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