Image Credit: Peter Lenardon
More Musings of an Energy Nerd
Do you want to build your home out of natural materials? If so, you can build your walls with adobe, cob, cordwood, rammed earth, or wattle-and-daub. Although all of these walls have a long history, their thermal performance is poor. If you want a well-insulated wall, one natural material is the clear winner: straw bales.
A 23-inch-thick straw-bale wall has an R-value of about R-33. Moreover, since virtually all straw-bale walls are plastered on both sides, these walls are relatively airtight.
If you are an owner/builder with lots of time on your hands, and you want to build your walls out of natural materials, straw-bale construction makes a lot of sense.
Nebraska roots
The first straw-bale houses were built in Nebraska in the late 19th century. (I’m proud of this Nebraska connection. Both of my maternal great-grandparents settled in Enders, Nebraska, in the 19th century, while my paternal grandmother grew up in a sod house in South Dakota.) According to Bruce King, the author of Design of Straw Bale Buildings, the oldest standing straw-bale building in the world is the 108-year-old Burke house in Alliance, Nebraska.
Many people confuse straw and hay. Hay is dried grass — the same stuff that your lawnmower spits out. Hay is used to feed cattle, horses, and other farm animals. Straw, on the other hand, consists of the dried stems of grain plants — for example, wheat, oats, barley, rye, or rice. Since hay is more vulnerable to rot and mold than straw, hay bales should not be used for wall construction.
Straw is often sold for use as…
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21 Comments
mudsill
insulation where the sill meets the concrete?
Response to Shane Claflin
Shane,
Look at the photos.
Image 3 shows 2x4 bottom plates installed on a slab-on-grade foundation; rigid foam has been installed between the 2x4s to provide insulation.
Image 4 shows the use of Perlite for the same purpose.
If you are worried about air sealing (rather than insulation), use any conventional sill-seal product between the 2x4 toe-up plates and the concrete. Sill seal can be supplemented with caulk.
Many thanks
Thank you very much Martin for this very interesting post.
I'm from Sonora Mexico (not too far from Tucson) my parents just bought a nice lot where we're planning to build a small country house and we're looking over the possibility of wheat straw-bale and adobe as our main building materials.
I really enjoy your blog and this entry on particular was trully motivational. I work as an environmental consultant/building-energy-modeller and I don't really have that much building experience so I hope it does not bother you much if I directly ask for some book/web-site recommendations.
Finally, I'm also a devoted Passivhaus fan (I got the opportunity of touring the German houses on the Solar Decathlon Europe competition and I was delighted), so, is the concept of an adobe/straw-bale Passivhaus in the Sonoran desert just plain crazy?
Best regards!
Response to Mauro Contreras
Mauro,
Thanks for the kind words.
If you are interested in more information, you should check out the books and Web sites listed in the "More Information" box above.
As to whether it's possible to build a straw-bale Passivhaus in the Sonoran desert: I don't know. Hot-climate Passivhaus details are still being developed -- cold-climate Passivhaus examples are far more common -- and I'm not a certified Passivhaus consultant. You'll have to hire a consultant to get a full answer to your question.
Corrections and Clarifications
While addressing the preference for infill straw bale (SB) in wet climates and the fact that most codes no not allow load-bearing SB walls, much of the description of construction methods and techniques is relevant only to structural SB construction in dry climates. For instance, you say "at the top of the wall, builders usually install a wide wooden top plate or a concrete bond beam." This is necessary only for load-bearing walls. A timber frame already has a rafter plate.
For SB infill, bales can be laid either flat or on edge. There are advantages and disadvantages to each, but the overall R-value is about the same. To cut a bale in half, your instructions are backwards: each half bale is first tied together tightly with baling twine or plastic pallet binders and then the bale’s strings are cut. Otherwise the compression and density is lost.
The easiest foundation option for infill SB is not "the thickened-edge concrete slab on grade", which requires a toe-up and capillary break, but a conventional foundation to support the timber frame, with a wooden floor deck extending out beyond the foundation to support the exterior SB thermal envelope. This puts the bales on wood rather than concrete and raises them well above the exterior grade to avoid the splashback that is inevitable at this most problematic of areas.
"It’s a good idea to leave rebar pins sticking out at the perimeter of the foundation …many straw-bale builders have concluded that staking bales is unnecessary." Not only unnecessary but dangerous in a wet and cold climate, as this will almost certainly result in condensation on the steel in the middle of the bales.
I believe you've misapplied Straube's quote to window sills, since he's addressing cladding drainage systems. “The fundamental requirement [for window sills (sic)] is that there be an outer layer of cladding, with a drainage gap below and a waterproof layer below that,” says Straube. “You also need an opening to let the water out. You can use a Benjamin Obdyke product to create the drainage gap, or in a dry climate, you can use two layers of building paper. You also need some type of flashing with a drip edge and an exit slot for the water.” In fact, the exterior plaster is keyed directly into the bales and must be in contact with the straw to act as fire-proofing, rodent-proofing and moisture barrier. No housewrap or drainage mat should be used between straw and plaster. Flashing is needed wherever the plaster meets another material, and window sills need to be sloped and include a drip groove. Wooden cladding requires only a slight gap outside of the plastered bales created by the furring strips, which should not be screwed to an earthen plaster but attached in a more secure manner – either to framing or embedded stakes or tied through the bales .
"While some straw-bale builders apply plaster directly to the straw bales, Straube recommends attaching chicken wire or wire lath to the bales before plastering." Again, this may work in a very dry climate, but everywhere else it's best to avoid metal on the exterior.
"Laura Bartels advises against the use of chicken wire." For transitions over framing or other members, burlap soaked in clay slip works well. There are also fiberglass mesh products. Chicken wire is sometimes used under the interior (warm side) plaster to create curves at window and door returns.
As for other, more reliable, resources, I would suggest looking for a soon-to-be-published book by Jacob "Deva" Racusin (the instructor for the Natural Building Intensive and Straw Bale classes at Yestermorrow) on the Chelsea Green imprint. Though I am not a straw-bale builder, I edited several chapter of Deva's book and have consulted with him on his projects. Deva is also writing up the results of energy auditing and blower door testing on a number of northeastern SB homes. The primary challenge for SB builders in a cold climate is air-sealing details. Wrapping a timber frame, particularly one with knee braces, presents many transitions which require very careful detailing. It's at those transitions and between walls and ceilings where air leaks and consequent moisture accumulation is often found.
Mauro,
Straw bale
Mauro,
Straw bale Passivhaeuser are totally feasible, even in the Sonoran desert. There are a number of straw and rammed earth Passivhaeuser - and even a SIPs-esque panelized system w/ insulation and panels made from straw (like agriboard).
You shouldn't need triple pane, nor too much insulation if designed properly. Detailing w/ rammed earth or adobe will be a little trickier, as you'll mostly likely need to add insulation (so in effect, a kind of sandwich wall as done by the SIREWALL folks).
My old boss worked at TU Darmstadt where the solar decathlon projects were built. Some really great resources at that uni. In terms of design, the 2007 version was much nicer.
Clarifications
I have been building strawbale buildings for 13 years on the West Coast and mostly in Vermont. I applaud Martin for covering this great building method, but I am afraid I would not recommend this introduction. I would agree with most of Roberts corrections and begin by saying that any into to SB that does not clarify a climate is not worth reading. The details for Vermont vs. Arizona are very different. On top of this there were some major omissions such as the discussion of material to use for plastering, as well as the good hat and good pair of shoes rule that dictates a min. 18" from grade separation for a SB wall and the same overhang if not more depending on the exposure. There are some universal best practices, but using cement stucco plaster in Vermont for example, is a recipe for disaster.
And as much as I respect Straub's work, I would not refer to him on SB construction practices. I find that he generally has a decent understanding of the building science of SB's but it seems that he hasn't been around many projects.
My recommendation to anyone looking to build with Strawbales would be to find local builder swith SB experience and look at their work, watch a project being built, and talk to other builders from the area. A strawbale "expert" in Oregon could cost you 15 grand in plaster mistakes on a project in Maine. I have seen it happen.
The most exciting development in SB building for me is the development of high performance detailing that has developed over the past several years in cold climates. Strawbale structures have achieved PassiveHouse standards in other countries. Achieving PH air tightness will be quite difficult with the plaster method described in this article. There are many ways to build with SB that involve stud framing and proven air barriers that will make it much easier to achieve the PH standard, and more affordable.
It would be great to see another article on SB focusing on cold climates, with a little more research. There is a professional trade network called Natural Builders Northeast, (www.nbne.org) that is a great resource.
Response to Ben Graham and Robert Riversong
Ben,
You wrote, "The details for Vermont vs. Arizona are very different." I certainly agree with you that climate matters -- I have been emphasizing that point for many years. (Of course, the same could be said about conventional construction details. Even for a stick-built home, details in Vermont are quite different from those in Arizona.) Thanks for reminding readers that the same principle applies to straw-bale construction.
I'm a little surprised at one of your points: "There were some major omissions such as ... the good hat and good pair of shoes rule that dictates a min. 18 in. from grade separation for a SB wall and the same overhang if not more depending on the exposure." In fact, I mentioned those details when I wrote, "It’s vitally important to keep liquid water out of a straw-bale wall. To accomplish this goal, ... the foundation must be raised above the surrounding grade, ... [and] the roof must have generous overhangs on all sides." I'm not sure what I could have written that was stronger than the phrase "vitally important."
This introductory article did not allow a full discussion of plaster specification, which is why I directed readers to the sources provided in the "More Information" box. Thanks very much for referring readers to Natural Builders Northeast.
Response to Mike Eliason
Thanks for the feedback!
The houses from TU Darmstadt were awesome, in the competition in Madrid last summer my personal favorite was the house from HTW Berlin.
Cheers!
Ok call me crazy!
I love this! I have herd of it but never actually gave it much attention. I need to do this at least in once in my life.
Here is where everyone will hate me but, what about 2 inches of spray foam on the exterior, than coat it with stucco/mud/whatever??? This will act as a vapor retardant and with the stucco over top of it I think the straw will be plenty protected.
I get this is a total natrually green thing, BUT a little nudge from some spray foam would make this rock!
I am totally on board with this, I just do not know if we have a big straw inventory here in NJ (there are still lenty of farms in the area plus with PA so close...)
I do not know what do you guys think?
Thanks for the cool article!
Good intro
I'm also a builder with bale experience in the Southwest. I applaud your condensed but fairly thorough introduction. The one error that stood out for me is that we have installed bales on edge in order to save wall width and it works fine, with some issues having the strings out.
I'm a big fan of load bearing and wish to promote it. It's much easier and more "elegant" way to build. I also build adobe and they both seem much more suited and actually stronger in the load bearing form. Still, I agree, the post and beam approach is often worth all the extra hassle for the convenience in the building process. Tradeoffs.
About pinning. Perhaps the trends have changed, but we found exterior pinning with bamboo (or...rebar!) to much stronger and more workable than interior pinning. Whether pinning is necessary is debatable, but it does rigidify the walls before plastering.
While it's apparent that Ben and Robert have very solid experience and/or information and well developed opinions, I did not find the faults they did, perhaps because of the region I've worked in. In any case, you did cover the critical "hat and boots" well, I thought. And I'd like to know more about why stucco is a recipe for disaster in the Northwest. I suppose it's just too wet too often? Snow against the walls? Also, is there documentation about stucco/chicken wire actually attracting moisture or is that an idea going around? I've missed that while living out of country.recently.
Also, about flashing: Certainly all the straw should be plastered, but flashing can still be applied. I like to detail windows by smooth mudding around them up to the finish plane during prep, then, once dry, applying an adhesive flexible membrane flashing right to the mud around the window. It sticks and stays on long enough to finish, anyway, Expanded metal lathe goes over that for the plaster. This has worked well in the SW, allowing innies without moisture issues. It does require careful attention to the detail.
Really, that's the final point about Straw I'd like to make. It's been promoted in the past as a do-it-yourself, easy-up kind of approach, but I find that deceptive. Walls can go up quickly , but straw is very detail oriented and can be a recipe for disaster if you or your builder are not well aware of the need for intelligent attention to the details. And lots of them. Not a piece of cake, not cheap, but a great material when done right.
Look forward to more articles about advances and differences in detailing SB for different climates.
Frank Bovio
Frank,
I'm glad to hear that you are enthusiastic about straw-bale walls.
I don't recommend installing spray foam on the exterior of a straw-bale wall, because the foam will reduce drying to the exterior. In the case of a flashing failure that allows moisture to enter the wall, you want the wall to dry quickly in both directions.
Response to Steven Lewis
Steven,
Thanks for the useful tips and kind words.
I think it's fair to say that straw-bale builders are a fairly opinionated lot. Different techniques have arisen in different areas; some of these differences are due to differences in climate, while other differences simply reflect local practice that has solidified into dogma over the years.
Writing about straw-bale construction is difficult, because a few builders are likely to contradict almost any statement. In any case, thanks for sharing your techniques.
Structural issues
Thanks for the article. I've been reading the links as well.
That Building Science article is helpful, but it leaves me with a few questions about the structural aspects for load-bearing walls:
If 95-99% of the load is carried by the plaster skin, why does the wall need to settle or be compressed before plastering?
What kind of plaster details are needed to transfer the load at the sill, and around window openings? I understand the idea of a raised footer, but it seems like the stucco/plaster has to extend to the slab surface, forming a capillary wick (?).
Response to Kendall Willets
Kenneth,
The reason that a Nebraska-style wall needs to be pre-compressed is that if a newly-laid wall is plastered immediately, the settling (which is driven by the weight of the upper courses of bales plus the roof load) will pull the plaster down with it and cause the plaster to crack. After all, the plaster is intimately bonded to the straw bales.
The plaster is an important structural component of a finished wall, if it is installed after compression. However, if you plaster too early, the thin plaster is not able to resist the forces generated by settlement, and the plaster cracks.
Concerning your other questions: I invite GBA readers who are experienced at straw-bale construction to reply.
Straw bale housing
When you include rammed earth as an unlikely alternative to strawbale, you may not be aware of a design system called Sire-Wall; which is a system incorporating insulation within the the envelope of rammed earth. The system is, albeit quite costly, a very energy-efficient and long lasting alternative to strawbale construction. Check them out.
regards-
smalld
SIREWALL also claims their
SIREWALL also claims their walls are the 'performance equivalent' of an R-50 wood-framed wall...
More on Sirewall
Mike,
Thanks for the warning about performance exaggerations. The Sirewall system appears to use 4 inches of "rigid insulation." I can't find anywhere on the company's website where the insulation product is described (that's a danger sign right there). But if it's XPS, that would be about R-20 insulation -- plus some dirt on either side.
Good article
I think straw bales are a great option. But the plaster part is completely unnecessary. The only reason it was invented was out of necessity. Think about this. Any insulation you put into a wall is going to need to stay dry. If it doesn't, the wall will have to be opened up and fixed. So no matter what you use, it is assumed that you are going to know how to make a cavity air tight, waterproof, and keep out critters. SO it doesn't matter what you put in there as long as it has a good R value. Too much fear mongering going on. People are too worried and rely on "expert" advice too much. Use your own mind people. They tell you not to use grass. BUT YOU CAN. It isnt going to rot because it will be dry. And its easier to make then straw bales. But the experts wont tell you that because they cant make any money off it.
Joe, what do you mean?
What do you mean that the plaster is unnecessary? How else will the bales stay dry?
To Joe M,
Something has to stop air movement and still allow vapor. Plaster does this well.
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