Hybrid Questions and Answers Involving Cement

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.

Questions and Answers


Tire/Concrete Block

Styrofoam/Concrete Blocks

Strawbale or Earthbag/Concrete Block




Q: My husband & I are looking into building a ferrocement dome (modular units). What do you feel is the feasibility of sandwiching a layer of papercrete blocks between 2 layers of ferrocement? Since we are potters, ultimately ceramic tile and grout will be used for the finish. The wall would probably end up over 12' thick. I am aware that papercrete wicks moisture. Will this effect the ferrocement? I envision the dry papercrete wicking moisture too quickly from the outer layer of cement as the cement is applied. Then again. if the papercrete is moist as the last layer of cement is applied, will the papercrete ever have a chance to dry out being stuck between two layers of ferrocement?

A: It is an interesting concept that you propose. There would certainly be advantages to the arrangement, if it could technically be accomplished. With the ferrocement on the inside you should get plenty of thermal mass, and the papercrete would provide the insulation. The trick would be to get the papercrete to dry out after you poured the final coat of cement, and this might be quite difficult; some formulas of ferrocement are virtually water proof. My suggestion might be to forgo that final coating of ferrocement, and allow the papercrete to be the outer layer. This papercrete could be done in a couple of pours, so that the first one is highly insulating, with mostly paper and little mineral material. The next pour could be more much more sand with the paper and cement, to give a more durable, fireproof layer. This is basically what I did with my house, and it has worked out well. If the initial ferrocement shell is mixed to be waterproof, then you could simply allow the papercrete layer to breath and evaporate moisture as needed. Sealing it to moisture would be another option, but any water that does manage to get through might be trapped there for some time. This might take some experimenting, perhaps trying it unsealed at first, and the sealing it later if this seems necessary.

Tire/Concrete Block

Q and A: I am living in Baja California by the Sea of Cortez. Most of my indoor living is in two concrete block rooms which are what was here when I moved in. The immediate problem is the west wall which becomes as hot as an oven even this early in the summer. I have been thinking of adding a second wall of tires (Mike Reynolds style) outside the concrete block, followed by stucco. Do you think this would work well or do you have any other ideas?

This would definitely help, mostly because of the thickness of it. Tire walls provide mostly thermal mass and not much insulation, but the thicker the wall, the more stable the temperature will be. If you do this, you might stack the tires a bit away from the block wall to provide an air gap that will serve as an insulated barrier in transferring the heat.

Besides insulation, the humidity gets almost unbearable; should I use a plastic barrier between the tires and the concrete block?

I am more of a fan of maintaining breathability in wall systems, as I believe it is healthier for both wall and inhabitants. The humidity is in the air, and is therefore going to find its way inside anyway. A plastic barrier will just trap the moisture that gets into the wall. You might investigate forms of dehumidifiers to deal with this.

Besides heat and humidity we have some really nasty bugs here so I have to do what I can not to provide homes for them. A good solid stucco should stop the bugs from entering the wall. Any quick ideas or recommendations of reading/videos would be appreciated. I have a fairly extensive library about alternative building but, as often happens, I am left wondering about specific questions for my particular situation.

A much simpler solution that you might want to try as a first step is a paint-on insulation. I have recently found out about this, and in fact have a link to a company that makes it: the Insuladd product.

Q: I current live in a small ranch style home built in the 50's. It is a true brick house, which means it radiates cold in the winter right through the walls. Is it possible to clad the outside with the tire wall technique?? I do realize this would require a redo of the roof system to accomplish this.. Has anyone else done a conversion like this??

A: What you really need to make this house comfortable is to wrap the bricks with something that will insulate them from the extremes of temperature, both cold and hot. While a tire wall would help, this would not be the best solution; a more insulating material, such as strawbales, earthbags filled with scoria or rice hulls, or even commercial rigid insulation would do much better. All of these solutions do require a subsequent plaster on the outside to protect them, as well as a likely roof extension and a proper foundation...so this is not a simple job. Many people have done this.

Styrofoam/Concrete Blocks

Q: I am about to build a house in Thailand, where there are three seasons: hot, hotter, and hottest (100-110 F). A friend of mine built a house using bricks that are made of a mixture of cement and recycled styrofoam. He recommends a cheaper alternative for our exterior wall construction: a 'sandwich' made of two vertical layers of cement blocks with a recycled styrofoam in the center of the 'sandwich.' He has made one wall like this and it stays very cool. The white foam pieces he uses in the center are recycled lids for coolers. He uses two of them, back to back, for each piece of insulation, sliding them down between the cement block walls. My primary concern is toxic gasses in the event of a fire. But I would appreciate any thoughts you might share.

A: This sounds like an excellent strategy for building in your region. Isolating the two layers of thermal mass walls like that will keep the heat from migrating into the inner layer, while allowing the inside mass to remain cooler and help buffer interior temperatures. Your concern about toxic fumes in a fire seem like a minor likely problem to me. First of all, the masonry blocks obviously will not burn, and the relative lack of oxygen in the wall cavity would keep the foam from burning much either. Furthermore, tests done on SIPs that have EPS cores have shown that the foam does not tend to burn when encased even in wood fiber materials, and when it does, the fumes are no more toxic than wood smoke.

A: (Owen Geiger) I agree with Kelly's summary of your building system.  One additional consideration is ceiling insulation.  Most roofs in Thailand are not insulated and get very very hot.  If roof insulation is not addressed, the house will quickly over heat.  One good technique for the region is to fill polypropylene rice bags with rice hulls.  Rice hulls are fire resistant, don't attract insects, provide excellent insulation, and are virtually free in Thailand.  The bags can be suspended from the roof structure with wire, wood poles, etc. and covered with reed or bamboo mats, or plaster, etc. if fire is a concern.

C: Our house is going to be a one-story, Mediterranean style, U-shaped with a central courtyard. You are correct about most Thai houses lacking insulation. We plan to have ridge vents and soffit vents. We had planned to use conventional insulation, as well, but the rice hull idea sounds great. The living room & dining room will have a cathedral ceiling, so we would have to use conventional insulation there, with a ventilation chute above (linking soffit vents & ridge vent). The other rooms could certainly have rice hull insulation in the attic (in addition to soffit vents & ridge vent).

A: (Owen Geiger) Consider using natural building materials, such as adobe or pressed earth block, instead of concrete block.  If you're concerned about moisture damage to the exterior, consider using natural materials just on the interior. You can use the rice hull idea on cathedral ceilings.  One option is to build parallel trusses where the top and bottom chords are parallel.  Another option is scissor trusses.  Both create adequate space for the earthbags/rice hulls.  Be sure to leave an air gap above the earthbags to vent the hot air.

Q: How do you cut out windows in a cinder block concrete house?

A: This is easier said than done. Cinder block construction usually involves the creation of a grid pattern of steel grouted in concrete to fill at least some of the voids. If this is the case where you want to put the window, then you will need to cut through all of that. This is possible if you have a saw that is designed for cutting stone or concrete and steel. The other consideration is that the window may need a lintel above the opening to support the weight from above. You should probably talk to an engineer or a builder who knows about these things to find out what kind of extra support will be needed, since it depends on the size of the window and what loads will be applied from above.

Strawbale or Earthbag/Concrete Block

Q: I want to insulate the exterior of an old cinder block garage that's now a living space (too narrow for interior insulation): straw? earthbag? other? Also needs to be something a 62 y.o. woman can handle!

A: Both strawbales and earthbags filled with insulating material (crushed volcanic rock, rice hulls, perlite) would conceivably do the job for you. Strawbales would need to be set on  their own foundation; earthbags could perhaps be put on the ground if the fill is not vulnerable to rot. Then either of these would also need to be protected by an overhanging roof, and the details around doors and windows can be tricky. A simpler approach might be use conventional rigid insulation (like blueboard) attached to the wall and then protected with a stucco or something.


Q: StrawBale construction in Alabama (surrounded by farmers) seems like a very affordable technique. Has anyone combined this with Shotcrete? The strawbale makes for a nice permanent form, with shotcrete applied to both sides. Gaps in the bales can allow rebar connections and additional crete supports with thick caps for flooring support. Any concerns, references, or advice for this mixture of techniques?

A: Actually I have heard of people doing something like this. There is a fellow in Canada who has been manufacturing Strawbale SIPs (Structural Insulated Panels) using bales with cement stucco. His website is http://www.genxsystems.com/strawbale.htm  and there is more information about a specific plan that utilizes these at http://dreamgreenhomes.com/plans/sbsip.htm  . One thing that I might caution you about is that the general consensus among strawbale builders is that the bale walls are much more likely to survive over time if they have breathable skins. Shotcrete is typically rather high in cement content, which tends to render a rather waterproof membrane, so this could be a problem. A weaker mix might make a more breathable wall. The problem is that if moisture enters the wall for any reason (leaks, poor design, condensation, etc.) then it cannot dissipate, and the straw may rot.


Q: I am exploring building a 20-30' round structure. The structure will be dug in the side of a hill (it's a deep hill maybe 40-50 ft high, 20-30% of the south wall will be exposed on the hill, the rest will be underground, with 4 ft dirt over the roof. So far I plan on using earthbag walls (aprox 8' high) because the soil on this hill is very good (8' of topsoil at site and lots of sand on property). I will wrap the exterior walls with foam board, plastic vapor barrier and then possibly ferrocement the outer shell as well to give extra lateral strength since I cannot find much case studies on underground earthbag walls. The top of the wall will have a cement bond beam and then a ferrocement dome. I was thinking the cement dome because it would be safer to build under ground, but now I have been told the ferrocement may be a big undertaking. Any suggestions? I plan to build a floor/platform at the top of the wall across the entire span to accommodate a sand form for the dome.

A: (Owen Geiger) What you're proposing is possible. The most difficult and dangerous part is the dome. 20' would be challenging. 30' would be even more difficult. Adding 4' of earth could be deadly. Even 2' of earth is extremely heavy. At the bare minimum you need to read a book on ferrocement. You don't need a sand form. Just bend the frame into shape and hold in position with poles. The dome must be integrated into the bond beam, and the bond beam must be securely joined with the earthbag walls by pounding rebar down through the walls a few feet at least. The foam idea is fine.

Based on your comments, maybe I will be better off making two 18' diameter.

Great idea. Make two roundhouses. Much safer, much easier. It wouldn't hurt to add a center support to just be on the safe side...


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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 LLC.