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Frequently Asked Questions - Technical

We are in the process of compiling and questions and answers for our website’s Frequently Asked Questions Page. If you have a question that you don’t see here, please email it to us for inclusion.

The information contained within this website is based on compilations of research, laboratory testing, published articles, field experience, discussion with peers and observation of the service history of existing buildings. It is intended to be used for informational purposes only and is not intended to be a substitute for the development of a site-specific design by a qualified engineer.

FAQ – Natural Materials

1. I want to use a natural material. Can you engineer it?
2. How do I evaluate the quality of published results? Are there some test results that are more trustworthy than others?
3. Is there an overall macro scale or medium scale for a sample that is more appropriate to evaluate than a small-scale sample?
4. Is the age of a test considered in determining its value and relevance?
5. How do I extrapolate test results when designing for on-site field conditions?
6. How long will a natural material last? What is the long-term performance of a material?
7. What aspects of a material are critical to its performance? Which variables are important to control?
8. How much on-site testing is required? How do you perform testing on site? Is full-scale testing required? How much testing is required, and what is the minimum number of tests?
9. What factor of safety is prudent?
10. How long does it take to build a structure using natural materials? Do I need to enclose and complete the building in one season? What is typical?
11. Are engineers more or less conservative when engineering designs using natural materials?
12. How do natural materials compare to conventional in terms of thermal and building envelope performance?

FAQ – Hybrid Systems

1. Can I combine straw bale walls with earthbag walls? For example, could I use straw bales to insulate my north wall, and build the south and west walls out of earthbags?
2. What are some considerations when combining materials and systems?

*Differential thermal expansion and contraction
*Differential expansion with exposure to water
*Differences in dew point (ie, metallic materials used to pin through courses of stacked straw bales will accumulate condensation, eventually causing rotting of the straw)
*and many, many more…

3. Do natural or hybrid buildings require expansion joints?
4. Can you combine natural materials with more common building materials? For example, can straw bale be combined with steel and/or concrete?
5. What are some pitfalls to avoid?
6. Are there examples of successes and failures?

FAQ – Earthbag

1. What is earthbag construction?
2. Can I use earthbags for foundations?
3. How tall can an earthbag wall be?
4. What kinds of bags can be used?
5. What is a standard size for earthbags? Are tubes available, and are they better or worse than individual bags?
6. How does the type of fill affect the strength of the assembly?
7. What does the barbed wire do? Do I need more than two rows of barbed wire? (How do the barbs interface with the compacted bags?)
8. What amount of compaction is required? How is the compaction accomplished?
9. What types or grades of fill can be used? Is one type better than another?
10. What is the in-plane strength of an earthbag wall?
11. What is the out-of-place strength of an earthbag wall?
12. What is the compressive strength of an earthbag wall or foundation?
13. Is it necessary to plaster or otherwise protect the earthbags?
14. Do the bags degrade, and if so, are they necessary once the fill is compacted?
15. How much does an earthbag wall or foundation settle over time? Should it be detailed like a log-cabin which allows for settlement as the logs/cure season?
16. How long should the bags be seasoned?
17. Do you have to insulate the earthbag wall?
18. Are the walls waterproof?
19. How do you anchor fixtures, shelving, and framing for openings and top plates to the walls?
20. Do you need a bond beam at the top of a wall?
21. Is there a minimum length for an earthbag wall? Is it necessary to avoid walls that exceed a given ratio of height to length?
22. Is an earthbag wall stronger when it is straight, or stronger when it is curved/stepped?

FAQ – Straw Bale

1. What is the insulation value of a straw bale wall?
2. Is it necessary to use a reinforcing mesh or lath behind the plaster? What does the reinforcing do? What is Tenax? Is chicken wire still used?
3. Can I use clay plaster?
4. Does cement-lime plaster “breathe”? Is it important for a straw bale wall to “breathe”?
5. What connection details are used to join straw bale walls to foundations, and to join roofs to straw bale walls?
6. Do bale walls need headers over window and door openings?
7. What is the minimum length of a bale wall? What typical length of wall will act as a shear wall?
8. How thick should the plaster be?
9. If I have wet bales, will they dry?
10. Does the wet plaster cause rot of the bales?
11. How long does it take for the intrinsic moisture in the wall to reach equilibrium?
12. Should I monitor the moisture in a bale wall? Is there such a thing as a moisture monitor?
13. Should this measurement be part of the overall monitoring of the HVAC system?
14. Do I need to be concerned about seismic forces on bale walls?
15. What is the out-of-plane strength of a bale wall?
16. Can I plaster only the exterior side or vice versa? Does that affect the strength?
17. Can I use clay plaster on the inside, and cement plaster on the exterior?
18. Why do I need to have a notch in my foundation or piece of hardware to bear the weight of the plaster?
19. What is the load path of a bale wall? Does the plaster skin need to bear on the sill plate? What if the exterior plaster skin overhangs the sill plate?
20. Is it better to use straw bale as a non-bearing wall? Does it still need the same detailing if the structure is a timber frame?
21. If the bale walls are non-load bearing, do I need to have it engineered?

FAQ – Plaster Skins

1. How do I apply flashing and waterproofing around an opening in a straw bale wall?
2. If the plaster cracks, do I need to re-plaster the wall? How do I repair cracks? Do cracks allow water into the wall? Do they affect the strength of the wall?
3. How do I prevent or minimize cracking at the corners of my building?
4. How do I prevent or minimize cracking at the corners of window and door openings?
5. What is a cold joint?
6. Does cement-lime plaster crack less than clay-based plaster?
7. Please tell me what is meant by “autogenous healing.” What kinds of cracks are sealed or mended in this way?
8. Can I add pigments to change the color of my plasters?

FAQ – Prefabricated Straw Panels

1. Is prefabricated, tilt-up construction faster, cheaper, suitable?
2. What types of connections are used to join panels?
3. Can you stack the panels one on top of another?
4. What types of connections are used to join panel to other building elements?
5. What size should the panels be? Are they stackable like building blocks, or masonry construction?
6. Does the orientation of a bale affect the strength, thermal performance or moisture properties of the panels?