Slope pan flashing to outside.

Included in TLS #49 (Myths and Realities, Spring 2005) was a discussion of ways to deal with moisture at the bottom of windows. David Eisenberg shared a written design detail for a pan under the window to carry water away from rather than down the wall. We wanted to share a drawing of this detail and David kindly provided one for us to share in Tech Tips.

Here’s the portion of the discussion in which David details this design idea.

“Protecting the bales beneath the windows requires that you catch the water under the window and make sure it gets all the way out of the wall. In other words, ideally, you would have a pan of sorts under the window, sloped slightly to the outside, extending a bit beyond each side and with a lip at the back and on each end (so water can’t just run off the ends), and extending out beyond the exterior wall surface, with a drip edge – so that any water that leaks through or runs down the sides of the window ends up in this pan and is shown the exit. You can make these pans out of metal, plastic, ice and water shield, cast this shape into a concrete sill, anything that will keep the water from leaking through it, but the principal thing here is to make sure that the water can’t get into the wall below the window. You can put your window sill material, whatever it is, on top of this pan flashing being careful not to punch unsealed holes when you install the sill. It can take a little thought and ingenuity to do this, but it assures you that, when the windows leak, the water leaves the building.

Concept of pan flashing turned up at back and sides extending beyond exterior finished wall with drip edge. Extending behind finish or trim at each side of opening.

“That old practice of just putting roofing paper or plastic over the top of the bales and setting your windows on it and then plastering over it just leads the water down inside the plaster to the bales wherever the water protection ends unless it runs continuously down the wall under the window to below the bales (and we don’t recommend doing that).  It just temporarily moved the problem down, didn’t solve it.”

This article does not appear in The Last Straw and is original content.

Prepared by Joyce Coppinger, Managing Editor/Publisher, The Last Straw Journal

402.483.5135, <thelaststraw@thelaststraw.org> www.thelaststraw.org

INSULATION

The R-value used for straw-bale walls is R-30. Most conventional stick-built construction has an R-value of around 15 with as high as R-30 in ceilings.

Testing under controlled conditions allows the researcher to estimate the thermal resistance to heat flow through the material. This is expressed as an R-value. (R = resistance) R-value is the inverse of U-factor, or conductivity. U-factor is a measure of Btu/(hr. s.f. °F), or British thermal units per hour, per square foot of material, per degree Fahrenheit of temperature difference between the two sides of the material.

Conclusions from the document Thermal Performance of Straw Bale Wall Systems available at www.ecobuildnetwork.org/strawbale.htm

Tests have shown a range of values from R-17 (for an 18-in. bale wall) to R-65 (for a 23-in. bale).

Analysis at Oak Ridge National Lab, among other places, has shown that R-values for insulation materials used in standard walls are generally much higher than the R-value for the wall as an assembly of disparate materials. Joe McCabe recently postulated that the same phenomenon could account for the difference between the high values from his testing of bales and the lower values obtained in the 1998 Oak Ridge test of a straw-bale wall system. While it is possible that the relatively low densities where bales abut each other might contribute to greater heat loss than would be measured through an individual bale, it is unlikely that this would account for the entire difference. This difference between bales and bale walls is nothing like the difference between standard insulation and what is found in stud framed walls (insulation voids, thermal bridges, uninsulated headers, and other faults).

It is noteworthy that all tests of straw-bale wall systems prior to the Oak Ridge test in 1998 had potentially significant shortcomings and should not be considered particularly reliable. The last Oak Ridge test had no identified deficiencies and is considered by most to be an accurate determination of the thermal resistance of straw-bale walls. ORNL determined the R-value to be R-27.5 (or R-1.45/inch), or R-33 for three string (23-in.) bale wall systems. Shaving a bit off the top just for conservatism’s sake, the California Energy Commission officially regards a plastered straw-bale wall to have an R-value of 30.

A final note is a reiteration of a point made earlier: it matters little whether the final truth about the R-value of straw bales walls is R-33 or R-43 or even R-53. Above R-30, the differences are minor and will usually be overshadowed by windows, floors, doors and ceiling/roof details.

Whatever the value, it is at least three times better than the average -in.R-19-in. wood stud-wall system.

FIRE

In July 2006, the Ecological Building Network in California funded and oversaw the following ASTM E119-05a – Straw Bale Fire Tests done in Texas. Both walls withstood the fire and hose stream tests, as described below.

(Documents are available at www.ecobuildnetwork.org/strawbale.htm)

One-hour Fire Resistance of a Non-Loadbearing Wall w/ Earth-Plaster Coating.

A 12 ft x 14 ft non-loadbearing wall constructed with 7.5 pcf rectangular wheat straw bales stacked in a running-bond pattern, clad on each surface with 1-inch of earthen-plaster, produced, assembled and tested as described in the documentation, successfully met the conditions of acceptance as outlined in ASTM Method E119-05a Fire Tests of Building Construction and Materials for a fire endurance rating of one hour.

Two-hour Fire Resistance of a Non-Loadbearing Wall w/ Cement-Stucco Coating.

A 10 ft x 10 ft non-loadbearing wall constructed with 7.5 pcf rectangular wheat straw bales stacked in a running-bond pattern, clad on each surface with 17 GA stucco netting and 1-inch of cement/stucco, produced, assembled and tested as described in the documentation, successfully met the conditions of acceptance in ASTM Method E119-05a Fire Tests of Building Construction and Materials for a fire endurance rating of two hours.

The EcoBuilding Network’s Board of Directors is currently Ann Edminster (Pacifica, California) Architect, author of Efficient Wood Use in Residential Construction-in., and co-chair of the development committee for LEED(TM) Residential standards.

Bruce King (San Rafael, California) Director, Founder, Structural Engineer, author of Buildings of Earth and Straw-in. (1996), -Making Better Concrete-in. (2005), and Design of Straw Bale Buildings-in. (2006); Sarah Weller King (San Rafael, California) Secretary and Treasurer Peter Loafer (Boulder, Colorado) Attorney and property developer; Drew Moran (Palo Alto, California) President, Drew Moran Construction; Anne Tilt (Berkeley, California) Architect and partner, Akin-Tilt Architects; Carol Vilonia (Santa Rosa, California) Architect, contributing columnist for Natural Home magazine, co-author of Natural Home Remodeling (2006).

MOISTURE

From House of Straw – Straw Bale Construction Comes of Age, U.S. Department of Energy, Energy Efficiency and Renewable Energy, April 1995. www.eren.doe.gov/buildings/documents/strawbale.html

Will the bales rot? Without adequate safeguards, rot can occur. The most important safeguard is to buy dry bales.

Paint for interior and exterior wall surfaces should be permeable to water vapor so that moisture doesn’t get trapped inside the wall. Construction design must prevent water from gathering where the first course of bales meets the foundation. Even if straw bales are plastered, the foundation upon which the bales rest should be elevated above outside ground level by at least six inches or more. This protects bales from rainwater splashing off the roof.

From Moisture properties of straw and plaster/straw assemblies by Dr. John Straube in Canada as a result of testing done there. John holds a joint appointment as Associate Professor in both the Department of Civil Engineering and the School of Architecture at the University of Waterloo and teaches courses in structural design, material science, and building science to both disciplines. At the university, John is also the director of the Building Engineering Group. John is a founding principal of Building Science Consulting, a frequent contributor to buildingscience.com.

Based on the test data and literature review, several conclusions can be drawn:

1. A 450 mm (18-in.) thick straw bale should have a vapor permeance of approximately 110 to 220 ng/ Pa•s•m2 (2 to 4 US perms).

2. Cement: sand stuccos are relatively vapour impermeable. In fact a 38 mm (1.5-in.) thick cement : sand stucco may act as a vapor barrier (i.e., have a permeance of less than 1 US Perm).

3. The addition of lime to a cement stucco mix increases permeance. As the proportion of lime is increased, the permeance increases. Pure lime: sand stuccos are very vapor permeable. The permeance of a 38 mm (1.5-in.) thick cement : sand stucco can be increased to 5 or 10 US Perms by replacing half the cement with lime and to 15 to 30 US Perms by using a pure lime : sand stucco. The addition of even a small amount of lime (0.2 parts) may increase the permeance of cement stucco dramatically (e.g., from under 1 to 3 to 6 US Perms).

4. Earth plasters are generally more permeable than even lime plasters. The addition of straw increases the permeability further. A 38 mm (1.5-in.) thick earth plaster can have a permeance of over 1200 metric perms (over 20 US Perms), in the same order as building papers and house wraps.

5. Applying an oil paint to a moderately permeable 1:1:6 stucco will provide a permeance of less than 60 metric perms (1 US perms) and thus meet the code requirements of a vapour barrier.

6. Earth plasters were not found to have significantly different water absorption than cement and lime stuccos. The earth plasters, regardless of density and straw content, resisted 24 hour of constant wetting easily, although the topmost 1/8-in. of surface became quite muddy. In a real rainstorm this behavior may cause erosion.

7. Lime washes appear to be somewhat useful for reducing water absorption while not reducing vapor permeance. The lime wash over earth plaster did not dramatically lower water absorption but will increase the mechanical strength of the plaster after wetting, i.e., they will increase the resistance to rain erosion.

8. Based on Minke’s and Straube’s earlier tests, siloxane appears to have little or no effect on the vapor permeance of cement, cement:lime, lime, and Moisture Properties of Plaster and Stucco for Strawbale Buildings EBNet BalancedSolutions.com 34 earth plasters while almost eliminating water absorption. The use of siloxane can be recommended based on these earlier tests.

9. Sodium silicate did not seem to have much impact on water uptake or vapor permeance. This additive may hold earth plaster together, or increase its erosion resistance, but as tested it had no noticeable impact on moisture properties.

10. Linseed oil at 2% in an earth plaster mix is not a very effective water repellent and does act to restrict vapor permeance somewhat. It may add some strength to an earth plaster in the wet state. Heavy applications of linseed oil to the surface of finished earth plaster will, based on Minke’s tests, reduce the water absorption to almost zero, but will markedly decrease vapor permeance.

11. The test methods described here appear to provide repeatable results, and in general compare well to previous tests on different samples by both the same (Straube) and different researchers (Minke).

Gernot Minke founded the Research Laboratory for Experimental Building at Kassel University in Germany in 1974, studying straw-bale construction and other sustainable building techniques, low-energy and passive house construction, and green roofs. He is also an independent architect and adviser for building ecology.

The full article can be accessed at the EcoBuilding Network’s web site www.ecobuildnetwork.org/strawbale.htm

INSECTS, VARMINTS AND VERMIN

Will the bales rot? Without adequate safeguards, rot can occur. The most important safeguard is to buy dry bales. Fungi and mites can live in wet straw, so it’s best to buy the straw when it’s dry and keep it dry until it is safely sealed into the walls.

Paint for interior and exterior wall surfaces should be permeable to water vapor so that moisture doesn’t get trapped inside the wall. Construction design must prevent water from gathering where the first course of bales meets the foundation. Even if straw bales are plastered, the foundation upon which the bales rest should be elevated above outside ground level by at least six inches or more. This protects bales from rainwater splashing off the roof.

Will pests destroy the walls? Straw bales provide fewer havens for pests such as insects and vermin than conventional wood framing. Once plastered, any chance of access is eliminated.

House of Straw – Straw Bale Construction Comes of Age, U.S. Department of Energy, Energy Efficiency and Renewable Energy,

April 1995

www.eren.doe.gov/buildings/documents/strawbale.html

CODES

There are provisions within the building codes that allow building with bales. David Eisenberg, Development Center for Appropriate Technology (DCAT) in Tucson, Arizona (www.dcat.net), shares these citations from codes that pertain to straw-bale design and construction as an alternative materials, design and methods of construction and equipment. David has been involved with codes issues related to strawbale and other natural building materials and methods for 15 years or more, and as a member of the board of UBC, USGBC and other organizations working with building codes and green building programs.

You can use this information to answer questions codes officials and other regulatory agencies may have.

From the 2003 International Energy Conservation Code (IECC)

SECTION 103

ALTERNATE MATERIALS — METHOD OF CONSTRUCTION, DESIGN OR INSULATING SYSTEMS

103.1 General. The provisions of this code are not intended to prevent the use of any material, method of construction, design or insulating system not specifically prescribed herein, provided that such construction, design or insulating system has been approved by the code official as meeting the intent of the code. Compliance with specific provisions of this code shall be determined through the use of computer software, worksheets, compliance manuals and other similar materials when they have been approved by the code official as meeting the intent of this code.

From the 2006 IECC

SECTION 103

ALTERNATE MATERIALS — METHOD OF CONSTRUCTION, DESIGN OR INSULATING SYSTEMS

103.1 General. This code is not intended to prevent the use of any material, method of construction, design or insulating system not specifically prescribed herein, provided that such construction, design or insulating system has been approved by the code official as meeting the intent of this code. 103.1.1 Above code programs. The code official or other authority having jurisdiction shall be permitted to deem a national, state or local energy efficiency program to exceed the energy efficiency required by this code. Buildings approved in writing by such an energy efficiency program shall be considered in compliance with this code.

From the 2003 International Residential Code (IRC)

R104.11 Alternative materials, design and methods of construction and equipment. The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code. Compliance with the specific performance-based provisions of the International Codes in lieu of specific requirements of this code shall also be permitted as an alternate.

R104.11.1 Tests. Whenever there is insufficient evidence of compliance with the provisions of this code, or evidence that a material or method does not conform to the requirements of this code, or in order to substantiate claims for alternative materials or methods, the building official shall have the authority to require tests as evidence of compliance to be made at no expense to the jurisdiction. Test methods shall

be as specified in this code or by other recognized test standards. In the absence of recognized and accepted test methods, the building official shall approve the testing procedures. Tests shall be performed by an approved agency. Reports of such tests shall be retained by the building official for the period required for retention of public

records.

From the 2006 IRC

R104.11 Alternative materials, design and methods of construction and equipment. The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code. Compliance with the specific performance-based provisions of the International Codes in lieu of specific requirements of this code shall also be permitted as an alternate.

R104.11.1 Tests. Whenever there is insufficient evidence of compliance with the provisions of this code, or evidence that a material or method does not conform to the requirements of this code, or in order to substantiate claims for alternative materials or methods, the building official shall have the authority to require tests as evidence of compliance to be made at no expense to the jurisdiction. Test methods shall be as specified in this code or by other recognized test standards. In the absence of recognized and accepted test methods, the building official shall approve the testing procedures. Tests shall be performed by an approved agency. Reports of such tests shall be retained by the building official for the period required for retention of public records.

From the 2006 IBC

104.11 Alternative materials, design and methods of construction and equipment. The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved.

An alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety.

104.11.1 Research reports. Supporting data, where necessary to assist in the approval of materials or assemblies not specifically provided for in this code, shall consist of valid research reports from approved sources.

104.11.2 Tests. Whenever there is insufficient evidence of compliance with the provisions of this code, or evidence that a material or method does not conform to the requirements of this code, or in order to substantiate claims for alternative materials or methods, the building official shall have the authority to require tests as evidence of compliance to be made at no expense to the jurisdiction. Test methods shall be as specified in this code or by other recognized test standards. In the absence of recognized and accepted test methods, the building official shall approve the testing procedures. Tests shall be performed by an approved agency. Reports of such tests shall be retained by the building official for the period required for retention of public records.

INSURANCE AND FINANCING

See http://sbregistry.greenbuilder.com – the International Straw Bale Registry sponsored by The Last Straw journal, Greenbuilder.com, Development Center for Appropriate Technology and the Texas straw-bale association as a resource and research database pertaining to straw-bale building, including buildings open for tours and visits, descriptions of design, construction, materials, special features and those who were involved in the building project, including homeowners, owner/builders, insurance, mortgage lenders, builders, architects and others.

When planning and building a straw-bale building, it is best to make contacts early in the process about liability insurance coverage during construction as well as homeowners coverage after the building is completed.

Homeowners insurance is available for straw-bale homes and insurance coverage for other straw-bale buildings is available also. Independent insurance agents and companies may be more likely sources, but many other companies offer homeowners and liability insurance.

Many straw-bale buildings are owner-financed or built on a pay-as-you-go basis, but as strawbale and natural building become more popular and generally accepted many structures have been financed through mortgage lenders, banks, credit unions, state and federal funding for housing, and other sources.

Contact your local sources to determine your best options. You may want to prepare detailed financial calculations and a budget for the project before approaching these groups and institutions. And you will need to be well versed about straw-bale projects in your immediate area, identify comparables from real estate companies, if possible, and have already contacted your local codes official about regulations and permits so that you know the project can be permitted.

by Chris Newton – Queensland, Australia

Can you design and build straw-bale homes for a hot and humid climate? Living in Queensland, Australia, I am frequently asked to identify an invisible line on the map where “she’ll be right” applies on one side of the line and “don’t go there” applies to the other. The part of me that fears litigation wants to respond with “ask me in 20 years time,” the technical part of me feels it has to be evidence based, and the logical part knows the answer already exists in the local environment. So I take on board here these three points and discuss how I attempt to find that line on the map in our building history, current research and the observation of the environment we live and build in.

Macro Climate

Queensland extends from 10 degrees south to 29 degrees south of the equator, covering more than 1.72 million square kilometres. Queensland is more than twice the size of Texas. Within Queensland, we live in monsoonal, tropical, subtropical, grassland and desert climate zones.

The table below represents summer (December though March) in the climate zones of Queensland. Summer is dominated by the monsoons making this a hot, wet and humid season. All zones in Queensland have mild and dry winters.

Microclimate

We can create a microclimate in and around our homes. Changes in air movement, moisture load or sunshine can significantly change the wetting and drying potential of a section of the building. When designing the house and gardens in a humid climate, we need to be aware of creating microclimates that cannot dry out.

Relative Humidity

Humidity is the water vapour held in the air. This is the ratio of the actual amount of water vapour in the air to the amount it could hold when saturated; it is expressed as a percentage. The capacity for air to carry water vapour increases as the air temperature increases. Air with a temperature of 30°C/86°F can hold more than three times as much water vapour as air at 10°C/50°F.

The dew-point temperature is temperature in which air must be cooled in order for dew to form. Droplets of water can be deposited within the straw-bale wall when air cools below the dew point and water vapour condenses.

Wood can absorb moisture content up to 25% from a relative humidity 98% (See Straube report in Resources at end of article). Straw is hygroscopic with its large surface area and internal pores having the ability to absorb moisture. A bale whose moisture content is at 8% will weigh less than the same bale with a moisture content of 20%.

Wetting Potential

Table Daily Humidity in relation to Temperature Changes Source: Australian Bureau of Meteorology

We have a copy of an 1860 encyclopedia. It’s only damage is some yellowing and a few small brown spots (mold). This book had no special storage other than to sit on a bookshelf in subtropical Brisbane. So it seems that humidity alone may not be enough to cause decomposition of straw bales. However, I know through talking to people from Cairns that it is the norm to have molds growing on curtains, furniture and shoes throughout their summer. Newspapers and photos curl from the moisture they absorb. So humidity alone is enough to support mold growth in the tropics.

Historically, bathrooms have remained an area with high failure rates from moisture; this is true in any building type. Protection for straw-bale systems in wet environments exists. This can be in the form of vapour barriers, water barriers, design considerations, and attention to detail. It would be fair to say that, over the life of a building, some houses despite best efforts will experience elevated moisture levels in part of the wall system. Concentrated moisture only becomes a problem if the ability to dry is not timely for the given climate conditions. Remember that molds grow rapidly in hot and humid conditions, and are dormant in cold conditions.

Drying is the balance for wetting. The measure to ensure this includes a capillary layer below the bottom straw bale and a render with high permeability. Water vapour moves from low concentration to high concentration. High humidity will reduce the ability for the wall system to dry. In the tropics, rain may persist over several days. Attempting to dry clothes in the shade will take a long time during which they will acquire a moldy smell. You can not expect a wall system on the south side of the building to dry as efficiently as those on the north. High humidity will further compound this. (Note that we live in the southern hemisphere.)

Can you build with straw bales in a high humidity climate?

The line that removes high risk for straw-bale construction is unlikely to be a latitude line. Maybe it is a line that farmers have already identified. Grain farmers look for a climate dry enough so the grain dries adequately before harvest. The dry grain is then suitable for storage. Humidity is not a problem for the sugar cane growers who harvest the crop with high moisture content and send it straight to the mills where the juice is squeezed from the cane. So maybe the invisible line is found on an agricultural plan.

Resources

How Straw Decomposes, Matthew D. Summers, Sherry L. Blunk, Bruan M. Jenkins. www.ecobuildnetwork.org/pdfs/ How_Straw_Decomposes.pdf

Straw Bale House Moisture Research, CMHC (Canadian Mortgage and Housing Corporation). www.cmhc-schl.gc.ca/ publications/en/rh-pr/tech/00-103-E.htm

Moisture Properties of Plaster and Stucco in Strawbale Buildings, Dr. John Straube. www.ecobuildnetwork.org/pdfs/ Straube_Moisture_Tests.pdf

Monitoring the Hygrothermal Properties of a Straw Bale Wall, Dr. John Straube and Chris Schumacher. www.ecobuildnetwork.org/pdfs/Monitoring_Winery.pdf

Bureau of Meteorology–Australia. www.bom.gov.au/ weather/qld/

Chris Newton, Earth-n-Straw, Queensland, Australia, 0413 195 585, <chris@newtonhouse.info> www.newtonhouse.info. Chris, an owner/builder, educator and trainer in strawbale, plasters and other aspects of natural building, is the new President of AUSBALE, the Australia and New Zealand straw-bale building association.

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

Here is a detail of a window sill that we (Odisea) did on a project in 2000. Of course I can’t find any pics of a flagstone sill, but this one shows a slope of the sill and drip kerf. The CAD detail shows our flashing as a faint green line under the sill, which is extended beyond the plaster. The flashing was our “pan.” This project was done in my early days of really paying attention to the details, so there are subtle things I would do different, but the basics remain the same.

Window sill in Siberia

This second picture is a sill that we did in Siberia back in 2005. I built the window frames from scratch and then installed them into our window bucks. I placed a piece of thick building paper around the window and extended it out under the sill as you can kind of see in the shadow under the sill with the kerf. We were in the middle of nowhere and the people we were working for did not have any money so we used what was available.