Loose Strings: Technical Discussions
by Jeff Ruppert – Colorado, USA
T e c h T i p s

A little known fact in the bale building realm is that a handful of people scattered across different continents have experimented with the idea of finishing their bale walls with wood or some type of manufactured siding. The technical term for siding over a bale wall assembly is a “rain screen.” The use of a rain screen (sometimes referred to a “multiple defense assembly”) on a bale wall plays the role of keeping rainwater off of the bale portion of the wall. This is in contrast to the standard way of finishing a bale wall with plaster and allowing moisture to come into contact with it on a regular basis (also referred to as “faceseal” walls). In fact, almost all of the literature to date on bale-wall construction makes the assumption that they are faceseal assemblies.

In this article, we are going to take a look at the pros and cons of in-stalling siding over a bale wall. To some people the idea of not having a plaster finish on a bale house would seem weird, mainly due to aesthetic reasons. However, for those who have chosen to use siding, aesthetics take a backseat to function due to high rates of rainfall throughout the year, as well as constant high humidity. The option of allowing bale walls to even get wet in the first place is not an option and therefore other systems must be considered.

For those of us who live in drier climates, the consideration of moisture is not as dire, therefore giving us more choices. However, doesn’t the siding option make sense if you are concerned about moisture at all? If you would like to design a building with mixed finishes, such as a combination of plaster, masonry and siding, this would open up the opportunity to include bale walls as an option on those projects. In fact, by installing a rain screen over bale walls are we not greatly reducing the potential for moisture damage, as David Eisenberg puts it, by “designing problems out of the project” from the start? We will explore these issues and hopefully offer you another choice in your search for solutions.

Rain Screens
In the old days, a rain screen was simply an exo-barrier that was attached to a building to catch rainwater and shed it before it could hit the structure behind it. The Norwegians titled this approach the “open-jointed barn technique,” since originally it was used in conjunction with the construction of barns¹.

With tighter construction and newer forms of finishes, the technology of rain screens has evolved into a science. One of the advantages of using a rain screen on a bale wall is that, no matter
how you do it, it will probably add a significant layer of protection that would otherwise not exist. This assumes that you do not install the siding to accidentally direct water into the wall. The potential exists for this to happen, so just like any other type of finish, pay attention to the details!

Siding over bale walls

No matter what type of wall you build, the driving forces of moisture will be:

• Air pressure difference (gradient)
• Gravity
• Surface tension
• Capillary action
• Rain drop momentum.

The dominant force acting on your walls will be the difference in air pressure across the siding itself.  As the wind blusters around your house, there are pockets of less and more pressure ever changing within and around your wall assemblies. The main goal is to minimize any pressure differences so water is not accidentally driven into the wall assembly. By minimizing pressure differences, the main force acting on nearby moisture will then be gravity, drawing water down to the ground where it belongs, before it reaches your bales.

In order to equalize pressure, an air gap behind the cladding (siding) needs to be well ventilated to the atmosphere. This can be achieved through different methods, but whatever you do, make sure not to create a gap for wind to blow rain behind the cladding. This means providing ventilation behind the siding so air can pass through easily, but including a barrier at the points of ventilation to keep wind-driven rain from entering.

The advantages of using a rain screen are:

• Adds another option for finishing bale walls (aesthetic),
• Keeps moisture completely off the bale portion of the wall assembly,
• Provides replaceable/changeable finish,
• Has low or no maintenance (depending on material),
• Uses local materials in northern climates near forested areas.

The disadvantages of using a rain screen are:

• Plaster finish is not an option on a bale wall,
• May not be as durable as some types of plaster,
• Materials may not be sustainable or even available in your area,
• Aesthetic of siding may not match your project.

Rain Screen Concept on Bale Walls

It is important to remember that no matter how we finish bale walls, they must be sealed with plaster. This means that even if we choose to use a rain screen, we must apply at least one coat of plaster. One way to install siding on bale walls is to first install nailers for the siding. These can be in the form of 2-in.x2-in. wood strips attached to the sill plate and beam at the top of your bale wall.
We recommend attaching the nailers before stacking the bales, but you can do it afterwards if you like. Once the nailers and bales are in place, one coat of plaster is applied between the nailers. A rough coat of plaster over the bales is all that is necessary. Little or no troweling is required because no one will ever see the results. After plastering, building paper is stapled to the nailers and the siding is then installed, leaving a gap behind the paper for ventilation and drainage.

One issue of concern with this method is the gaps that can occur between the plaster and nailers as the nailer wood shrinks over time. These gaps can allow air to ?ow in and out of the bale wall, creating a loss of insulating value, as well as a path for insects and/or rodents. Extra care and/or the application of caulk can take care of these gaps. Also, these gaps can be eliminated if the nailers are installed after plaster is applied. Whatever you do, be sure that a gap remains between the back of the siding and the plaster.

This is but one way to install siding on to a bale wall. There are variations to this concept, but the goals remain the same – keeping rainwater and back-splash off your bale walls. Pay attention to the details and remember the forces that are acting on water that comes into contact with your walls. Holding these basic concepts in mind will help you design your wall system. And most important, do your homework first!

Happy wall building!

Resources
1. Rainscreen Cladding: A Guide to Design Principles and Practice.Anderson, J.M. & Gill, J.R. Butterworth-Heinemann, 1988.
www.shildan.com/Rainscreen/History.htmlhttp://irc.nrc-cnrc.gc.ca/pubs/ctus/17_e.htmlwww.greenhomebuilding.com/pdf/RainScreen.pdfwww.cmhc-schl.gc.ca/en/inpr/bude/himu/coedar/loader.cfm?url=/commonspot/security/get?le.cfm&PageID=70139

Ed.Note: Jeff encourages TLS readers to send in questions and comments to The Last Straw. There may be outstanding issues that builders are dealing with that most laypeople may not aware of. There are always many questions from people new to straw-bale construction. With this in mind, this column is offered and intended to encourage everyone to educate themselves to the fullest extent regarding building construction, and we are here to help in any way we can. This forum endeavors to offer the best of our knowledge, with no claim to its completeness, but to the spirit of bale building as a continuing evolution of one form of habitat within the larger realm of natural building. We offer this forum for dialogue, with no implication of being right or wrong. This forum is for you, the learner, artisan and teacher.

Jeff Ruppert, P.E., Principal, Odisea LLC, Ecological Building, Engineering and Consulting, P.O. Box 1505, Paonia CO 81428, 970.948.5744  <jeff@odiseanet.com> www.odiseanet.com
Jeff has been in the construction trades for over 25 years, beginning as a laborer and draftsman on his father’s construction projects. He has spent many years working on construction projects he designs, and is a licensed engineer in Colorado.

by Habib John Gonzalez – British Columbia, Canada

This article appeared in a slightly longer version in TLS#22/Spring 1998.

Here are the simple steps and materials needed to build your own bale wall moisture sensor:

1. Determine what depth of the bale you want to monitor and cut the 3/4-inch PVC pipe to that length.

2. Make the white pine sensor disk 1/8-in. thick to fit snugly into one end of the pipe.

3. Solder two lengths of telephone wire to two pairs of small bolts. One end of the pair of wires is bolted to a PVC pipe cap so the tips will protrude from the finished interior wall. The other end of the wires will be bolted to the sensor disk.

4. Use epoxy to glue the disk to one end of the pipe; run the wires through the pipe and fasten the other pair of bolts to the interior wall end cap. Glue the cap to the pipe.

5. Glue a perforated pipe cap over the sensor end of the pipe.

6. The sensor is ready for installation in the bale wall.

7.The TimberCheck moisture meter is available from www.leevalley.com

8. A number of bale wall moisture studies were sponsored by the Canadian Mortgage and Housing Corporation (CMHC). You can get a summary of all of the CMHC moisture work on their web site www.cmhc-schl.gc.ca/publications/en/rh-pr/tech/dblist.cfm?mode=year.  Scroll down to the bottom of the list for 00-103 (year 2000, document 103) on straw-bale moisture monitoring.

1. Outer end-cap
2. Perforated PVC pipe
3. Wood disk with screws
4. Wires
5. PVC pipe
6. Inner end-cap
7. Screw contacts

Build our house out of straw?

by Stephen MacDonald – New Mexico, USA

This article appeared in The Baley Pulpit,TLS#7/Summer 1994.

“May we look upon our Treasures, and the furniture of our Houses, and the Garments in which we array ourselves, and try whether the seeds of war have any nourishment in these possessions, or not.”

- from the Journal of John Woolman an 18th Century Quaker

“Build our house out of straw?” When our neighbor suggested the idea as a solution to our housing problem, both my wife, Nena, and I reacted similarly. “You must be kidding!” Even when he showed us a copy of Fine Homebuilding with an article in it by Gary Strang (1985) on a studio built out of straw bales, we were dubious. It was just too weird (images of rotting hay, mouse hotels, and pig stories readily came to mind). The idea was too simple and straightforward to be believed.

Try as we might, however, we kept returning to the idea of it. It did seem to fit our condition: Using straw bales was 1) low cost…we were near broke, having used the last of our meager savings to buy a small piece of land; 2) a way to stay cool (and warm)…having just moved to southwest New Mexico from Alaska, I was scared to death of the heat; 3) fast and physically easy to build…I just couldn’t face the slow, heavy work of adobe; and 4) ecologically sound…besides being energy efficient, a straw-bale building uses a renewable resource (often viewed as a waste product) that was locally available. Done right, building with straw uses very few trees.

In the end, we decided to go for it. Seven years later, we have no regrets. Just the opposite. We didn’t know it at the time, we were not the only ones interested. Through Strang’s article and newly formed friendships with Susan Mullen, a permaculturist and close neighbor, and an enthusiastic Matts Myhrman in Arizona, we learned of a small but dedicated network of straw-bale aficionados. Nor were any of us particularly innovative. The true trailblazers of straw were the folks from the Sandhills of Nebraska who, out of necessity, started a tradition of building their homes out of native hay and straw beginning back in the late 1800s and continuing up through the early 1940s.

The work of the Nebraska homesteaders remains the key. It took a fact-finding journey to Nebraska in 1989 by Matts and Judy Knox, his wife, to finally convince us that we, like most of those early Nebraskan straw-bale builders, could further simplify our technologies by using straw bales as load-bearing walls without the time and expense of poles or posts. We modern practitioners of straw have come to call it building “Nebraska style.”

It is this style of building that has captivated my imagination and been the thrust of our most recent building endeavors. Much good work needs to be done to revitalize the straw-bale building tradition and get it accepted into common practice. Tackling the building codes is part of that work along with trying (and sharing through The Last Straw) new and innovative techniques. I have no doubt in my mind that sooner, rather than later, this Earth will demand it of us.

Nena in front of the MacDonald’s straw-bale home.

Meanwhile, here are some of this straw-bale builder’s rules of thumb.

I. Keep it small. How much space do you really need? Be honest. Be creative with your space. Small is easy to heat and keep cool. It’s easier to keep clean. It takes up fewer of the earth’s resources and takes up less of its space. You finish the job, at a lower cost, so you can devote money and energy to more useful work. If your teenagers need distance, have them build their own outbuilding or addition. They need to learn the skills, anyway.

2. Keep it simple. Control your impulses to make your house a complicated, “artsy” statement. Simple, small and rectangular houses are beautiful when made of straw and other natural materials. Let form follow function. Again, spend all the time and money you saved by being – out in the woods, feeding the poor, or playing with your children.

3. Build it yourself. Trust yourself. You can do it, especially if you build with straw…and especially if you follow rules 1 and 2. Read building books and magazines.

Ask questions of builders. Build it on paper and as a model first. Track the details. Use your common sense. Be creative with your mistakes. Don’t be intimidated by the “experts.”

4. Stay out of debt. Pay as you go. Assemble the parts as you have the money and time.

5. Use local materials. Use more rock and adobe. Use locally milled lumber and poles. Your neighbor needs the work and you need to know firsthand what demands you’re asking of the forests and the fields.

6. Be energy conscious. Build to maximize passive heating/cooling strategies. Superinsulate your ceiling. Stay off the electric power grid if you can. Put up a windmill or use a solar pump. Build a composting toilet. Raise a garden.

7. Make yourself a home. Don’t just build a house, make yourself a home. Learn to be at home. Do no harm.

6 June 2005 Update

It is hard to believe that 18 years have somehow flowed by since Nena and I first built our little house of straw here in rural New Mexico. Two kids fledged, one now married and making his way at the edge of the Adirondacks in New York, the other just back in the United States after months of solo travel through Europe and western Russia.

Our little house continues to do well. We finally added a small greenhouse to the southwest corner of the place, and several years ago built a really first-class outhouse off the shop. I keep meaning to replace the salvaged (and very inefficient) casement windows we have, and one of these days I’m going to get around to finally plastering the outside of my Nebraska-style office/former teenage daughter ‘cabin.’ “Margosh, margosh” as my Mongolian friends would so often say – tomorrow, tomorrow.

Stephen MacDonald lives with wife Nena, son Orien (and co-author with his father of A Straw-Bale Primer), and daughter Aili, in their owner-built houses of straw in Gila, New Mexico. Steve and Nena live their Quaker faith in numerous ways including active participation in The Friends of the Gila River, working to create a cooperative ecosystem-based Gila River Ripiarian Management Plan with all stakeholders. Steve returns to Alaska each summer to continue biology mammology field work in the bush, and touch base with his northern home for over 14 years in the 1970s to early 80s… and stay cool.

“Somehow Nena and I have survived our various mid-life crises, finding new balance as we age and continue along our now 32-year journey together. I am still very much engaged with my work on far northern mammals (now through the Museum of Southwestern Biology), while Nena, having let her nursing license lapse, spends her days here at home.”

[Guest Editor’s Note. Stephen and Nena’s small straw-bale house has been an inspiration to many. It still inspires because it so effectively embodies the basic principles Stephen outlined of what a home should be - a nice place to live in.]

Post Editing Note: If this information is valuable to you, there is much, much more in the published, official version of The Last Straw.  Please subscribe at The Last Straw online.

Ramp next to pump

Merry Mudders

(Stucco Pumping Iron)

by Habib John Gonzalez – British Columbia, Canada

This article, under the title “Stucco Pumping Iron” appeared in TLS#33 (Plaster special issue). [See note at end of article.]

The system I’ve been using for helping owner/builders stucco their bale homes comes from New Mexico. Folks there hire the stucco pump and operator, rent a big (125 cubic foot per minute) compressor with two 50-ft. lengths of air hose with Chicago fittings, two mortar (paddle type) mixers and enslave seven or more friends to help. Two people run the mixers, one runs (literally) the wheelbarrow, three trowel the mud, and one helps me with the hoses. This method worked well for the projects around here.

Experience shows us that nine to twelve workers make the jobs run smoother, with less burnout from the heavy work of mixing and wheel barrowing the mud. It is easier for the trowellers to keep pace with the spraying if there are more hands to help.

If you hire the four legendary Merry Mudders to mix the materials, we usually spray about 2,000 square feet/185.81 square meter of wall per day. These hard-working apprentices are the building and spraying crew for Sustainable Works.

Since most of the trowellers will be inexperienced and tend to overwork the material to make it look “professionally” finished, there is less stress and work if the owner/builder uses a hand-rubbed finish to the walls. As the brown (second) coat begins to harden, the crew rubs the walls with heavy damp sponges, rubber gloves or burlap rags. The trowel marks are removed and the wall takes on a soft porous “sand” finish, like a traditional adobe building. It is beautiful and easy to do.

The equipment rental, stucco materials costs (use prescreened mortar sand to avoid pebbles jamming the spray nozzle which makes me cranky), and prep work are added to the price of the spraying. I helped an architect friend do his load-bearing cottage in Sandpoint, Idaho, and he calculated his cost was half of a commercial job.

When you have evaluated doing your job this way, please let me know what you decide, and we can take the next step from there.

Masking

An important job before spraying is to mask carefully everything you do not want stuccoed with 2-mil. plastic. Where fine, straight stucco lines are desired around posts, window and door bucks, and such, use duct tape to hold the plastic in place. Otherwise, careful stapling will do.

Sifting

In the 1999 season, we introduced a homemade screen to sift the stucco mix as it pours into the wheelbarrows. Owners build a 2×4 frame, large enough to cover most of a wheelbarrow with a brace in the middle to keep the 1/2-in. hardware cloth (galvanized screen) from collapsing with the weight of the mud. This screen catches rocks, sticks, and dry lumps of unmixed materials before they reach the stucco pump to cause clogging stoppages. This saves heaps of time and work clearing the pump, hose and nozzle. I remain a happy camper without all these dramatic “lock-ups” plugging up the works.

A Ramp

The 18-gallon/68.117-liter hopper on the pump is 32 in./81.28 cm high, so we need a ramp and table for the wheelbarrow operators to be able to dump the mud into the machine.

The ramp is generally 16 ft. long and 20 to 24 in. wide. It is built strong enough to carry an adult and a full wheelbarrow of material – about 400 lbs./181.4kg, depending on how well in shape the operator is. A bale or two placed under the ramp keeps it from flexing as the barrowers run up to the table. The well-braced table is built of 2×4s and scrap plywood, 32 in. high, 32 in. wide and 32 in. long.

Buckets

Have lots of 5-gal./18.92-liter plastic buckets on hand (a dozen or so) to measure sand and water and to soak trowels and sponges at the many work sites along the walls. Additional supplies to make life easier are: eight large cellulose sponges for rubbing the brown coat, at least four heavy wheelbarrows to empty a full load of stucco from each mixer (frees the mixer for immediate refill), and three large heavy-duty garbage cans for water (dipping water for the mix is much faster and accurate than waiting for a garden hose).

Estimating Materials

In stick-frame construction, one bag of mixed stucco covers about 80sf./7.43m2. Stucco covers about 35sf/3.25m2 of bale wall due to the thick layer applied, absorption of material by the fibres and the irregularities of the wall surface. One bag of stucco mix (a commercial cement lime stucco product) has a volume of one cubic foot. Measure the length and height of the exterior and interior walls to determine their total area. This figure is the total area per 1/2-in./1.27-cm coat of stucco.

Divide this figure by 35sf/bag to get the number of bags of cement (or stucco mix) needed per coat (scratch and brown). If you are spraying two coats of stucco, multiply this figure by two.

The first or scratch coat is mixed with sand at a ratio of 3:1.

This converts to shovels of material as follows: one bag of stucco mix equals seven shovels in volume, therefore the scratch coat is mixed with 21 shovels of sand per bag of mix. The second or brown coat is mixed at a ratio of 4:1, or 28 shovels of sand per bag of mix.

To calculate the total amount of sand needed, we use the brown coat ratio and multiply the number of bags of mix by four. This gives us the amount of sand in cubic feet; sand is sold by the cubic yard, so this figure is divided by 27. Order the sand by the next greater dump truck load; if you need 7 cubic yards for the project and the supplier has a 10-yard truck, get 10 yards since sand is cheap and a lot can be lost in dumping, and during building.

During the work, water and sand will be added by pre-measured buckets to keep the density of the mix consistent. This avoids wet mixes running off the walls or dry mixes plugging the pump and hose. Anyone seen randomly adding material to the mixers (i.e., “eyeballing” a little extra water or sand) will be drawn and quartered and then considered for serious punishment!

Cold Weather Work

As the daily temperatures drop in the fall, the experts from the Portland Cement Association say to heat the stucco mix water when the daytime temperatures reach 4oC/39.2oF. The other strategy is to enclose the building while stuccoing and for a few days afterwards.

The PCA Stucco Manual states “Water heated to 130oF to 140o F/54o C to 60o C and combined with cold but unfrozen sand will produce plaster in the 70oF to 100oF/21oC to 38oC temperature range. Heating water to raise the mortar temperature has the effect of accelerating the plaster set which protects against early freezing.”

The water can be heated using a large residential water heater to fill the buckets at the mortar mixers.

Another very useful heater is the propane-fired demand water heater; this only burns fuel when a hot water tap is turned on, there is no tank to be constantly heated. Boche and Paloma are two popular makers of demand water heaters.

A third option is to find an old metal trough or several heavy 45-gal./170.29-liter drums which will be heated by a wood fire below. The warm water will be dipped out by the mixing crew.

To ensure against a cold snap, it is helpful to have enough tarps, plastic sheeting, and similar materials available to hang from the eaves to enclose the whole building. This allows the stucco to cure without the threat of spalding (flaking caused by severe frosts).

Lubrication

Buy two large bottles of Joy dishwashing soap. Old Russian plasterers told us a little bit in each mix acts as a lubricant for the pump and they are right!

A Review of Stucco Options

After reading the stucco handout, a contractor still asked what kind of interior finish I trowel onto the bale homes. Here is my reply: The answer is nothing….we spray and mix, advising the owner/builders of their options.

Many are happy to have the trowelling done as a work party to save money, promote community, spread the SB song, etc. The results are not professional, which is why I encourage clients to do a sponge-rubbed “sand” finish on both exterior and interior walls to remove the less than perfect trowel marks and bring out the soft sand texture. If the clients want something more controlled, then I encourage them to hire a professional plasterer to lead the volunteers; otherwise, we cannot help them and ask them to hire a professional crew.

The work party developed as a way to help the owner/builder around here get the plastering done quickly, to protect their building and make it livable sooner. Some larger homes were taking two years to plaster; one commercial building, built with soft shaggy bales burned (according to the fire chief, the fuzz carried the fire up into the rafters and through the 3,000 sf./278.71m2 roof in 23 minutes). The owner delayed plastering a long time while deciding whether or not to use stucco wire.

One compromise is for us to spray the scratch coat only as a work party; the work is less exacting than the final coats and we leave the site knowing the bale wall surfaces are safe. Several friends have done two work party spray coats with us and followed them with simple gyprock mud, sometimes tinted with latex paint. This interior finish is either satiny smooth or textured to taste.

Another interior finishing variation is to dilute the drywall mud to the consistency of heavy cream, add sand, and brush it on the walls (bale or drywall) to continue the pattern used on the exterior walls. This worked well for matching the texture of the drywall with the bale walls and required no sanding.

In the summer of 2001, a client decided to use colored finishing stucco as the brown coat. We sprayed it on 1/2 in. thick instead of the usual 1/8 in. depth. The results were astonishingly fine. Imasco brand in Canada works well and their Technical Services Department remained silent when we inquired about using their material in this manner. La Habra in the U.S. discourages the practice but clients have opted to use it anyway and with good results. One job cracked a lot on the south wall but I believe it was the result of too few workers rushing and thereby overworking the material.

In the spring of 2003, a client, who works for LeFarge Cement in southern British Columbia, used two cups of Bayer brand ferrous oxide colourant per 90-lb. bag of stucco to colour the brown coat we sprayed on his house. We used a half bag of colour additive for 2,000 sf./185.81m2 of wall area, the walls were given a sponge finish and the result is a soft, earthy red tone.

Resources

For more information on the Bayer Ferrox Pigments, contact Bayer representative Kieren O’Conner at 1.412.777.3180 in Pittsburg, PA.

Another source is Jennifer Cohill in Phoenix, Arizona, at 602.266.0462 or check with your local stucco supplier.

Concretech in Delta, B.C., supplies a full line of concrete colouring products from www.concretech.com or toll free at 877.952.0157.

Habib John Gonzales is director of Sustainable Works, a consultation service, teaching the fundamentals of straw-bale construction in British Columbia, Alberta and the Yukon Territory, Canada. Habib is a pioneer of load-bearing strawbale in western in Canada, and developed a low-cost, homemade moisture meter (see TLS# 22 for plans as well as the article on p. 10 of this issue.) This article, under the title “Stucco Pumping Iron,” appeared in TLS#33 (Plaster issue) – copies are still available as a back issue order or on the TLS #1-40 CD. The original of this article in TLS#33 has additional info on the stucco pumping iron not present in this updated and condensed version.

Habib’s contact info is <habibg@netidea.com> Sustainable Works, 615 Cedar Street Nelson, British Columbia, Canada V1L-2C4 Tel/fax: 250.352.3731