from around our property with a lime mortar.  

-The wall has 30# felt, going up and over it underneath the bales
-battens, then fabric for a air gap
-battens, then lath
-stone mortared to the lath + wall ties.  

-while my main focus was getting the bales above grade with the pony wall I got too creative when I realized I had plenty of stone. In hindsight I should have kept the exterior pony wall in plane with the outside of the bales.  Instead I set it in, and while I kept the stone mostly below the top of the pony wall, due to compression the bales to contact the stone/mortar in many places with no break.  

-it sounds like moisture wicking is my biggest concern here.  All of your weep screed recommendations make sense to me.  Clay plaster is already up on the interior so let in 2x4s may have to be wooden pegged into the bales?

-the big unknown is between the top of the stone and where the bales are overhanging that portion.  Would it make sense to slide some galvanized roll flashing on flat back to the pony wall between those two and hang the screed in front of that?

If there’s a way to add more pictures I’ll gladly do that

Figured it out.  Not as many photos as I thought but It’s something.  I can sketch a wall detail too if that helps.  

Eric, thanks for the additional information.   A wall section drawing showing how all of the wall elements relate (foundation/stem wall, bales, veneer stone wall, box beam or top plate, rafters or trusses, etc.)  would be helpful.  A few more questions.  What is the stem wall core made of--rebar reinforced concrete masonry units that are core-filled?  Or reinforced concrete?  Also, this appears to be a load bearing straw bale building, correct?   And, do the bales actually overhang above the veneer stone wall, or is that just their fuzzy surface that seems to lap onto the stone, but they are actually stacked so their edges are more-or-less in line with the sill plate?

Although not related to your original question, what is handling the shear in this wall assembly (in-plane and out-of-plane forces like wind and earthquakes)?

Jim
Many Hands Builders

For sure, its a bit rough and ready but it at least gives the concept.

-The overall structure is a timber frame wrapped with the bales so not load bearing.  The bales are tied to the frame with lath ells stapled and staked.
-From foundation up, you have:
-rubble trench
-8"x24" grade beam poured rebar reinforced + footers and pads under posts
-double stud pony wall: two 2x4 walls anchored to grade beam plywood gussets spanning studs every 4' with rockwool insulation infill
-photo gives the best detail on the stone veneer, but its sitting on a -1" ledge let in to the beam
-bales on pony wall on nails
-It's not shown but the bales are notched around the rafters and sqeezed in under a plywood "box beam" we nailed in level between rafters and the plate.

so yes, the bales do indeed overhang the stone, extending past the sill plate about 3".  I was planning filling the gap with plaster but obviously thats a terrible idea for moisture wicking.
So seems like my biggest issues to solve are thus:
-forming a capillary break between top of stone and bottom of bales?
-forming a break between stone and plaster and creating a stop/support for exterior plaster to land on.

Good detail drawings Eric, very clear.  

A couple of facts.  1" of plaster weighs around 15 lbs, and most straw bale walls have between 1" and 2" of plaster on each side.  A column of plaster 8' tall weighs at least 120 lbs.  That's a lot of weight to cantilever 3" beyond the sill plate for the straw fibers to hold up.   What might happen if you just plaster the straw bale surface as-is and hope for the best?  Maybe nothing, except for the previously mentioned concern for moisture wicking from the stone veneer wall, and possible problems with the plaster at that joint.  I haven't seen exactly this situation before--but I'm guessing that the plaster weight would be too much for the straw, and you'll see sags on the plaster surface, or long horizontal cracks over time.  Straw bales make for a great lath, but they usually don't carry that load cantilevered several inches from the sill plate.

Options?

Shear the bale surfaces back 3" so the bale surface that will receive plaster is directly above the sill plate.  Then let in the 2x I mentioned earlier as a support for the weep screed that would both support the bottom of the plaster edge, and provide some separation between the bales/plaster and stone wall.  You'll lose some R-value but you're still around R-21.  The challenge will be to remove a uniform 3" of straw--if I were taking on this project I'm sure an angle grinder with a chainsaw blade would be involved (and a respirator, gloves, and eye protection!).  I'd try to figure out some way to gauge that depth using a truth stick (straight edge that reaches from sill to top plate), or let in 2' every few feet to serve as a depth reference.  It's a novel challenge I haven't worked through before.  It'll take at least a day and will result in a very large pile of short straw fibers--great for clay plasters!

Or, try to level the top of the stone wall as best you can by grinding away the stone and mortar and place another sill plate either in-line with the stem wall sill plate, or perhaps above it.  The sill plate should be made of a naturally decay resistant or pressure treated wood.  If the "sistered on" sill plate aligns with the stem wall sill plate secure it with 5"+ timber screws driven through the edge of the new plate and into the existing one.  If the new sill plate rests on top of the existing you may, be able to secure it with 3" screws at the narrow gap between bales.  Then attach weep screed to the new sill plate, holding the screed at least 1/4" above the stone wall so moisture drips down instead of wicking up...1/2" would be better.

A "square foot" of plastered wall (imagine standing in front of the wall and drawing a 12" x 12" square on it--the "square foot involves all of the wall from outside to inside) weighs between 45 lbs. and 50 lbs.  That's between four and five times heavier than a 6" conventionally framed and insulated wall (insulated with fiberglass, rock wool, cellulose, or foam).   In modern times most timber framed buildings use much lighter weigh infill panels for insulation than straw bales.  When a straw bale wall "wraps" a timber frame there are really two separate wall systems.  The timber frame carries roof loads and lends some support to the bale walls, but the bale walls carry their own self weight.  They still need to resist lateral forces from winds and earthquakes.  

I'm not sure which lateral force governs in your area.  Where I live and work earthquakes are a greater force than tornadoes or high winds.  When I have worked on timber framed buildings with bale wraps the two walls are tied together as you have done, but they each require their own structural design.

Something to think about.

Jim
Many Hands Builders

Awesome I’m leaning towards getting a new sill plate screwed back to the original exterior one.  I think that should work out for us!  Trimming the whole wall back would be a simple solution except we do have 1 stud wall and windows that would be out of plane.  

The simplest choice would be to have the new sill plate as a 2x6 coming in on top of the old one.  That will leave about 1” at places between the bottom of the new sill plate and the stone/mortar.  I’ll drop the weep screed down over that larger gap, but any suggestions on filling/sealing that area?

As far as seismic goes without looking back I believe we are 1 step up from the lowest risk.  Wind is probably the bigger factor: last year I think we clocked 60-70 mph gusts and not nearly the tornadoes that middle Tennessee gets but looking at historical data small ones can happen here.

I agree.  Having windows out-of-plane would look strange, and be difficult to detail when plastering.

Here’s how I’d approach this if it were presented to me as a renovation project on a straw bale building:

I’d use a chainsaw or angle grinder with a chainsaw wheel to remove straw at the bottom of the wall surface, careful to not hit the stone wall!  I’d also be prepared to further level the stones and mortar with an angle grinder and diamond wheel for cutting stone.  Since this could throw up some sparks best to have a hose ready, set on the mist setting, because the bale surface is vulnerable until a coat of plaster is on it.

Once the straw is removed so a 2x (pressure treated or of naturally decay resistant wood) of whatever depth required to cantilever 3” under the bale wall edge and also provide enough purchase on the existing still plate I’d temporarily place the 2x to see how much space there is beneath it, and identify areas where I can secure the sill plate extension to the existing.  

I’d probably use both polyurethane glue and 3” screws to secure the sill plate extension.  I might also trowel a relatively dry mortar or concrete mix on top of the stone wall to the level of the existing sill plate, then “wet set” the sill plate extension after first stapling a foam gasket (brand name Sill Seal) to the underside.  This functions as a moisture barrier and helps block gaps in the foundation surface that insects might use.  

Placing mortar or concrete into a narrow gap beneath a straw bale wall will be tricky, and the material could set up faster than I can work it.  If I was working alone I might use a mix retarder so it stays workable for longer.  I'd probably also work with shorter sections of sill plate—no longer than 8’—and small batches of mix.  If the gap is small enough I might also use a grout bag or grout gun (like a caulk gun but with a refillable tube) to squirt a relatively wet mortar mix into the gap...

Off-the-shelf 7/8" weep screed “J” channel  (called "J" channel because the profile of this 10' long galvanized steel material resembles a "J") is usually 3 1/2” tall, so I’d probably let in a 2x on edge to give the channel more support.  This would sit above the sill plate extension.  Removing the extra straw near the surface would also make it easier to get the mortar or concrete mix into the narrow space above the stone wall.  I might also have screed custom fabricated to the plaster depth and leg height I need.

I have a couple of other concerns.  One I mentioned earlier—the wall’s ability to resist lateral forces. There’s also the issue of condensation running down the underside of the roof surface and directly into the bale wall if there’s no other barrier to prevent that, and also fire—the building code requires a fire barrier at the top of a straw bale wall separating it from the roof cavity (IRC Appendix S Strawbale Construction Section AS104.2 Finishes),

A straw bale wall stacked in a running bond has some ability to resist lateral forces without failing, and plaster skins help reinforce that.  However, an otherwise unreinforced wall could still fail if subjected to a large enough lateral force, and periodic tie-ins to the interior timber frame may not be enough (unless these were designed by a structural engineer who did the force calculations?).

The building code offers some prescriptive methods that address this (see IRC Appendix AS Strawbale Construction Section AS105.4 Out-of-plane resistance methods... and also AS106.13 Braced wall panels), but both rely on some framing like a box beam or top plate.

Load bearing straw bale walls usually have a box beam at the top to distribute the roof weight, but since the timber frame is handling that your drawing shows bales stacked to the underside of the roof?  You mention that the bales are notched into the rafters and squeezed beneath a plywood “box beam,” but its not shown in the drawing.  

Is there any 2x or 4x material at the top of the exterior side of the bale wall running under the rafters?  And is the roof vented or unvented?  

Jim
Many Hands Builders

Great detail thanks again Jim, currently seeing if I can track down weep screed but it’s not something stocked in my area.  Gonna call the local roofing supply tomorrow to see if I can get it custom bent.

Sorry I dont have the top of wall detail drawn in, quite busy lately between a timber framing job and trying to get power run to our site…
There are box beams essentially built out in every rafter bay above the bales but they are slightly narrower than the bales due to the geometry.  The bales do contsct the 2x6 roof decking on the exterior edge for about 2-3 inches before the roof pitch hits those box beams.  So I’ve been treating those box beams as my fireblocking.  Not sure if that’s correct or not.  

The roof currently is still just 2x6 t&g with 1/2” ply sheathing over that for diaphragm and then a 9 month synthetic felt.  Once we finish plaster and get power run I plan to “wrap and strap” basically an unvented built up rigid insulation over the decking with a metal roof on that.  The metal may end up “vented” if I have two perpendicular layers of strapping over the foam.  Still working that out.  

Thanks for the roofing explanation Eric--that should work.  I'm not sure what you area's roof insulation requirement, but the building code is for a minimum acceptable level.  Exceeding it anticipates a future when you might wish you had more, and it's very difficult to add insulation to an unvented roof unless there's also an attic that you can access.  Most heat is lost (and gained) through roofs; best to exceed R-value when you have the opportunity.  Around here we have an R-39 minimum--we try to install upwards of R-50 when budgets allow.

Weep screed.  I mentioned that the screed has a "J" profile.  The depth should reflect the plaster you expect to apply--probably 1" or so.  The short leg is going to be around 1/4" high, and the long leg (that attaches to the sill plate) can be as long as your sill plate is tall.   The material can be galvanized or powder coated steel.  For weep screed we gang drill 3/8" or 1/2" holes every foot or so.  

You might also consider having some casing bead made as well.  It's the same profile as the weep screed but doesn't need holes.  Casing bead surrounds the sides and top of a plaster field to both help the plasterers gauge the plaster depth (scratch coat fills 1/2 the channel, brown coat fills most of the rest, finish can lap over onto the short leg of the "J" channel, or tuck behind it if the screed/bead are a color that accents the project.  It also "cases" door and window trim, and any exposed exterior timber framing.  I did a lot of remodel work on older (and not so old!) straw bale buildings where the original builder ran exterior plasters right up to the wood timber framing or door and window trim.  You can usually get away with that on interiors, but exterior conditions are usually more extreme, and wood swells and contracts with humidity changes.  This movement causes the edge of the plaster in contact with the wood to fail, leaving a gap.

Which leads to a concern I have for the top of your wall.  A box beam made of several layers of plywood with fire resistant insulation sandwiched between is a great fire barrier.  It's role in this regard isn't to make the house fireproof--just slow the spread of fire from roof to walls, or from walls to roof.  Slowing the spread allows occupants to escape, and maybe for a fire department to save the structure.  Having straw in contact with the underside of the roof deck isn't ideal but it's probably not fatal.  

However, it sounds like you'll be running the exterior plaster directly up to the underside of the roof deck?   Just know that this joint is likely to open up.   Unless the bales were compressed when you stacked them they'll probably settle a bit, and most plasters--especially clay plasters--shrink when they are dry.   The weight of plaster can also cause a gap to open at the top of the wall.  This gap may be small--1/8".  It could also be 1/2" or larger.  Both offer an entry point for insects.  Where I live Acorn woodpeckers love to stuff acorns into gaps and cracks like this--their granary for the winter.  

To avoid this you could let in 2x material on-edge at the top of the wall just under the roof deck, cut to the roof's slope, then glue and screw it into place.  This helps to "capture" the top bale" so it can't shift outwards, and it also gives you a place to attach a frieze board--a wood trim at the top of the wall that covers the joint between the soffit or roof deck and the wall finish.  This can be a time-saving feature when you're plastering directly under the sloping roof.  Your hand and trowel need some space to work, but sloping eaves constrain that space quite a bit.  Frieze boards drop the plaster field down a few inches.

The let-in horizontal framing at the top of the wall gives you a place to attach the frieze board, and also the casing bead that borders the top of the wall.  And, if you want to add any of the out-of-plane resistance methods described in the code, you now have a way to attach it at the wall top.

The two most common are:

(1) pair poles (bamboo, rebar, 1x lumber) on either side of the wall then tie them trough tightly with baling twine.  The poles are usually let into the bale surface so the plaster field won't have long vertical bumps in it.  The poles are usually on 2' centers.  This really stiffens the wall.  

(2) stretch a suitable mesh (Cintoflex deer fencing for clay plasters, 17 ga. or 14 ga. galvanized mesh for lime plasters) on both interior and exterior wall surfaces  attached at the sill and top plates, and again sewn through and tied tightly with baling twine.  

The sew-through with baling twine is usually at every bale course.  You mentioned that you have already plastered the interior.  If it's just a scratch coat you could still let in the poles or apply a mesh over the top of that.

The first method essentially "splints" the wall, the second creates a basket.  Both are quite effective at resisting out-of-plane lateral forces, but there needs to be something at the sill and top plate for them to attach to.

Both methods are detailed and described in a book by the California Straw Building Association, Straw Bale Building Details: An Illustrated Guide for Design and Construction. New Society Publishers 2019).

Jim
Many Hands Builders