Light Straw-clay
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Q: My question is about Light Straw-Clay. I am building a timber structure and I am going to do Light Straw-Clay infill. I attended one of 2 of your workshops and have a question as to how long I should let the clay slip straw sit around before tamping into forms. And should I use green or dried branch supports inside the walls?

A: The slip-coated straw can either be packed immediately into the wall cavity, or else left to set up for up to 24 hours. This supposedly helps the clay soak into the straw so the wall will hold together better, but I have not noticed that it makes a huge difference. In any case you want to keep the coated straw well covered so that it doesn't dry out, or you will need to moisten it again before packing it into the wall.

As far as the horizontal pins go, I have used both green and dry, both with good results. Probably dry is better so you introduce less moisture and movement into the wall as it dries. On the other hand, you want the sticks to be quite flexible, not brittle, or they may break when you tamp straw-clay over them.

Q: We are thinking about building a post and beam straw bale house and we are looking at options for the interior walls. Light straw-clay sounds like it could be a good option. Could it be built on a traditionally framed floor?

A: Yes, certainly. Light straw-clay can be built as a conventionally-framed stud wall with straw-clay infill. It is a bit heavier than a standard wall with fiberglass insulation and sheetrock, but not terribly so.

Drywall is such a pain to work with and we want something that will give a similar look to the exterior walls. Would it work OK for bathroom walls? I am assuming you couldn't have plumbing or electric inside the wall.

Sure you can put plumbing and electricity through a light straw-clay wall. There are some special considerations, but it can be done easily enough. As far as bathrooms go, I would recommend a lime plaster to protect the straw from moisture. I have a wet sauna with lime-plaster over light straw walls and there have been no problems so far with moisture.

Q: I'm working with Builders Without Borders on housing designs for Afghanistan. One possibility is building hemispheric domes out of straw-clay blocks. One or more coats of straw-clay would help tie it together. Lime plaster seems like the best finish. I know you have some experience in this area, so maybe you could answer a few questions. Does this sound like a feasible idea? Do you know the compressive strength of straw-clay? Has anyone done this before? Any ideas? By the way, I really enjoy your website and have spent a number of hours reading your articles. You have what most websites lack - usable information. Keep it up.

A: (Kelly) I certainly believe that you could make a dome out of straw-clay blocks and that it would be inherently well insulated; the question is how long would it last? Straw-clay is vulnerable to moisture penetration, and domes are especially open to all of the weather. Straw-clay is typically used as infill in timber-framed structures with substantial eaves to protect the wall from rain. This means that some way of sealing the dome would be essential. I'm afraid that lime plaster would not do the trick, since it is quite breathable and does absorb moisture readily. The other concern is that even if you managed to seal the exterior completely, then you have the potential problem of condensation forming on the inside and not being able to escape through the wall. All in all, I would look to some other method of building that eliminates these problems. Domes are great because they are so resource efficient...you just need to build them with materials that can handle the weather, naturally, such as lightweight cement, stabilized adobe or cob, earthbags filled with mineral material (is there volcanic rock in Afghanistan?) and covered with a stabilized plaster.

Comment: I was surprised at how little clay was involved in this light clay sample. This sample was very light and felt very strong. Of course straw is the best for light clay because the long fibers interweave, and then the seeds sprout and tie it together even more. Only completely useless where termites prevail.

Q: I want to build a clay/straw house. I wanted to know how tall I can go with it. Can I do a tall 2 story, or 3. Or is that an overload on it.

A: I'm wondering what you mean by a clay/straw house. Are you referring to the German technique known as light straw-clay or slipstraw in the U.S.? If so, it is certainly possible to build a multi-story house using that technique. The straw/clay mixture is used as an insulating infill within a wooden framework. You just need to make sure that your beams etc. are strong enough to support the weight of the walls on the higher stories, since light straw clay is considerably heavier than standard U.S. lightweight construction.

Yeah slip straw is what we are wanting to do. We actually have been looking at using grain bins for the outer shell because we live in farm land and have access to used grain bins for very cheap. Sort of a grain bin castle design :) Well I have been reading about light straw clay for a bit now and really want to do it on the inside of the bin for the look and the insulation value. But from what I read I can't build it against the inside of the steel because it will not dry...?...or maybe its that it will dry to long and rot? I'm not exactly sure, I just know I am reading it needs to be open on both sides. So I am trying to figure out a way to do it with slip straw inside still. Or maybe its that the walls have to be breathable, and if its on the steel wall it wont be. So I don't know if I can figure out a way to dry it, or the heat of the steel itself in the summer after we would erect the inner wall would help dry it? Or really how we could do it. Maybe I'd have to make straw bricks ahead of time, and stack them so they have a gap between them and the steel. I don't really know at this point how to make it work.

In my opinion, it is never a good idea to have a straw-rich material (e.g. straw bale or slipstraw) in direct contact with a non-breathable surface, especially one (like exterior metal) which will get cold and become a condensation surface for moist air moving through the wall. This is a recipe for potentially severe water damage inside the wall over time.

What would make me feel comfortable is if there were an air space between the slipstraw or bale walls and the metal, vented at the bottom and the top (carefully of course to avoid rain penetration) so that moist air could make its way out of the building rather than condensing in contact with the wall. This would be relatively easy to achieve with bales; harder with slipstraw, as you would need to devise some sort of forming system that was rigid enough to hold the air space while the slipstraw was being tamped in place, but also breathable. Perhaps you could do it with hardware cloth. You could attach vertical 2x2s to the interior of the silo, spaced fairly close together (12" ?), stretch and staple 1/4" hardware cloth to the inside of those, use the 2x2s to anchor your interior framing (to which you could attach a bendable form made of thin plywood, for example) and then fill the cavity between with your slipstraw. I think this would probably work, although it would be quite a project.

I don't recommend the straw/clay block idea because I think it would be hard to get enough insulation that way. Even with very light slipstraw (not much clay and not much compression), you are only getting about R-1.5 per inch of thickness... perhaps slightly more. To make a mixture that will hold together in block form you need to add a lot more clay, and your R value probably declines to close to R-1 per inch.

So I could do something like have bales walled up but just spaced off the inside wall of the bin? Am I understanding correct? So there is a gap between wall and straw.

Yes, I think you could stack up the bales, leaving a narrow air vent between the bales and the silo wall, which would be vented to the exterior of the building. Then of course you would plaster over the interior surface of the bales.

Q: I am curious as to using corn stalks as insulation. I was walking through a part of a corn field just picked and I was looking at a stalk and wondering if it could be used in slip straw so I grabbed it to take a closer look and it seems tough like straw but then a core that is like Styrofoam, so I thought hmm that stuff seems like it would be really good. So that's my question: you guys ever try anything with corn stalks?

A: I don't know whether corn stalks would work well in a light-clay application, although I suspect not unless they were chipped up. I think that full-length stalks would be too rigid and elastic to bond together well. If you were to run them through a chipper/shredder and then let them dry, they might work fine, although I don't know how durable they would be. If you try it, let me know how it goes.

Q: I want to know if I could do anything to speed up the drying time, like if I did a thick light straw clay. I thought of running a few pipes through it with holes to blow in hot air or something like that.

A: The best that I know to do is to make sure you have really good ventilation. Electric fans can help.

Q: Can you expect good results by packing a light straw clay mix into the six inch gap left between the two six inch CEB walls of an 18 inch double wall structure? Will lime stabilization added to the clay slip improve the insect and decomposition resistance? Does an expansive bentonite clay have any problems that you know of for a light straw clay mix? I have easy access to an almost pure supply of the stuff. Experiments with both the lime and the bentonite seem successful, and a very light panel is the result.

A: If you pack a damp straw-clay mixture between 2 layers of CEBs, I would be concerned that you could get a lot of decomposition in the light straw clay before it dries. I always use wooden or plywood forms that are removed as soon as they have been filled. If you leave the forms on for even a day or two before stripping them it is common to see mold growing at the edges of the light straw clay just inside the forms. Your CEBs will be more "breathable" than plywood, but I doubt that they would let moisture escape quickly enough to eliminate the possibility of rotting in the infill. One way to address this would be to coat the straw with slip and let it dry completely in the sun before packing it into your wall.

I have not used pure bentonite for building, but I would think that if there is one building technique where its expansiveness would not cause big problems, it would be light straw clay. If you have made some test samples and they look good, I would trust your testing process.

As far as the addition of lime goes, I don't know whether it would increase resistance to either insects or decomposition. My guess is that it might, but I wonder whether any increased resistance would be worth the additional expense, risk, and embodied energy involved. I have also heard (not confirmed) that the causticity of lime can break down cellulose over time.

Q: I read an article about light straw clay and was wondering if this has ever been used as an insulation layer for cob buildings? If so, how is light straw clay layer attached to the exterior of the cob wall?

A: I can't remember ever seeing or hearing about LSC being used to insulate a cob wall. Part of the reason is probably that, compared to more lightweight materials, LSC is only a moderately good insulator per inch of thickness. For example, to get the equivalent of R=12 insulation (which is what you would get from a 3.5" thick batt of fiberglass, cotton, or wool), you would need between 6" and 8" of LSC. If you add that to the exterior of a load bearing cob wall which is already 16" thick, you end up with a 2' thick wall. Not only would you have the problem of attaching the LSC securely to the cob, as you mention, but also you would need to build a form for the outside of the LSC which would mirror the shape of the cob wall. One of the things people most appreciate about cob is its sculptural nature - it is so easy to make curves, arches, etc. But how to match that with a formwork rigid enough to withstand the force of LSC being packed inside? It would be much easier to make the walls straight. So then you end up with a straight wall 2' thick. For much less work you could have built a straw bale wall, and ended up with much better insulation. I'm sure there is a way to solve the technical problems of how to insulate a cob wall with LSC; I'm just not sure that it would be worth the effort. If your goal is a sculptural wall with good insulation, I would lean towards a straw bale wall with really thick earthen plaster.

Q: I am interested in building with straw-clay slip walls but have a 2 story portion. What are the drawbacks, if any, to height?

A: The only drawbacks of building straw-clay 2 stories high are the same as building anything 2 stories high. You'll have to come up with a good system for lifting so much material high into the air. Scaffolding is indispensable. Also, because straw-clay walls are considerably heavier than conventional framed-and-insulated walls, you can run into some minor engineering complications. If all of your second story walls are directly above first-story walls, that will lessen the need for gigantic beams and so on. Good luck!

Q: I have a house in Southeast Oklahoma which is traditionally built with siding. I want to have a finished look of earthen plaster. Could a person use a light straw clay to cover siding or even pull siding and use the technique of LSC then plaster over it? I need some way to change the aesthetics of this house to meet eventual houses I plan to build that will surround it. I plan to use cob, earthbag and strawbale.

A: If you want the look of earthen plaster on a conventional building, there are (at least) two ways you could approach it. One would be to remove the siding and the interior sheetrock, fill the spaces between studs with light straw-clay, and then plaster over both the interior and exterior surfaces. Keep in mind that straw-clay is not as good an insulator per inch of thickness as conventional insulation materials. Depending on your mixture and your technique, you can expect to get about R=1.5 per inch of thickness. So to get the insulation you need in Oklahoma (I'm not sure how cold your winters get) you may need to build out the thickness of your wall framing to get a thicker straw-clay wall.

If you decide to leave your existing insulation and siding in place, then you can probably apply an earthen plaster over the siding. You will most likely need some sort of lath to get the plaster to adhere well. I don't recommend using metal lath or chicken wire, because the expansion and contraction of the metal will tend to crack the plaster. Some effective options could include: plastic deer fencing stapled to the siding; natural fiber burlap glued to the siding with a mixture of clay and flour paste; reed mat screwed to the siding; or lots and lots of nails pounded partway in. The best lathing technique will depend on the condition of your siding and how thick you want to build out the plaster; it may even vary in different parts of the house. Over that, you can apply a base coat made of clay soil, water, and lots of chopped straw. And over that, if you want, you can apply an earthen finish plaster.

Don't forget that earthen plaster is one of the least durable exterior finishes available. If you get much wind-driven rain onto the walls, or if water from the roof drips down the walls, it will quickly damage the plaster. Because your house has not been designed with earthen plasters in mind, there is a good chance that the roof overhangs are too short. You may need to extend them or do something else to keep the weather off your walls. You will certainly want good gutters if you don't have them already.

Q: Could you tell me if there is a concern with termites in slip straw applications?

A: I don't have much direct experience with termites, but I imagine that if you have a bad enough infestation, they could eat anything made of cellulose, which includes straw. Slipstraw is almost always used as an infill within a wooden frame, and I believe that the wood itself would be more attractive to termites than the clay-impregnated straw. By the time termites get into the slipstraw, my guess is that you'd already have a problem with your wooden structure.

Q: We are building an off grid 2 story home on piers, post and beam style. I think I would like to use slipstraw for insulation between the beams. Will I be able to use cedar boards on the outside walls or would I have to use some sort of earthen plaster?

A: Typically, slipstraw (also called light straw-clay) walls are made by filling the cavity in a framed wall between removable forms. The forms should be removed immediately to allow the straw-clay to dry from both sides. I have seen that, depending on the weather, mold can start to grow at the junction between straw-clay and wood within a couple of days if the forms are not removed. After the straw-clay infill is completely dry (several weeks to months), it is usually plastered, but could also be protected on one or both sides with wooden siding. The detail I have seen for this is to fur out the framing with strips of one-by lumber, then attach the siding horizontally to the furring strips. This allows for air flow between the straw-clay and the siding, so that any moisture in the wall can still evaporate out rather than condensing between the straw-clay and the wood where it could lead to mold growth. Also you should detail your siding in such a way as to prevent water running down the outside of the wall from getting between the boards, just as you would with any siding project. Also keep in mind when framing your building that slipstraw is many times heavier than conventional insulation such as fiberglass. You will need to take the extra weight into consideration when engineering your floor girders, beams, etc.

Q: We have a 1900's (dilapidated) house in the North of The Netherlands. When first moving in a year ago, we stripped the gypsum boards, which were soaked and very moldy. Despite the bone chilling cold last winter (it never got warmer than 13 °C inside) we love the "breathability" of the house. However, we need to insulate before this coming winter, and we're very interested in the "clay straw" techniques. Question: our roof is sloped and constructed (outside-inside) of clay roof tiles on a wooden frame, with wooden boards underneath. Would it be possible to use the clay straw as an insulation medium there? And would a layer of 4 inches "do" anything? Also, we're trying to figure out a way to go about it. Personally I've been thinking about constructing wooden frames with chicken wire and filling those with the material, leave them to pre-dry in the barn for a day or two, and then attaching them to the inside roof. Would that be feasible?

A: Much as I love straw-clay, I can't claim that it's a really good insulation for a cold climate like yours. 4 inches of light straw-clay might give you about the equivalent of R-6 or so. For roof insulation in a cold winter climate I would recommend closer to R-30, with R-20 being a minimum. If I were you I would be looking into wool or cotton as roof insulation, and also (if you only have a space 4" thick) planning to increase the thickness of the insulation cavity by adding additional framing.

That being said, even 4" of straw-clay would be a lot better than no insulation at all. And if you can make the house tight, so that you are not losing all of your hot air to the outside through cracks and windows etc., you will be able to keep it warm if you're willing to burn enough wood or electricity or whatever it is you're using as a heat source. However, heating a poorly-insulated space is just not ecologically sound in this age of climate change.

If you do decide to use straw-clay, you should know that it takes a while to dry out. In Germany, the straw-clay experts recommend allowing a week for every inch of thickness for a straw-clay wall to dry, and that assumes that moisture can escape from both sides of the wall. So pre-drying for a couple of days as you suggested would not be enough to make much difference. I always recommend doing straw-clay projects at the driest and warmest time of the year, and making sure you have lots of air flow (open windows with fans) if you are doing it inside a structure. Also, adding boric acid to the clay slip as a mold-retardant is a good idea when working inside, or in less-than ideal drying conditions.

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