Straw and Clay...

Hey all.. is there an easy way to mud.. we have some straw bales that are not even and some have a 4" run.. would like to get the wall somewhat even and was wondering what wire mesh or something I could use to help even the walls out?  Maybe level it with LSC and then float it?

Hi Kyle,

A little more information would help.  It sounds like you have a straw bale wall...for a building?  garden wall? that you want to flatten because some of the bales are 4" out of plane?

There are a couple of ways to flatten a straw bale wall, but a lot depends on context.  Let's start with how tall the walls are, what the wall is for, and whether there's any framing in the wall. Also, what kind of foundation does the wall sit on?  

Before applying any kind of mesh or mud I'd try to flatten that wall--but do tell more about it.

Jim
Many Hands Builders

Photos as well.

Hi Kyle,

You might check out the brief educational videos relating to keeping bale walls plumb at the YouTube Channel of the California Straw Building Association.

https://www.youtube.com/user/strawbalebuilding

There's one on flattening walls to make them (MUCH!) easier to plaster, and also on keeping bale walls plumb during the bale stack.  

Although the rough straw bale surface is an excellent lath for plaster, there's a limit to how thick that plaster can be.  A square foot of plaster 1" thick weighs around 15 lbs.  At 2" thick it's 30 lbs.  IRC Appendix AS Strawbale Construction, the building code available in the U.S., actually limits plaster thickness to 2" unless engineered for greater thickness.  That's because the thicker the plaster build out to accommodate a lumpy wall, the more likely the plaster will sag before it dries or cures, develop cracks due to the force of the extra weight pulling on the straw fibers, or if unsupported at the base, shear off.  Mesh over the bale surface does little to make the wall flatter, though it can reinforce the plaster.

This video series is a public service provided by CASBA; we're trying to produce around four videos each year.  CASBA is an all-volunteer non-profit organization that advocates for the use of straw as a building material.  Our members are builders, architects, engineers, and homeowners interested in using more sustainable building materials and methods.  We function as a trade association of sorts:  our members have

--written the model building codes in the U.S. for straw bale, light-straw-clay, and cob construction (among other more-natural building materials).  

--funded and/or conducted research on the fire resistance, seismic, and acoustic characteristics of different wall assemblies using straw

--produced educational materials like CASBA's Straw Bale Building Details: An Illustrated Guide for Design and Construction, videos like those on the channel link, and workshops.

--been involved in determining the life cycle analysis and carbon storage potential of straw as a building material.

There are about twelve videos so far, each treating an important step in the design and construction process.  We produce around four videos each year.  I help to select topics based on priorities.  The first subjects were chosen because the are among the mistakes seen most often in the straw bale building construction process--keeping wall flat during the bale raising, or flattening them after the bale raising--is a common concern.

Good luck!

Jim
Many Hands Builders

Yeah, the walls were of straw bale but I ended up mudding with clay and then packing with LSC and additional mud on that.  Seems to be the best method to fill in the wells.

Sorry for posting and being late but I am good so far.  I don't mind the wells in the walls and the walls not being fully "FLAT" but some people do and was just interested.  Since this is a living wall I can't really sheet rock it due to the risk of moisture and mold within the straw.  Some of the house was packed using LSC but it sounds like they wanted something a little lighter up top.  Walls are 8ft tall and supports were fed through the wall.  It looks good, I'll get some pictures!

What is LSC please?

Sounds like it worked out for you Kyle.

I like the way light plays off of the gently uneven walls we've been able to achieve with straw bales, but we work at making them that flat, mostly just keeping the bales plumb during the bale stack, and if the strings aren't exposed, shaving the walls with a Lancelot blade or chainsaw to achieve a less-bumpy wall.  

The aesthetic we aimed for has perhaps a 1/2" variation from plumb over the height of an 8' wall, which is pretty darn flat from a plasterer's perspective, but not so flat that you’d mistake it for a conventional wall.  Not "sheetrock" flat, but also more understated than when entire bales jut out of plane several inches many times across the wall.
A cautionary note for other readers of this post. Coating a straw bale or LSC wall with wet plaster or wet filler material like straw-clay needs to be done with some awareness that deep layers of wet material or deep cavities filled with wet material will dry out very slowly--sometimes so slowly that microbes become active and begin to eat until the moisture content of the material drops below 20%.    

When I was building, I tried to stuff gaps and cracks with dry straw rather than wet straw-clay or plaster unless we were able to let these stuffed or built-out areas dry for several weeks before covering the walls with plaster.  

Adding wet plaster to a dry substrate like straw bale or light-straw-clay introduces around a quart of water to each square foot of straw bale (or LSC) wall surface during a three-coat plaster process.

As a rule of thumb, lime plaster mixes need as much water as binder (by volume), and usually more. Clay plasters need somewhat less. A cubic foot of plaster covers 12 sq. ft. of wall surface at 1” thickness. A typical 1:2.5 binder : aggregate lime plaster mix is made by combining 7.5 gallons of sand with 3 gallons of lime (dry) and 3 gallons of water. (Most of the lime and water disappears into the voids between sand particles, so 7.5 + 3 + 3 = 7.5+!). Divide 3 gallons of water by 12 sq. ft., and you have .25 gallons, or a quart of water spread over each square foot of wall during the three plaster coats.  This doesn’t include moisture introduced during damp curing required for lime plasters.
Most of the water in plaster evaporates out of the plaster and away from the wall within a week of application, but some will soak into the exterior layer of straw and hold there until coaxed back out through the plaster by optimal drying conditions.

Any wet materials stuffed deeply into the wall—say a few inches or more—are going to take much longer to dry out of the wall than damp materials immediately adjacent to the plaster surface.

General advice: while you can build out very thick layers of plaster or fill material like straw-clay in order to flatten a straw bale wall (I have done it many times!), it’s much easier to keep the wall relatively flat while building it, there’s no risk of adding too much wet material to the wall, and a flatter wall is easier to plaster.  

Jim
Many Hands Builders

Jim,

I don’t want to hijack this thread but I’m just hoping you can clarify something that you said about damp curing lime plasters. I’ve read much of your contributions to the natural building world and I’m so grateful we have you! I think it was in the LSC Essential series plaster book, you have some great information in it and I applaud you for sharing.

How important, in your experience, is the damp curing of type s lime plasters? My house has mostly clay plasters, but I did a tadelakt shower and some shikkui-style lime plasters inspired by Kyle Holzhueter’s work. All my lime plasters have been sprayed twice a day for at least 7 days, and they’re great. But it’s the worst part of lime, boring and tedious. I plan on doing some tests comparing damp curing vs a do-nothing cure, but I’d like to hear your thoughts. I know some professional accomplished natural builders whose business and money is on the line and it seems like they take a more do-nothing cure to a non-pozzolanic type s lime plaster.

Thoughts from the expert? I appreciate your insight.

Sorry John, here in the U.S. "LSC" is an acronym for light-straw-clay a wonderful blend of clay and straw that goes by other names in other places.

Scott, I'll try to get to your question about damp curing Type-S lime in the next few days.

Jim
Many Hands Builders

Hi Scott,

Sorry for the long delay--a nearby wild fire has kept my attention!  

RE the book Light Straw Clay Essentials, you might be thinking of my friend and colleague Lydia Doleman, who wrote that book. I contributed only encouragement and a few pictures.

As for expertise, I have worked with different kinds of lime plasters and have lots of thoughts on how to apply them to straw bale and light-straw-clay walls, and also conventionally sheathed buildings. Still, I wouldn’t describe myself as an expert on lime plasters—plenty of people have much more experience.

My understanding is that all the lime plasters—the “hydrated” limes like naturally hydraulic limes (NHL), straight Type-S lime, and artificially hydraulic limes (AHL) like Type-S with a pozzolan added—all benefit from damp curing.

Some years ago I read Building With Lime: A Practical Introduction by Stafford Holmes and Michael Wingate, a couple of UK based experts on using lime in building.  On page 125 of their book, in a chapter about lime renders (plasters) and why they fail, they write:

“Carbonation and chemical reactions that give final strength take place best in moist and warm conditions that dry out slowly.”  

There’s more to this—conditions also can’t be too hot or too humid, and of course freezing is going to cause problems, too. My understanding has been that limes carbonate best when temperatures are between 45 degrees and 85 degrees Fahrenheit. Too hot or too cold, and the carbonation process shuts down.

To your specific question RE damp curing a Type-S lime plaster. Yes, I recommend damp curing, if for no other reason than I didn’t want an expensive do-over. My crew and I usually applied Type-S exterior lime plasters that were made artificially hydraulic by adding a ¼ part metakaolin (a kaolin clay that has been calcined—fired at a high temperature) or pumice.  This introduces a mild hydraulic set to the Type-S, which is an “air” lime—it sets only by absorbing carbon dioxide from the air. An air lime plaster at 3/8” thickness can take several weeks to set hard enough (green hard) to support a subsequent coat, so adding something like metakaolin or pumice reduces the time between coats, and makes the plaster a bit stronger.

I have only heard of straight Type-S limes that were applied but not kept damp—never done it, nor have I tested samples to learn what might happen if a lime plaster dried out before it cured.  My guess is that it might be friable (powdery) or crumbly, but I don’t know for sure. I don’t know exactly how the chemistry works, but I understand that damp curing helps the lime—as Holmes and Wingate say—to fully carbonate (convert from either quicklime--CaO or a hydrated lime like Type-S, NHL, or AHL—Ca(OH)2 to calcium carbonate—CaCO3.

So what does it take to “damp cure” a lime plaster?  It depends.

When I have applied Type-S or NHL limes to interiors where it was relatively easy to control both temperature and humidity I have often been able to just let the plaster set without adding moisture to keep it damp.  On a few jobs I misted (not hosed!) the walls once or twice during the entire five-to-seven day curing time recommended both by manufacturers and application guidance found in lots of places—books, on-line, etc.

If we applied any of the limes to exteriors during cool, overcast, calm, and drizzly days that were expected to last for the week-long-curing period we didn’t need to supplement with daily misting because the weather conditions didn’t favor rapid drying.

Unfortunately, here in S. Oregon the building schedule often had us applying exterior lime plasters when conditions were hot, dry, and windy (e.g., 90 degrees Fahrenheit, 20% humidity, 30mph winds). Under those conditions moisture in the wet plaster would evaporate quickly, well before the lime had time to cure, so we intervened to keep the plaster damp. Sometimes misting once-a-day sufficed. I have also worked on projects that required misting the walls as many as five or six times each day.

When I say “mist” I mean keeping the wall surface damp without water running down it. This can be accomplished on the “mist” setting of a garden hose spray nozzle, or by using short bursts of spray on a “gentle rain” or “shower” setting. Just enough to wet the wall. It’s important to not “fire hose” the wall—drive so much moisture into the plaster that it runs down the wall surface or worse, erodes the freshly applied plaster.  Also, applying too much liquid water can saturate the plaster and soak into whatever substrate you are working over. If that substrate is wood sheathing covered by 2-ply stucco paper (a typical conventional wall assembly)—no worries—liquid water sheds down the lapped building paper and away from the wall interior. But if the plaster is applied directly over straw bale or light-straw-clay walls (or some other cellulose based insulation) you might introduce more liquid (read “bulk”) water that can take some time to migrate back out as water vapor through the permeable plaster. Best not over-do it!

To reduce the number of times someone had to walk around a building dragging a garden hose for a week after each coat we usually draped shade tarps from roof fascia wherever direct sun would linger on the freshly plastered wall for more than a few hours. Here, north and east facing walls aren’t a problem so much as south and west facing walls.  Shade tarps also shield the wall from the drying effects of wind. As a plus, on a hot day it’s more comfortable to work in the shade! (Tip—after working under dark-colored tarps for several years I switched to using white tarps—they reflect sunlight so are much cooler to work under, and it’s easier to see what you’re doing!).

Not sure if that helps, but it’s one (retired) plasterer’s view. Err on the side of caution unless you can afford to do it over.

I look forward to hearing what your tests show!

Jim
Many Hands Builders

Jim,

Thank you for your detailed and informative response. I have read a bit about using metakaolin to create a mildly hydraulic lime, and it’s been on my list. I’ll bump it closer to the top!

I keep testing and experimenting and keep you in the loop. I did a shikkui inspired lime over clay and lath and so far it has been awesome. Super sticky, well bonded, incredibly flexible.

Thanks again!  

I was kind of hoping you were going to tell me something like if I squirt a little comfrey tea in the lime plaster mix, I don’t need to damp cure it… that was my secret wish.