Jim Reiland

The short answer is yes, we should avoid placing impermeable materials against wall assemblies made with materials like straw bale, straw-clay, etc., unless we provide other means for moisture to escape. The only difference of opinion that I’m aware of is whether some materials or combinations of materials are vapor permeable enough for a particular climate. My earlier mention of plywood sheathing may be the source of your question about impermeable materials?  

In natural building “best practice” is to design wall assemblies that allow moisture to escape. However, that depends on how much and in what form moisture can be expected to enter the walls, as-well-as the materials in the wall assembly itself. That’s why “best practice” can take on a regional quality—what works in arid Nevada may work not as well in humid and wet Florida; what works in mild coastal California may not work in frigid Vermont.

My experience is almost exclusively with straw-based insulation materials covered on both sides with vapor permeable plasters—clay and/or lime. A fairly recent development in straw bale construction has been to use clay plasters on the interior and ½” or 3/8” plywood sheathing on the exterior with a vapor permeable plaster or some other siding over the plywood sheathing.

Much of western-central Oregon south through central California presents a climate suitable for this system, at least according to the materials modeling done by members of the California Straw Building Association. Buildings in Eugene, Ashland, and Jacksonville, OR and Vallejo and Lake Tahoe, CA have employed this wall assembly. This analysis, called a WUFI (an acronym for the German words ”wärme und feuchte instationär”) shows how heat and moisture move through different materials of a wall assembly given varying climate conditions. Regions with mild winters and warm dry summers are well suited to this kind of wall assembly as this thickness of plywood is vapor permeable enough to not trap moisture. The wall assembly is more permeable if whatever siding system that covers the plywood—metal, cement board, wood, or plaster—has an air-gap.

When I replastered my straw bale house last year I used this method.  After removing the existing plaster I let 2x4 studs into the bales (which had been laid flat many years ago so no strings were cut) on 2’ centers. I gang-drilled ½” holes on 12” centers through the stacks of the 3/8” plywood panels that sheath the straw bale walls. On my project the plywood isn’t structural—it’s there to support the conventional plaster regime, i.e. a 2-ply building paper/drain mat stapled to the plywood, lath stapled to the 2x studs the plywood attaches to, and a three-coat lime plaster built out to 1” thickness.  

I’m guessing that the ½” holes (that total 4.2 square inches per 4’ x 8’ plywood panel) increases vapor permeance; I haven’t seen tests to confirm that. If the plywood were part of the building’s shear it’s also unknown whether holes of that size and location in the plywood would compromise any structural function. Still, it’s a promising system as most builders and building code officials are familiar with plywood sheathed stud walls, and any kind of siding can cover the sheathing, both lowering costs and increasing aesthetic options.

A longer explanation of vapor permeability might begin with the question “how does moisture get into walls?”

For another time?

Jim
Many Hands Builders

Hi Emily,

Thanks for the clarifications.  Either wall system should be protected from wind-driven rain by either or both adequate roof overhangs and a suitable plaster.  

Note that both straw-clay and hempcrete could be plastered, but they could also be sided if you were concerned about wind-driven rain. (Addendum. Straw bale walls can also be sided, but it's best if the exterior bale surface is plastered first. At the very least a scratch coat provides an air barrier, resists fire, insects, and rodents. Adding a brown coat is better as it supplies a better air barrier.  Adding siding to a straw bale wall requires some planning. One method is to let 2x ledgers or posts into the bales on whatever centers the siding requires, usually 2', and make a note of where they are.  Once the plaster covers the wall secure furring strips through the plaster to the ledgers or posts, then secure the siding to the furring strips. This results in a "proper" rain screen with an air gap (the thickness of the furring strips).  When stacking the bales against plywood sheathing--a wall system that works great in some climates plastering the exterior isn't possible, and may not be critical as the plywood takes on some of the function of the plaster.  Add siding to the exterior surface of the plywood according to your area's best practice. Rain screens are a great practice, but may not be absolutely necessary in some climates.  Be sure to install insect screen at the top and bottom of the wall so it's not an open space for critters to occupy.)

Clay plasters are lower cost and user friendly but can erode from the wall if hit by too much rain.  Sealing them with some silicate based product or linseed oil helps, but also makes it difficult to fix as a repair plaster won't stick as easily to the treated plaster.  

Lime plasters are more durable but they aren't water repellent so much as water reservoirs. In natural wall assemblies like straw bale, straw-clay, or hempcrete walls exterior plasters  function to absorb liquid moisture (wind-driven rain), then release it as water vapor. This mechanism can fail if there's too much rain and not enough dry time between rainfall events. When that happens the water soaks into whatever substrate the plaster is on--straw bales, straw clay, hempcrete--where it stays until exterior conditions pull the moisture back out in the form of water vapor.

How long that takes depends on exterior conditions--warm and dry summer weather is best for evaporating water from the walls, though very cold winter conditions help to prevent microbes from becoming active. I have seen straw bale and straw-clay walls that were wet on the exterior surface survive for months in S. Oregon winters without apparent damage because it was too cold for the microbes to start eating. Some of the repair work I did on straw bale structures in S. Oregon involved only the lower few feet of the wall where the plaster absorbed liquid water from rain splash and roofs without gutters.  

It costs more, but larger overhangs protect walls better.

RE your comment about "adequate dry time." The handed-down wisdom in the straw-clay building world of "one week of dry time per 1" of wall thickness" really depends on optimal drying conditions.  I have tested straw-clay walls many months after they were placed and found readings well over 20% moisture content.  It's possible that the measuring tools we use (moisture reading equipment designed for hay) isn't appropriate for a material like straw-clay, but lacking other reliable methods to evaluate the interior of a straw-clay wall we waited for the wall moisture content to come down before plastering either side. We began exterior plasters nine months after the walls were placed, and completed the interior plasters almost a year after the straw-clay was placed.  We also didn't rely entirely on warm, dry weather to coax moisture from the walls. Over the winter the owner used a wood burning stove to raise the interior temperatures, and also ran dehumidifiers.

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

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

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

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

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

I appreciate what you're saying Joshua, it's frustrating.

People were getting sick from a number of things. Some buildings were tight and the finish materials were off-gassing into the interior air space.  Other buildings weren't tight and water vapor carried on the leaking air condensed inside the walls and mold grew. I have been in poorly air-sealed buildings on sunny, windy days and noticed dust clouds dancing in the interior air. It was bits of fiberglass dust sifting through the T&G wood ceilings, which had been installed without any kind of air barrier between the wood ceilings and the insulation.  Imagine breathing that air for a generation or two!

The mechanical ventilation systems recommended for airtight buildings are called Energy Recovery Ventilators (ERVs) or Heat Recovery Ventilators (HRVs) and are designed to exchange the heat from the outgoing air so you're not bringing in cold or hot outside air which you then need heat or cool. Energy Recovery Ventilators also exchange humidity.  Extreme cold climates generally use HRVs, but a quick internet search offers several sources that offer guidelines for which one is best for any given location.  A relatively inexpensive and easy-to-install (in new construction) HRV unit from Panasonic works well for small homes.  It cost around $600 in 2022.  Placed in the central area of a home--say a kitchen-living area with bedrooms on either side--they do a pretty good job.  

I'm set up to install an HRV here, but for now my wife and I are running a portable HEPA air filter whenever we close all the windows. Our straw bale house has R-30+ walls and R-50+ ceiling insulation.  I haven't had a blower door test done, but since it was the first straw bale house I built I wasn't paying as much attention to air-sealing as I did during my building career. I'll guess we'd blow at around 3 or 4 ACH.  Could be better, could be worse.  On hot days when we closed the windows the space stayed cool but the air seemed stale; the HEPA air filter changed that.  It does nothing to address moisture (neither does an HRV). To help expel moisture we try to remember to run the kitchen range vent when we cook, and also the bathroom fan when we shower.  

But as you said, people forget to open and close windows, or turn exhaust fans on. Still, I'm not too worried about moisture inside our home. We have 1 1/2" thick interior clay plaster that functions as a hygric buffer--absorbing and releasing interior moisture. I also office in a straw bale woodworking shop where a hygrometer hangs on the wall.  In fifteen years the needle hasn't moved much.

Jim
Many Hands Builders

Good that you checked around Gregg,  It may be different everywhere, but I expect most jurisdictions issue permits based on exterior dimensions.

I have never used house wrap, but then I didn't do much conventional construction--almost all of my work was on straw bale or straw-clay buildings, or structures insulated with blown-in cellulose and sheathed in plywood.  All of these were plastered on the interior and exterior with lime and/or clay plasters.

I understand there's a vapor membrane used in extreme cold climates on super energy efficient structures.  Jacob Deva-Racusin at New FrameWorks in Vermont was telling me about it--can't recall who makes it....Mento?

The only "paper" I have used is the 2-ply paper (top layer is asphalt paper, bottom layer is a dimpled or wrinkled vapor permeable synthetic membrane of some sort) commonly used for lime or cement plasters. The paper is only 39" wide so it's lapped in 3' horizontal strips over sheathing--not really an air barrier. But that, combined with a well detailed exterior stucco and carefully applied interior lime or clay plaster did a pretty good job, along with air sealing electrical and plumbing perforations, gaps and cracks around windows and doors.  

The handful of my projects that were blower door tested had pretty good results for not trying that hard--between 1 and 2 ACH (air changes per hour).  For readers not familiar with this metric, it means that the air inside the home that we spend money heating or cooling doesn't escape the home very easily through leaks in the building envelope, only to be replaced by outside air that we need to heat or cool.  The Passive House standard is .6 ACH.

For reference, the average Oregon home built in the 1990s and early 2000s tests at 7 ACH, which is pretty leaky.  I'm not saying that all buildings made with natural materials are inherently airtight.  I have also been in many straw bale and straw-clay homes that had blower-door tests much the same or worse than 7 ACH.  They often still felt comfortable because the distributed thermal mass of interior plasters moderates temperature fluctuations, but they weren't nearly as energy efficient as they might have been had more attention been paid to air sealing.

Many in the natural building world would prefer a leaky building to installing some kind of mechanical ventilation system.  It is more natural that way, but also wastes a lot of energy. I liked to make buildings as tight as possible, and prepare for if not install a mechanical system like an HRV (heat recovery ventilator) or ERV (energy recovery ventilator).  That way, people who initially didn't want a mechanical system could more easily add it later, or they could just crack a few windows if they felt the need for fresh air.  At least they have some control.  Leaky buildings always just leak.

Jim
Many Hands Builders

Matt,

I too worked mostly in a rural area with small towns, but my colleagues throughout the U.S. West report square footage for permit requirements measured as exterior footprint.  

A few relatively forward-thinking building code jurisdictions in California (e.g. Sonoma County) will permit a straw bale building with 18" or 24" think walls as though it had 6" thick walls--same as most conventional residential construction.  The rationale is that builders who construct super-energy efficient homes that store carbon using the lowest embodied energy material on the planet shouldn't be penalized.  They're not actually using all that wall thickness for living space, and they're doing the rest of us a favor. I have heard of builders and homeowners elsewhere getting an exception like this for thick-walled construction (interior square footage plus 6") but it's far from universal.

RE exceptions for structural permit requirements. I believe that Jackson County in S. Oregon is typical for the rural U.S.  There is no requirement for a building permit for structures under 200 sq. ft.  Most county planners and building code officials don't care if you plop a 200 sq. ft. or smaller shed on your multi-acre property to store tools, animal feed, firewood, etc. However, should those buildings be used for living space with electricity and plumbing, in Jackson County they require a structural permit and both electrical and plumbing permits.  I understand that some rural counties in California have a 400 square foot agricultural building exception.  

I'm guessing that more urban areas are much stricter about this. Lots are smaller and any size structure could have impacts on neighbors or block egress for fire fighters, etc.

Jim
Many Hands Builders