Questions and Answers involving Wood
Kelly Hart is your host at greenhomebuilding.com, and has built his own home using a hybrid earthbag/papercrete technique, which can be seen on the Earthbag page. He has adapted the concepts popularized by Nader Khalili and his "superadobe" building, by filling the bags primarily with crushed volcanic rock. This creates insulated walls that are similar to strawbale, except that they are completely impervious to damage from moisture, insects or rodents. The earthbags are plastered both inside and outside with papercrete. Kelly has produced a video, titled Building with Bags: How We Made Our Experimental Earthbag/Papercrete House, which chronicles the adventure of building this house, and shows other earthbag houses as well. Another video program that he produced is A Sampler of Alternative Homes: Approaching Sustainable Architecture, which explores a whole range of building concepts that are earth friendly. One of the homes shown in this video is a hybrid strawbale/wood framed home. Kelly spent many years as a professional remodeler, and is available to answer questions about what he has done, or consult about other hybrid projects.
Brad Hankins has been designing and building unique projects for 25 years. He is hands-on with all the trades, believing that if he can't do the task himself, how can he expect varied and quality work from his crew and subcontractors? He studied engineering at the University of Washington (supported by carpentry work) until he figured out that engineers are schooled to look for problems while architects are schooled to look for solutions. After undoing the technical thought process by getting a Bachelor of Fine Arts from Washington State University, he received a Masters in Architecture from the University of Colorado at Denver. Brad is committed to reuse and recycling, designing small efficient spaces, and creating affordable housing. Questioning assumptions such as wants versus needs is an ongoing process. One of his recently published projects is a small home of 680 square feet known as Fredley. It is unique in its use of mixed materials and alternative design concepts. Brad works closely with the materials letting them inform the design. He currently runs a design/build business on Whidbey Island, WA (dbBrad) and is the author of Mister Greenguy answering questions about sustainability.
Kelly's responses below are not designated to anyone, whereas Brad's are directly attributed to him.
Q: I already purchased a foundation/framed/roofed house and am planning to use the bags between the studs. I have a question about the breathability of the walls and the papercrete application. You said you added Portland cement to the mix. I heard that the cement hinders the breathability of the wall.
A: There is a big difference between the breathability of concrete stucco and papercrete. They both use Portland cement, but papercrete is much more porous and breathable, whereas cement stucco doesn't breath very well.
Q: Also, what is the cost of the papercrete as compared to applying a cob mixture over the sandbags?
A: It is difficult to compare the costs because there are too many mitigating factors. Do have a mixer for the papercrete? Do you have good clay and sand nearby for the cob? Are you going to mix the cob by hand, or will you be renting equipment to mix it? I believe that the papercrete might need less maintenance over time, which might be a factor.
Q & A: My husband has built a small stick frame house. We built it before we knew much about alternative methods. We had always talked about building something different, but we were a bit wary of the learning curve and lack of experienced builders in our area (NW Louisiana). Since my husband's father built in a similar way we had his expertise on hand. We are now at the stage where we were about to put in fiberglass insulation in the walls and rough-cut cypress on the outside.
Having lots of good insulation in the walls and roof will be imperative for comfort. There are a variety of green insulation alternatives you might consider, besides the fiberglass: see this page.
Now, I may be crazy, but I am trying to convince my mentally and physically weary husband to switch gears and go for something that will help us utilize the energy of the sun for heating as well as keep us cooler in the many hot humid months without using as much energy. Did I mention that we are almost out of saved money and will soon need to return to work? We are looking for alternatives that are dirt-cheap.
Your motives are right on as far as wanting to heat and cool naturally, without paying for heat and air conditioning.
This is what I am proposing. 14 x 20 PP rice bags with local earth will wrap our 20' x 32', two-story house.
While doing this would probably help keep you more comfortable inside, I think there might be better ways. Putting all that thermal mass outside the insulated core won't buffer temperatures nearly as much as if the mass is located on the inside. I might suggest that while you are still at this framing stage, you might be able to redesign the windows some so that you have a somewhat passive solar house. In Louisiana you don't need very much solar gain...just a modest amount. You might study passive solar designs some.
And we would cover it with several inches of clay plaster mixed with rice hulls that we can hopefully get delivered cheaply (It doesn't seem we have much straw around here).
I don't think the rice hulls would help much with the plaster, since they aren't long enough to bind it. But you don't necessarily need the straw, either. If you want to proceed with the exterior earthbag idea, you might consider filling the bags with the rice hulls...then you would have some super insulation.
We currently have all of our windows and door installed, should we create arches or triangles over the opening or will wooden or metal supports work?
Any of those are options, especially if you use the relatively light rice hulls in the bags.
What would be the easiest and/or cheapest route?
Depends on several factors...certainly the triangles or arches might be the cheapest, but not necessarily the easiest.
How would we allow for other additions to connect to the structure (like a porch for example)? How would you start the foundation of the earthbags? We have a reflective metal roof and 8' rafters. The roof has a very gentle slop and we were considering no crawl space.
You have a lot of questions...perhaps you should find one of the earthbag books to help you with all of this listed on this page.
Do you have any insulation recommendations for the roof between the rafters?
See the above comment about insulation.
Do you think there are any extra considerations when creating an earthbag wall of this type--because, for example, one cannot access the inner side of the stacked bags?
Access to one side shouldn't be a problem.
How much overhang to your recommend beyond the earthen plastered walls? We get about 50 inches of rain here year.
Maybe 18" would be good.
One of my husband's main concerns about the earthbag wrap is structural safety. He is concerned about hundreds of pounds of earth stacked two-stories high falling away from the house and crushing someone or some thing. How can we safely and not too expensively connect the earthbags to the house structure?
I would still use the barbed wire between the courses to keep the whole thing as one piece...and then also periodically tie the bag wall to the wooden one with some sort of metal strips or wire tied around some of the bags periodically.
Have you heard of anyone else wrapping a house in earthbags?
No, but I have considered doing it myself.
He is also worried about the time it will take with one or two people and occasionally a couple others working on it (but mostly just my husband).
His concern is perhaps justified, in that doing this would definitely add quite a bit of time to your project, in terms of figuring it all out, teaching yourselves how to do it, making some mistakes and having to do it all over again, and then finally plastering the whole thing with earthen plaster, which is quite tedious in its own right.
If I were in your position, I would be tempted to do things in a simpler way: make sure that you have some passive solar heat gain for the winter, insulate everything as well as possible, place a fair amount of masonry materials on the inside, such as interior brick or rock walls, tile floors, etc....and enjoy your new place the way you originally intended, without too much more fuss.
Q: We have spent the last year working on our strawbale/conventional hybrid home. Our engineer who has worked with straw before suggested convention framing with OSB on the outside and notching bales with a chainsaw and sliding them up tight against the OSB then finishing the interior of the bale with natural plasters. This is what we have done.
We are about to apply one of our final coats and I have found a problem: we have had a sheetrocking crew mudding our sheetrock (interiors, ceilings etc) and consequently had a huge amount of humidity. We have as possible opened windows but with sub 0 temperatures the last week we can't leave them open long and have the house around 70 deg.
I have noticed that in a number of locations that have not yet received mud on the straw if I push my hand through the intersection of 2 bales I can feel the OSB on the inside and it is frosted with about 1/4 inch of ice is my guess. The straw seems to also be a little wet at that outside edge where it meets the OSB. Am I being over concerned that this is a problem or is it going to be just fine? Please help ASAP as I don't want to move forward until I have some definitive answers.
A: I know of a house that was built several years ago by an experienced carpenter who did something similar to what you describe, using conventional 2X4 framing and T-111 plywood as exterior sheathing, and then insulated the structure with bales laid on end (the studs were spaced to accommodate the bales). Rather than use an earthen plaster on the inside, he wanted sheetrock, so he had to attach nailers to do this. He was concerned about condensation forming on the exterior sheathing also, just as you have discovered, so he opted to place a plastic moisture barrier on the inside before he attached the sheetrock, rendering the house basically unbreathable, but least he felt this would save the bales.
This is a difficult situation, because the conventional wisdom says to keep it all as breathable as possible, but with such a hybrid structure where the exterior is not inherently very breathable, then these sorts of condensation problems can occur and you need to mitigate against this outcome. In your case, where you want the natural plaster inside, placing a moisture barrier there is not so easy, nor am I sure it is the best solution either, since you definitely don't want to trap moisture in the wall. It may be that the best thing in your situation is to go ahead as you plan and pray that it will work out in the end. Normal living in a house produces quite a bit of vapor (from breathing, cooking, bathing, etc.) so there will always be a source of moisture wanting to get through those walls which could condense where it meets the cold outdoor sheathing.
A (Owen Geiger): Well, as you've found out you've broken one of the most important principles of strawbale building -- one of the big things harped on over and over in all the literature -- that moisture must be able to pass through the strawbale walls. There is some leeway in the system if you live in a dry climate, use high quality vents and vent moisture at the source (especially a range hood in the kitchen and bath vents). A dehumidifier during construction would have been a good investment. Building during the summer to speed drying would have been best. I don't know all your climate details, but sorry to say it's likely you are at high risk of moisture damage with few good choices.
Q: Is there anyway I could contact Joe Michalak from Colorado who did the strawbale in-fill. All the information I read says to encapsulate straw with cob so it can breath and resist fire; however Joe used woodchip board exterior and sheetrock interior. I wanted to ask him if these manufactured products can breath also or if he even considered this?
Q: My wife and I are building an all wood home in Vermont. The walls will be constructed of three sandwiched 1x6 boards running vertically three feet apart. These 'beams' will be exposed on the inside. Outside of them will be 1x8 tongue and groove running horizontally up the wall. There will be rigid foam on the outside of the 1x8 for insulation purposes and then vertical ship lap boards outside this. My interest is closing in the space and creating a flat wall on the inside instead of a series of hollows between the 'beams'. Would this be possible using straw in the hollow and then plastering over the inside wall?
A: I know somebody in Colorado who used strawbales inside a wood-framed and clad building, and it worked out fairly well. He carefully framed the structure so that strawbales fit perfectly between the studs. He also had to put a moisture/vapor barrier on the inside before plastering (he actually used sheetrock), so that condensation would not form within the straw.
This regards straw bale with a wooden frame construction. I know usually you use 2x4 16" o/c for frame construction (platform construction). When using straw bale would you use the same (2x4 16'' o/c)? Or would you spread out the 2x4's wider to accommodate the large bales?
Q: I want to build a permanent tipi using tamerack poles. Could I cover the poles inside & out with chicken wire, sandwiching flakes of straw in between, & plaster the whole thing with papercrete?
A: This seems like an interesting idea that might well work, especially if you live in a relatively arid region. The main concern would be moisture possibly wicking through to the inside. I know of straw bales being successfully plastered with papercrete, so this should work. I would suggest that you make sure that the straw is thoroughly stuffed between the wire and poles, so that at no place is the papercrete solid from inside to out. If you try this and moisture does seem to be wicking through, you could try to seal the exterior papercrete with something, but this is not always easy to do. This would be an experiment... but one worth trying. Let me know how it works out.
Q: I'm thinking of building a 30' earthbag yurt style home in northern Maine, with a plywood roof and then covering the whole thing with 6 mil plastic and straw bale on top for insulation. You talk of the walls being able to breath, what do you think?
A: In general, what you describe sounds like a reasonable plan. The strawbale roof should give you plenty of insulation up there, but you will need to keep the straw dry for it to be effective and last over time. Also, in your climate you will want the walls to be insulated, so that may affect what you decide to fill the bags with. I filled mine with a crushed volcanic rock, but other alternatives might be perlite or rice hulls...or you can fill the bags with your local soil and then insulate the outside of the walls some other way.
Q: I know that the straw will break down over time and I'm willing to live with that. My idea was to cover the whole house with a 6 mil plastic roof and earth bags and then with straw bales for insulation. The idea came from the old Indian earth lodge's and from trying to keep costs down. Can you give me any more options other then plaster for covering the earth bags if I were to use conventional insulation?
A: My suggestion that you keep the straw dry is partly for your own comfort, in addition to the longevity of the roof...wet straw will not provide much insulation. You might consider layering the roof with plastic-straw-plastic-straw to maintain some real insulation. Another option for an insulated earthbag wall is to actually create a double wall, with a space between the two columns of bags, which can either be left as an air void, or filled with some other insulative material, such as straw, cellulose, vermiculite, wool, cotton, rice hulls, etc.
Q: I am considering building a timber frame house with straw bale wrap. I've been thinking of a earthbag vault roof. I like your idea of the light weight volcanic rock filling. My question is do you know any source(s) for this type of roof design and do you have any personal input in this design.
A: In general, your design concept sounds nice, but I would caution you against the use of earthbags for a vault...they do not perform well in this configuration. I made an 8 ft. span vault with earthbags filled with scoria, and I feel that this was pushing the limit of their capacity. A denser, more solid material will perform better in spanning larger distances with a vault.
Q: What would you recommend for the arch material? I prefer a self-supporting roof. Any ideas? Do you think cob would work or am I going to have to go to the (non-green) light weight concrete, maybe pumicecrete? I know the Auroville Earth Institue builds this type of roof using adobe type bricks.
A: Nader Khalili made houses using earthbags up to the roof level, and then switching to adobe blocks to complete vaulted roofs over them; I think the vaults spanned about 16' at the most. Cob could probably be used also, but it may require a form under it, because the cob is moist when applied and can slump with gravity. Then there is the whole question of insulating the roof and how to accomplish this. Poured lightweight concrete with some steel reinforcing does have some advantages.
Q: We live in a Pan Abode log house. I have been having a great deal of trouble finding out how to improve its acoustic and insulative aspects, and its rain resistance (which is lousy). The company itself will naturally swear that log houses are fine in all aspects, but they aren't, at least not in this climate of Nova Scotia. I know we will need much wider roof overhangs. My quandary is, how to effectively apply bales, or even earth bags (which may be the better choice because of wind-driven rain here) to the exterior, without eventually running into trouble with moisture infiltrating into the bales from the interior of the house, through the logs, which are only 3 inches thick (I understand what is needed for plastering the exterior). I have read some of your replies about coating the side of a bale that is in contact with siding of various homes, but my worry is about this coating, that would be against the exterior log wall surface, being able to dry out properly after installation. Would it work to give a coat of lime/clay plaster to the exterior of the log walls, let it dry, and then place the bales against that surface? Would moisture transference from interior to exterior still be able to occur? If I were to end up using earth bags, which I prefer to avoid because they are so heavy, there is still the fact that the polypropylene is against the log walls and would likely interfere with moisture transference. I want to maintain the internal acoustic properties that the logs have (not using insulation on the interior) because live music played inside is amazing.
Q: How can I attach wood to rock, or rock to wood? Are there special drill bits I need, or special screws?
A: During the construction of a rock wall it is possible to attach wood to the wall by embedding long lag bolts. If you place the bolts where they can be embedded in the mortar joint, then this is fairly simple to do.
If you are trying to attach wood to an existing rock wall, then you will need to somehow get the metal attachment into that wall. You can use a small masonry bit to drill a hole, either in the stone (if it is soft enough) or in the mortar joint. This hole should be sized to be able to insert an expansion anchor, so that you can then screw into this through the wood to secure them both together. Be careful in this process, though, because if you put too much pressure with the expansion it might crack the stone or the mortar.
In some circumstances you might be able to drive special hardened nails through the wood into the mortar, but this will not generally be as secure as screws would be.
Q: I am planning to build a log cabin up in the mountains in North East Mexico in the state of Coahuila, at an elevation around 8,500 feet. Up there, the typical summer temperatures range from a low of 47 F at night to 79 F in the day, and in winter, the typical temperatures are a low of 28 F and a high of 60 F. The lowest typical temperature reached in the winter is around 6 F. Some cabins up there are made with just adobe and others with logs. I have been thinking about combining both types, by using adobe as the thermal mass, with some insulation, plus an exterior with log siding and an interior with wood panels. Is this a better idea than simply using some insulation in between the siding and the interior wall? Or does the adobe need periodic maintenance even if protected in this fashion? What adobe thickness would you suggest? What supplemental insulation value would you suggest, if any?
A: Your plan sounds reasonable enough and should work fine. Depending on the species of wood, logs do provide some insulation (the less dense they are the better they insulate). An additional layer of insulation between the two will only improve this, and I would imagine that perhaps 1 - 1 1/2 inches of rigid insulation would serve this purpose. I suggest using the standard adobe blocks that might be available in your region, although these are becoming more difficult to find in Mexico as brick and concrete have taken their place.
A: (Quentin) I keep pondering this one and have not come up with an answer worthy of pushing the send button. In short, I don't think it is worth all the effort to combine materials. Log houses work fine. Adobe houses work fine. Insulated houses work fine. Log, adobe, insulated houses are a new breed of cat. I guess he might try it and report back to the rest of us in 25 years.
Q: Am needing to build a 24'x30' home for my little girl and myself as cheaply as possible. My married children are opposed to strawbale and sandbag construction but will support me in putting up a 2x6 structure? Question: once the foundation, sub-floor, 2x6 walls and roof are done, how would it work to put plywood on the exterior walls and use adobe on the inside between the studs for thermal mass? On the outside, add 4"-6" of rigid foam insulation covered with hardiboard siding to retain passive solar and wood stove heat generated during the day?
A: (Brad) It looks like you are pursuing how to build a house, what your options are, what the program is and doing investigative work and design work up front to solve the myriad of complex issues associated with building a house. This is good, because one can’t just, halfway through the construction of a house, make inventive changes and expect success. These issues really all want to be pinned down prior to beginning construction, especially if being built by someone who doesn’t have the decades of experience to solve the issues that will arise when you deviate from the building norm. But it can be done and you’ve come to the right place for an answer.
Traditionally, adobe is a load bearing wall made of smaller units, adobe bricks. Once complete, it receives a coat of stucco or plaster. My concern with your system is: will the extra work warrant the change in material and the additional cost? If you’re in the sun belt, then this idea is worth pursuing. If you live in an area prone to long wet winters or flooding, or very high humidity, you may reconsider your choice of materials.
Adobe, essentially a cob mixture formed in brick sized shapes will be difficult to fit between and around all the studs. It will also make electrical and plumbing more difficult. Cob houses are often formed as larger units (walls) and then allowed to air dry. Perhaps filling the entire bays with cob and letting them air dry in place would be a better method. This could be done after completion of electrical and plumbing but would be easier to do while the wall is laying on the ground, though standing the wall up will be more of an issue because of the weight. Regardless, doing it early on would be easier, but will slow down the framing process dramatically and extend the period in which you don’t have a roof.
If you are certain to use bricks, I may consider placing them on the inside of the wall and using traditional insulation in the 2x6 cavities which is about half the cost of rigid and also helps solve the issue of electrical and plumbing. The problem here is again a function of your climate; if the dew point is between the adobe and the insulation there will be a moisture problem that will need to be addressed. It is very important not to trap moisture in your walls, roof or floor systems.
Really, it is this possibility of moisture that concerns me the most about putting adobe bricks between wood studs with Hardi siding. The traditional hardi on building paper on sheathing has been in use for quite a while, but it will leak water, guaranteed. So how do we keep this water from turning your adobe bricks to mud? Current codes have addressed this by adopting rain screens which help to mitigate the moisture related issues and would be very appropriate for your house, especially since you’re looking at rigid insulation on the exterior anyhow. But doing a rainscreen with 4” rigid has issues too, so be careful.
Regarding thermal mass, having adobe walls will certainly help retain wood stove heat but won’t do much for solar. With solar gain, it is important to let the sun hit directly on the thermal mass. It will be almost impossible for the sun in your area to effectively heat up walls of adobe so there would be little gain here.
I hope this helps with your thought process and good luck on building your house. One last note, I highly recommend not using OSB anywhere on your house, especially in such an alternative setting. Plywood is negligibly more expensive than OSB and such a better product, particularly with the shear and moisture issues you will be dealing with.
Q: I am searching for information on how to retrofit my house using natural material. I am planning a retreat and want to build some small cottages using cob, earth bag, cordwood etc. I want to use my existing home as the retreat center and want to tie in the materials I will be using to build my cottages. Are there any good reference materials available on redoing exterior and natural plasters on interiors. Can I build cob benches in a house that has wood floors and sits on a stem wall? I'm sure I can remove siding from the house and apply the felt paper and chicken wire and do stucco but really do not want to use stucco. What about lime plaster?
A: (Brad) Retrofitting a house with natural material is a great idea if it already needs work. Taking out perfectly good materials and replacing with more natural materials just to do it is less of a good idea. Where do the old materials go, will they be recycled or will they end up in a land fill only to slowly dissolve into our groundwater?
I understand the need to tie buildings together, but similar materials is only one way. Here is an example of three buildings all of which have different siding materials but are very cohesive because of good design, similar details and site placement. Contrast is as strong a design concept as Similar!
With this said, my recommendation is to leave the siding on the house (assuming it’s still functioning properly) but to look for simple and clever ways to tie the buildings together. A strong paint color for trim elements, similar details at windows and doors, use of plants, garden and paths to create relationships and secondary and tertiary structures like fences, arbors and window shading devices.
If you really want to plaster the house and must replace the siding, and assuming you live in an arid climate that warrants the plaster, a few thoughts. What is your current insulation value? If it’s not at least R-21 (code), now is the time to fix that.
Doing plaster will also create issues at windows and doors that will need to be addressed. How will the plaster come into windows and doors? Will you need to extend the jambs and do you have vinyl, wood or aluminum windows, all of which will have slightly different details for plaster? Positive flashing and well taped (Vykor or blue skin type product) joints/corners are imperative.
You may want to consider a rain screen. Plaster when bonded to the felt and wire doesn’t allow for the water that gets behind the stucco to drain out well. A rain screen provides a positive drainage plane and could be a drainage matt under your chicken wire or could be rigid insulation held off on stickers which you would apply the plaster onto. The detailing will be the hardest part of plastering your house as the mixing of plaster and troweling is pretty easy, if not a good workout!
Regarding references for natural plasters I suggest: The Natural Plaster Book: Earth, Lime, and Gypsum Plasters for Natural Homes by Cedar Rose Guelberth and Daniel D. Chiras, 2002
The cob benches will want to have a barrier of some sort to protect the wood underneath during the curing process. Wet Cob has a lot of moisture that will move by conduction faster than convection. That means dry materials touching it will absorb moisture faster than the air. Once it’s dry, you should be good to go.
Q: This spring I'm going to build a cabin on a beach on Cypress Island in Puget Sound, (Washington State). I'm trying to use as much local material as possible because it's good practice —and because the only access to the island is a two-hour ride in a very small boat. My plan was to hew planks and beams from the plentiful supply of large, very high quality cedar driftwood on site; my buddy suggested building a sandbag house. I read quite a bit about earthbag construction a few years ago and like the idea so I'm thinking about combining the two.
I'm thinking of making a log-cabin-style structure with runs of sandbags between the logs. The sand bags would eliminate the need to notch the logs and would act as a mortar. Using the bags would also give one the freedom to use oddly shaped logs that one couldn't use in traditional log-cabin construction (I'm thinking of smaller bags to fill in the gaps).
Do you know of anyone that's experimented with this idea or something similar? Also, what can you tell me about building on a beach? Most earthbag structures I've seen are in the desert. I'll be building a ways from the water and using large boulders as-they-stand as a foundation so I'm not worried about erosion, just about moisture and salt-air. What sort of plaster/goo would you recommend for the exterior? I know that papercrete and other commonly used coverings for earthbags won't hold up with constant moisture. —I'll build an overhanging roof of some other material, so I'm not talking about direct rain, just very-high humidity and salt-air.
A: That sounds like a very fun project that you have planned. I can picture it very well from your description, and it sounds quite doable to me. I actually have not heard of anyone doing exactly what you propose. I would think that alternating logs and bags could eliminate the need for barbed wire, since the logs will certainly keep the bags from pulling apart or moving much over time. You might pin the logs to the bags with rebar stakes in critical places, if that seems necessary. It would be nice if the logs would still be visible to some extent, for aesthetic reasons, so that might mean using fairly small bags so the logs are not embedded too far. I would expect that the soil that is on site would suffice to fill the bags. For a plaster, you might consider earth/sand and lime, with a higher percentage of lime on the final coat, as this should hold up pretty well on a protected wall...and it is breathable, which is good in a humid environment.