Owner/
Builder

Foundations for Strawbale Buildings
ASK THE EXPERT ADVICE
Links to the Ask the Experts page

Dr. Owen Geiger, Ph.D.( in Social and Economic Development,) is the former Director of Builders Without Borders and current member of the BWB Steering Committee. Dr. Geiger is Founder and Director of the Geiger Research Institute of Sustainable Building (www.grisb.org). He is an author, engineer and licensed contractor specializing in strawbale construction and other types of sustainable building. He co-authored the Builders Without Borders Straw-Bale Construction Guides and contributed to Building Without Borders: Sustainable Construction for the Global Village. Dr. Geiger has consulted on numerous international housing projects, worked closely with Habitat for Humanity for seven years and mentored housing officials with the United Nations Institute of Training and Research. He is also a correspondent for The Last Straw Journal. Dr. Geiger's Global Straw-Bale Construction Certification Program provides high quality strawbale training via a unique program that combines hands-on experiences with research and assignments; this is a distance learning program for those within reach of the internet and with an adequate knowledge of English. See www.grisb.org for more information.

Q: I have a question regarding the foundations for a post and beam strawbale home. I specifically like the post and beam option because it appears to be much stronger and will be better received by the building officials, but I also like the post and beam option because of it's appearance when the beams are exposed on the inside surface of buildings. My question is what foundation options exist for a post and beam strawbale where the posts are on the inside of the bales? This approach doesn't appear to be common. Are there reasons why I should avoid it? Also if this approach is used what should be done to provide the lateral support which building codes require?

A: A wide range of foundation options can be used.  You'll have to get an engineer to evaluate your project and design the foundation accordingly.  This may seem too vague, but an engineer must evaluate your structure and local conditions -- seismic, soil conditions, frost depth, etc. A few ideas:
- Post and beam structures meet building codes if you use certified wood.  The posts carry the structural loads.
- One option is to buy a pre-engineered post and beam kit and wrap it in bales.
- Plastered bale walls provide lateral support, although your building officials will likely require additional strapping or tie wires.  Ask them what they require and/or work with an architect or engineer in your area with experience in SB construction.
- Be sure to insulate the foundation.  Heat will take the path of least resistance.
- Look for an engineer knowledgeable about low-cost, sustainable foundation options such as rubble trench, scoria-filled earthbags, pumice-crete, frost-protected foundations, etc.
- Placing the posts on the interior surface is quite common and also a good way to protect the wood.

Q: trying to find a formula for material ratio in bags for a earth bag foundation for a strawbale home. Specifically what materials and what ratio.

A: (Kelly) I recommend the use of washed 3/4 minus gravel to fill earthbags for strawbale foundations. This will keep any water from wicking upward and will provide a bit of insulation at the same time. Of course the bags need to be well protected from the sun and physical damage with a good plaster...and rather soon.

C: About the foundations, I downloaded a straw bale guide from Amazon. It states that the foundations can be from concrete blocks with rubble in the middle (the one I like best). As they don't use such concrete blocks here, I thought I would make a double concrete foundation each about 15cm wide and pour rubble (10cm) between the two, to provide drainage.

If you have space in the middle as you describe, consider filling the space with insulation such as lava rock.

Toe-up: The gravel-filled toe-up you describe is a good choice for raising the bales off the floor in case of a water leak or flood. It's often just 5-10 centimeters (2-4 inches) high. Don't confuse a rubble trench (which goes under the concrete foundation stem wall) with the toe-up.

C: What exactly is a toe-up? Is it the base plate that goes between the foundation and the bales?

The toe-up is the base plate (often filled with pea gravel for drainage) that goes between the floor and the bales.

Q: If I have a stem wall (required by Pima county code) do I still need toe-ups on top of that?

A: Raise the stem wall above the level of the finished floor and no separate toe-up is needed.  One option is to use earthbags filled with aggregate.

Q: Our land has almost no soil, but a lot of rocks. Our soil is officially (soil & water conservation dept. info) from 4" to 18" deep, and that small amount is mostly rocks barely held together by a little bit of dirt. That sits upon solid limestone/dolomite bedrock. For this reason, we plan to dig down only as far as we are able (probably about a foot at the site we have selected) and as wide as a 2-string bale; then back fill with gravel for a rubble trench foundation. Is a one-foot deep rubble trench good enough considering our ground conditions and the weight of a living roof (albeit a shallow one) on top of it? Alternative suggestions?

A: Clearing the soil and making a rubble trench foundation as described is a good idea.  All you need is to go down to solid or well drained rock.

Q: We had intended to pour a 4" tall, reinforced grade beam atop the gravel and top that with a full width, slip formed rock/cement stem wall (to lift the bales above grade enough to keep water out). We planned to put Styrofoam insulation outside the wall and below grade several inches since it would both help with any frost problems we might have from such a shallow foundation, and insulate the bottom of the bale wall a bit more than just rock and cement would do.) In retrospect, we feel that it may be overkill to bother with a grade beam when the stem wall will be as wide as the beam anyway, and equally strong. We also hate the idea of using something as un-environmental (and expensive!) as Styrofoam, but still need to deal with the whole insulation problem.

A: It sounds like you're mixing up the terms grade beam and stem wall.  Let's just call it a grade beam.  Pour an 8-10 inch grade beam on top of the rubble trench.  This raises the bales off the ground and allows for sloping the grade away from the building.  (4" isn't enough -- code says 6", plus you need drainage)  And rigid insulation on the outside is your best option.

Q: I am going to build with strawbale in Park County, Colorado. I am going with non load bearing system and post and beam framework. I want to utilize the rubble trench system under the bales. How deep should my trench be and should it be the width of my bales.

A: It's not clear exactly what you have in mind.  What will the bales sit on?  In general, a rubble trench should be the width of your bales and go to frost depth.

Q: My wife, Jamie and I are beginning a building project in Cass Lake Minnesota. We plan to build multiple buildings over the next few years - starting with a strawbale cabin this summer. I am very intrigued by your use of earthbags. The area that we are building in is all sand. Frankly I don't know how deep it goes but it goes a long ways for sure. It seems that a rubble trench with earthbags for the grade beam would be a great way to go. I want to use as little cement as possible, and I am very interested in the least toxic way to build.

A: Earthbags make an excellent foundation for straw bale walls.  In cold climates it would be best to use an insulating material such as scoria (volcanic rock) in the bags.  Bags of sand will wick away the heat rather quickly.  Another option is to use 2" of rigid foam insulation on the outside of the foundation, but this is more expensive and not as benign. You probably don't need a rubble trench, since the sand under the foundation would serve the same purpose (prevent drainage problems and heaving).

Q: I am an Architecture student at Kansas State University, currently working on my final project. I am doing haybale construction and our site is the Konza Prairie in Manhattan, Kansas. I was wondering if you knew of any alternate foundations besides the concrete foundation. I was looking into adobe, but I don't really know enough about it to be a good judge. This structure isn't going to be permanent, so I would hate to pour concrete on such a pristine piece of land. Let me know what you think!

A: I suggest using sandbags filled with gravel.  This system is now commonly called an earthbag foundation.  Earthbag foundations are becoming one of the favorite foundation systems among natural builders due to their low cost, simplicity and practicality.

Our website at www.earthbagbuilding.com has lots of pictures and projects.

Earthbag foundation details are at:
http://earthbagbuilding.wordpress.com/2007/11/01/earthbag-foundations/
www.earthbagbuilding.com/articles/slc.htm

In your project, the bags could be emptied and reused at the end of the project; leaving no permanent scar on the land.

Q: My wife and I are building a strawbale home with a gravel bag foundation. Our soil is mostly clay (perfect for plaster but poor drainage) and our climate wet. What kind of exterior finish would be suitable for the stemwall? I've heard that clay plasters should start 18-24" above grade. Also, if we berm or recess the house, what kid of waterproofing and insulation do you recommend?

A: Gravel bag foundations are great and rapidly gaining in popularity among straw bale builders (and others).  You can start building right on top of rubble (gravel) trench and save a lot of money. I agree with Kelly Hart about using scoria or pumice as bag fill material.  It's light and easy to work with, and it's the easiest way to create a frost-protected foundation. Some recommend adding cement to the gravel/scoria.  Unless it's a very expensive home or commercial building, I wouldn't bother.  Double bag it for peace of mind.

You'll probably want to use rigid foam board insulation if you use regular gravel.  Read up on frost-protected foundations (search Google for free guide).  Follow the recommendations for your climate: depth, thickness of insulation, type of foam board, etc.  It's really simple.

Keep earth plaster well above any moisture from snow or splashing rain.  Codes are just minimums.  Maybe add 6" to be safe.  And make sure you have good roof overhangs.  30"-36" or even more is recommended. You can plaster the earthbags with lime or cement plaster, or a combination of both.

Earth berming is highly recommended.  This helps insulate the foundation and encourages water to flow away from the building (if graded correctly).  A layer of 6 mil poly between the foundation and earth berm should do the job. Install a French drain if you get any significant amount of rainfall.  Install stucco flashing between the earth plaster and foundation.

Q: We are designing a home with a slipform foundation for the first level. The second level we are doing post and beam with straw bales. We have a few questions regarding one on the stone walls. In order for us to have post and beam for the straw bale I have to place the post on the rock wall. Is this possible by placing it directly on the wall with a mounting bracket? Or do I have to incorporate the wall with a builders tube with rebar coming out the side of the tube to secure it to the stone wall. Also, seeing that my second floor is straw bales my walls will be quite wide. So my 2nd question is: my footer; do I need one for such a wide wall, maybe 3 ft.? And if I do need a footing what would you suggest my thickness should be? 3rd, we were also reading about someone incorporating volcano rock into their stone walls for insulation purposes. Do you know about this? Do I place the volcanic rock in whole or would I have to crush it? How many parts of lava rock to a batch of mortar do I make?

A: The best is method is using the sono tubes as you described, especially with the added loads of a 2nd floor.

Footers: talk to your building code officials and/or a foundation company. Soils and building conditions vary widely around the country and even from site to site. This is an area you don't want to guess at.  In general though, you don't need full width footers for straw bale walls if you use a monolithic slab or if you start the bales on top of a wood floor frame. Do a Google search for pumice-crete. This is a great way to obtain an insulated foundation. The correct mix will be on the website.  Also, you can put pumice/scoria under a concrete slab or earth floor for added insulation.  Most builders use about 1/2"-3/4" size.  (Don't crush it.  Buy the right size to start with.)

Q: I'm in the planning stages of a 99 sq. ft. strawbale cottage. I'm using earthbags filled with gravel for the foundation (most likely without a rubble trench).To pin the first course of bales to the bags do I hammer the rebar straight through the bags into the ground? Or is there an alternative pinning method I'm not aware of? Also,I think I'm going with a post and beam frame. Since there's no foundation or slab to anchor the corner posts to, would you recommend digging the posts into the earth? If so,what would be the best way to do this if I end up doing a rubble trench, considering I would like to have the posts on the inside of the bales.

A: Yes, rebar can be used to pin bales to the earthbags.

Posts: Dig down 24" or so, wrap plastic around the bottom of the post and set it in concrete.  I would set the posts first before stacking the earthbag foundation (it's easier digging that way), but you can do it later.  Brace it from two directions to get it plumb.  Allow at least a week for the concrete to set up.  Leave a little extra length on top that you can trim off level.

Q: I am doing straw bale, post and beam, and interested in the earthbag foundation. I suspect I am talking about the stem walls/footings and bringing the bales far enough above ground to be clear of moisture. Here in Tom Green County in West Texas, moisture is seldom an issue, but when it IS, well, it is. Our soil is reddish brown, boot sucking when wet, and dries like a rock. Walking in it after a rain makes me feel like I should have a bolt in the side of my neck and a square forehead. Hope that tells you our soil type - used to have cotton fields here - no rocks at all. So, how far down should I dig? How many courses of bags? Should I use pea gravel for fill? How high should I get the bags? Should I fill the slab area with gravel up to the point of starting the floor pour? Should I pour right up to the bags, using them for forms? I am not in an area that requires inspections.

A: Soil science is a complicated field that goes far beyond simple descriptions, but here's one easy solution to help you out.  Talk to local foundation contractors (ex: ask for a quote) and learn what they do.  Ask them the questions.  You can also observe other houses being built, and/or talk to local building officials for general advice.  Do the same thing for earthbags (depth, height, etc.) as is being used for concrete foundations.  Yes, you can pour right next to the bags.  Yes, use pea gravel for infill and under the floor.  Also, I'd consider raising the building site to make sure water drains away from the building.

Q: Is it advisable to build with straw on a concrete basement structure? If not, what is recommended.

A: It's not clear what you are asking.  Most concrete foundations are only 8" or so wide, so I'm assuming there is a wood floor frame.  If this is the case, you can stack the bales on the wood floor.  (Foundations don't have to be as wide as the bales.) If you're trying to stack bales on concrete, you'll need to raise them up so moisture doesn't wick into the bales.  Most people build a 'toe-up' -- wood frame filled with perlite or other insulation or gravel.  Every straw bale book explains how to do this.

Q: I am planning on building a tire basement for a straw bale house, starting this summer. I am considering the use of 1 inch rock or gravel to fill the tires with instead of earth. Especially if this will be less labor intensive. My question is what would I use to prevent the rock from coming out of the tires when they are staggered. My tire walls will be about 8 feet tall.

A: You need clayey soil mixed with the gravel to hold it together.  The clay is like glue. I highly recommend using earthbags instead of tires and encourage you to do a sample of each.  Time yourself and compare the results.  But then again, maybe you're sitting on a pile of tires...

A: (Kelly) I suspect that you actually can use straight gravel packed in the tires. Because of the way the sidewalls of the tires close the gap in the central core of the tires, I don't think much will escape. I suggest that you lay out a few tires and try it out.

Q: My husband and I are planning to build a strawbale home in central Minnesota. We're thinking about using a pole barn package for the framing with a wood floor and straw as infill. However, we wonder how to set up the strawbales in this kind of structure. Can they sit directly on a wood floor? Would moisture be a problem? Or do you have other suggestions? We've also talked about a rubble trench.

A: Using a pole barn kit is a quick, easy and effective method of building with bales.  It will also help with the permitting process. The bales can be stacked 'flat' or 'on edge.'  Either way is okay, although on edge they take up less space and require fewer bales.  However, walls with bales on edge are a little unstable until everything is tied in and plastered.  Try a sample section of wall both ways and see what you prefer.  The R-value is the about the same either way, so that's not a factor.

You can stack directly on the floor, but the bales would be vulnerable to any plumbing leaks.  The safest way is to build a toe-up filled with perlite or vermiculite.  A good strawbale building book will explain how to do this.

Your choice of foundation will largely depend on your building codes.  Talk to your code officials and see what they want.  I like earthbag foundations for their low cost and low impact (filled with scoria,e etc), but they may not be accepted where you live.  They'll likely want a reinforced concrete foundation.  You can build a wood floor frame on top of this.  Whatever method you choose, be sure to build a frost protected foundation.  This reduces foundation costs and saves energy.  Also, insulate under the floor.

Q: Which book, CD or DVD do you suggest to learn how-to, step by step, using piers for strawbale - including how to make or mix fly ash, rammed dearth etc to reduce cement content in piers? I just read a book, Cabins, that suggests using Black Locust piers as the best, cheapest and most natural choice - that should work for strawbale instead of concrete piers, yes?

A: Pier footings are very simple and so you should be able to learn what is needed by reading on the Internet.  A few things to look for:
- build below frost depth
- use steel rebar correctly: size, spacing, 'chairs' to raise the rebar, method of overlapping and tying
- use correct concrete mix
- forms

Yes, black locust is good wood for piers.  Some use cypress, cedar, redwood, osage orange and other woods.

(Kelly) I'm pretty sure that this DVD covers the basics of a simple pier foundation for a strawbale structure:  The How-To Guide to Building With Straw Bales -- Load Bearing

Q: We're contemplating a small (240 SF) construction project using earthbags for the footers (strawbale above that). We don't need an entire bale of 1000 poly bags for that. Do you know of anyone in SW Colorado that offers smaller quantities of poly bags? Also, I'm having trouble figuring out what size bag to get, and how big a bag will be once it is filled with earth and laid in a running bond. If we have 18" wide bales, I figured we would need a bag wall that is at least 14" wide, if not a little wider.

A: You can buy small quantities of bags.  I suggest checking with local feed supply stores, farmers, etc.  They're used for many things now and so you should be able to locate used bags in perfect condition. 18" wide earthbags (sand bags, feed bags, etc.) are readily available.  This seems to be the most common size, that and 24" bags.  Dimensions vary an inch or so from one manufacturer to the next.  Try to find the 18" size, because that's the size of your bales.  Once filled and compacted, the bags will be about 16" wide.  Center the bales so they overhang an inch on each side and you'll be fine. Don't use 24" bags.  You don't want the bags sticking out.  And don't buy the small 14" bags or the foundation will be too narrow.

Also, I suggest using scoria (lava rock) in the bags to create an insulated foundation.  The scoria pits on the NM/CO border are not far from you.  This is what Kelly Hart used on his house in Crestone, CO.  Scoria is light and easy to work with, and even though it will cost a few dollars more than "fill dirt" you'll be thanking me later when you realize how much time you've saved (and money on long term energy costs).  While you're at it, buy a truck load and put the remainder under your floor.

Q: I've been doing some research into building a round pole frame/straw clay/straw bale hybrid studio and I'd love to use an earthbag foundation but have a number of questions about how to best make it work. First, I'm thinking the best way to set the posts would be to build the foundation from earthbags, gravel filled for first courses and road mix for upper courses, then pour a grade beam, and place the posts on this..I want the walls to be pretty plumb, do you recommend a grade beam or not? I live in the Pacific Northwest where the ground is wet! and I want to keep the bottoms of the posts above ground level, do you think bringing the lime plaster to the grade is a good idea here? Also do you have a preferred bag mix or external system for  providing foundation insulation?

A: (Kelly) I think that your idea of placing your poles on top of a grade beam poured above the earthbag foundation is good. This should adequately spread the forces on the poles over a larger area. Of course the pole frame structure will need to be well braced into place in its design, and I also suggest that the poles that bear on the grade beam be pinned firmly in place with pins embedded in the cement.

Lime plaster is pretty durable and should last quite awhile, even in contact with the earth at grade; cement stucco is even more durable and will last longer, especially if a mesh is used with it.

One of the easiest ways to insulate the bag foundation is by filling the bags with crushed volcanic stone (like scoria) if it is available. Ordinary gravel alone will provide some insulation because of all the trapped air space it provides. Commercial foam insulation board is a less natural solution.

Q: I am thinking of using earthbags filled with pumice or scoria for insulation to raise my bales 12" above grade.

A: (Owen) 6" above grade is standard.  This is what my drawing shows.  But it would be easy to add one more course of earthbags if you have a lot of rain or show.

I feel that for a non-load bearing structure this will be adequate support.

What I'm describing will also work for load bearing design. 

I will set the posts on small concrete footers inside the walls. I'm hoping that this option will allow me to avoid pouring a grade beam over the bags (it seems unnecessary and expensive for the bale walls anyway.

Yes, you can also use posts.  Posts allow adding more doors and windows.

My questions are related to the connection details between the earthbags and bales. From previous projects I've worked on I've always learned that it is a bad idea to "pin" the bales to the foundation with any metal and that there should be a vapor barrier between the foundation and the bales.

For small structures, I would just set the bales on the earthbags.  You could pin them with bamboo pins, use stucco mesh, etc.  No need for a vapor barrier between the scoria and the bales.  I put the vapor barrier on the outside of the bags and fasten to the top of the bags.   Also, I put a moisture barrier (6 mil plastic) under the floor.

I've also always used a pressure treated "curb" or "toe up" on top of concrete pad or stem walls with gravel in between so moisture doesn't puddle.

Pressure treated wood is a toxic material.  In California now you need to dispose of it in a special landfill for toxic wastes.  It's bad for the earth and it's not needed.

What do you think is the best way to tie the bales to the earthbags?

Besides the methods described above, you could tie them with baling twine.  Plan ahead and lay pieces of twine between courses of bags.  For larger structures, use poly strapping that goes under the earthbags and up and over the bond beam.  You can buy tools that cinch it down.  This is the same strapping used to secure loads on pallets.  Chris Magwood had an excellent article in The Last Straw a few years ago.  He tested various systems and gives his advice on the best brands.

Should I build a curb out of wood and place it on top of the bags, connected with a j-bolt or threaded rod driven through the bags? Should it float on the bags and let gravity hold it in place? If a vapor barrier is recommended, should it be on the top of the bags or should I carry it over to protect the inside of the foundation as well from moisture migrating in through the pumice?

I don't think moisture will migrate up through scoria.  Not sure about pumice.

I plan to use a pumice sub-floor with an earthen finished floor, topping out at 6" above grade.

Another important pont is to create at least two complete (continuous) courses of earthbags for the foundation that are connected with two strands of 4-point barbed wire.  This ties everything together.

As always the goal is to reduce the embodied energy of the materials, use what is natural and local and cheap and optimize the integration of different systems. It's challenging but rewarding to think outside of the box of traditional building and I can't wait to keep working on more projects like this.

This is exactly what I say and try to do.  Keep up the good work and please report back with photos so we can share your experiences with others.

Q: My husband and I are interested in buying property in the Colorado mountains. The specific property we are looking at has a flat piece of land with a hill that would be to the back of the house. Could this pose a problem with snow melt and drainage when considering a strawbale build?

A: Yes. This could be a problem no matter what way you build. I recommend building one or even two drainage ditches above the building site to divert water around the house. In addition, cut far enough into the hillside so you can slope the ground next to the house to drain water away. And you'll probably want a French drain, also.

Q: I'm about to build a jumbo straw bale home in Poland. I would like to use tires for foundations. The soil frost deep is 1m+ 30cm over ground level. Can You suggest me what is better for foundations strength as a filler inside tires? Should I use only earth or I can add cement or clay or gravel rock? My region is full of granite gravel, slate, river stones. What is better- stronger ?

A: Sand/gravel mix in the tires is best. That way it's totally waterproof even in a flood and will never settle. Research Shallow Frost-protected Foundations. There is a free design guide is on the Internet. This is 100% proven and an effective way to save time and money. You don't have to excavate to frost depth using this method.

In my opinion jumbo bales are overkill. I recommend 24" thick bales. Jumbo bales waste a lot of space, and require excessive foundations and extra roof materials. Heat will escape through windows and doors and gaps (the path of least resistance) and so 24" thick walls are plenty.

Disclaimer Of Liability And Warranty
I specifically disclaim any warranty, either expressed or implied, concerning the information on these pages. Neither I nor any of the advisor/consultants associated with this site will have liability for loss, damage, or injury, resulting from the use of any information found on this, or any other page at this site. Kelly Hart, Hartworks, Inc.

 

Home       Site Map        STORE

For Email contact go to About Us
Established in 2001, GreenHomeBuilding.com is primarily a labor of love. Kelly, and the GreenHomeBuilding team of experts, have answered thousands of questions for readers over the years, and we continue to publish up-to-date information about increasingly important sustainable architecture. If you feel moved to assist us in this work, your kind donation would be much appreciated; this can be easily done through our PayPal account:
Custom Search

VISIT OUR OTHER WEBSITES:

  [Solar Car]      [Earthbag Building]     [Dream Green Homes]