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 Magwood and Peter Mack-Canada

This article is an excerpt from the book Straw Bale Building: How to Plan, Design and Build with Straw (New Society Publishers, 2000), and is reprinted with permission.

Hitting a Moving Target. There is never a single point during the planning process when you can fix an exact budget for your project. Once your plans near completion, however, you have a chance to use them as a guide for estimating both materials and labor costs.

If you find you have missed your budget target by a significant amount, you will have to go back to your plans and start making adjustments. This can be disheartening, but it is better to catch such a problem early than to run out of money before there’s a roof over your head! You may be able to adjust costs without changing your plans, if you commit yourself to finding cheaper materials and hiring less labor. If you do change your plans to reduce costs, don’t forget to work in planning that will allow you to bring your building back to its originally planned size later.

You may discover that you have apparently created plans that will allow you to build for less than what you budgeted. Congratulations! This is every homebuilder’s dream.  Don’t change your plans, however.  When the project is over, you’ll be able to spend a bit more on detailing, furnishing, and landscaping.

It Always Costs More than You Think. The building project that is completed without going over-budget is rare. Your plans will allow you to create a budget estimate, but there will always be unforeseen costs, delays, and problems that will require extra cash to solve. Leave yourself with plenty of budgetary breathing room so you can deal with the inevitable. Try to reserve at least 10 percent of your total calculated budget to cover unforeseen costs.

Pre-construction Costs. The pre-construction costs of your project will not be evident from your plans. These include the price of property, interest on your property payments, building permit fees, driveway allowances, access roads, septic permits, service and utility hook-up and municipal development fees and taxes. Depending on where you are building, these fees can total several thousand dollars and take quite a bite out of your actual construction budget. Wells, septic systems, service entrances, and the excavation/groundwork must all be completed before you actually begin construction and will take another bite out of your budget.

Do you have a misting pump (above), a weed-whacker (right), or enough trowels and wheelbarrows for that big plastering party? If not, better add another $500 to your budget!

Other Hidden Costs. Before you start taking count of the dollars needed for materials and labor, don’t forget to consider other hidden costs you may need to cover.  The purchase and/or rental of tools can add up to a significant budget factor.  Working without the right tools is frustrating and slow, so think your way through the construction process and make a list of what you’ll need. From shovels and picks for digging to carpentry tools and plastering trowels, the list will be extensive and expensive. Keep a bit of your budget set aside for unforeseen specialty tools you’ll need to buy or rent. For specialized tasks– plumbing, wiring, heating, roofing, concrete form work, etc.– weigh the cost of acquiring or renting the appropriate tools and equipment against the costs of hiring labor.  It may be more economical to hire labor.

Storage. Any building project can involve lots of ‘tarping up’ to cover materials from the elements. This can be especially true for straw-bale projects. Invest in enough good quality tarps to cover the walls of the building and the mounds of straw.

Power. Depending on the availability of grid power at your site, you may require a generator for your power needs. Check the costs of purchase and rental to see which is the better option.

Transportation. If you are building yourself, you might find it beneficial to own a truck, van, or trailer that can be used to pick up and move materials. Such vehicles can be sold when you no longer require them, but you will need money to purchase, license, insure, and service them.

Toilets. Unless you are building in a well-serviced area, you will need some sort of on-site toilet. You can rent serviced units, or you can build an outhouse. Rental toilets are convenient and are removed when you are finished with them. They can also be expensive if the project is a long one. An outhouse requires an early outlay of time and money, but you get some building practice, and an outhouse is not a bad back-up facility to have in case of plumbing disasters in the future!

Work clothing. You will need proper clothing.  Buy good safety boots—spend extra for comfortable, well-fitting boots, gloves, and maybe a hard hat. If you are working in an inclement climate, warm and/or waterproof clothes will make a big difference to your ability to work efficiently.  Construction will wreak havoc on your clothing, so buy quality clothes or plenty of cheap, second hand stuff.

Insurance. Construction insurance covers your project in case of mishaps. Rates can vary tremendously, so get a number of quotes, and be sure you are covered for the risks that concern you most–fire, accident, damage from wind, rain, etc.

Sales tax. Don’t forget the tax man. Sales taxes can add a significant percentage to both material and labor costs. Don’t just total up pre-tax costs!

Cost of living. If you are doing your own building, don’t forget to include your cost of living while you are building. Rent and food must be covered, as will all your regular bills. If you are taking time off work to build, these expenses can take quite a bite out of your budget.

An Inexact Science. Unfortunately, budgeting is an inexact science. It is impossible to account for every contingency and glitch that may arise. The further afield you move from conventional construction, the more variables enter your budgeting equations. The only certain advice is spend plenty of time figuring out your budget, and leave lots of room for error.