Hybrid Questions and Answers Involving Stone

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






Q: I am looking at some land in AZ with red clay-sand heavily embedded with rocks. Have you seen any examples of earthbag building where they use stone facing on the outer wall?

A: I love stonework and suggest that you utilize this wonderful resource to some extent. It tends to be very time consuming, so bear that in mind. You could certainly face an earthbag structure with stones, but my suggestion would be to do this more on the inside where you really need the thermal mass, and then fill the earthbags with scoria (crushed volcanic stone) for better insulation...especially in AZ.

Q: I propose to build my house in the following way: Foundation with stones; walls with earthbag construction (rectangular); roof by vaults of adobe blocks. One problem I may face in future is rains, the effect of which on the vaults is unknown to me. Since I need an inner open verandah (courtyard), a circular design is out of the question.

A: What you describe sounds quite possible to me. A stone foundation is good, and earthbag walls would work well. The walls have to be thick enough to buttress the vault  (to keep it from expanding outward), so be sure to get some engineering advice about this. Your concern about rain on the vault is real. An adobe vault must remain dry, or there is the possibility of collapse. Sometimes people use stabilized adobes for the roofs, which will tolerate some rain. Here are some photos of the building of an adobe vault: https://www.adobealliance.org/photos/


Q: My question is whether or not strawbale would make an economical and practical inner wall (and insulation) with load bearing stone walls (either traditional or slipform)as outer walls...would the inherent condensation from the stone walls endanger the strawbales? Would a 1/2 inch space mitigate this danger, and have you ever seen this method used?

A: No, I haven't heard of this particular approach to building a wall. Your concern about condensation forming between the materials could be justified, since the cold rocks would tend to make this happen. An interior vapor barrier may be necessary. Trying to leave a small space between the rocks and straw might be difficult to achieve, given the unevenness of the materials. I always advocate putting the insulation material (straw) on the outside of the building envelope, with the thermal mass (rock) on the inside. If the straw had a somewhat breathable plaster on the outside, I don't think that condensation would be a problem with this arrangement, because the rocks would not get cold enough.

Q: Can anyone tell me if it is possible to build a strawbale home and slipform the front w/stone? I have searched through many book options, but I would really like to marry the two concepts I love, stone and strawbale, and would love to utilize as much of natural resources as possible. Would the stone be an option instead of stucco?

A: My recommendation would be to reverse the order and put the stone on the inside so that it will act as thermal mass in the house, with the strawbale insulating it. In this way you would also eliminate the potential problem of condensation affecting the straw. Put a breathable plaster on the straw and you will have an extremely efficient house that will last a very long time. In this way, I don't think that you need any gap between the stone and the straw.

A: (Owen Geiger) Stone/straw combo is not common and seldom discussed, so there's little if any body of knowledge to draw from.  My guess is it's similar to wood siding. I think a small 1/2" gap between the stone and bales would allow moisture to escape/drain. One obvious drawback to this idea is it creates excessively massive walls.

Q: I am interested in building a stone house. I have questions about what the cost will be. We have the cement mixer, and land- but probably not enough stone. I would like the house to have a full basement- out of cement possibly but was hoping something natural like strawbale might work. I was thinking that I could set the ground level outside the perimeter of the basement level to avoid the added stress of the structure. However, we will only be able to hire out for less than 1/2 the work. I wanted to basically do a 2 story house w/ basement with at least 3 bathrooms.

A: While it is true that stonework can be inexpensive, it is also very time-consuming. This fact, coupled with the fact that the stonework is really only a fraction of all the work and material that goes into to making a house, the cost of making such a home will not likely be much lower than a conventional home, if that. Certainly doing much of the work yourself can reduce the cost, perhaps in your case by as much as 1/4...

Strawbales are not suitable for any basement construction; they are too vulnerable to eventual rot. But they are great for a well-insulated shell of an above-ground home. My suggestion would be to design your home with concrete or stonework for the basement, and do the first floor with strawbale. Some interior stonework on the first floor will provide needed thermal mass to keep the home comfortable.

That makes sense for the basement. Also for the first and second floor- straw bale for an interior as an insulator will work? Keep everything warm... Is 3500 sq feet  feasible - structure wise? I would like it to be 2 story. My concern from what I've read so far is that the further up you go the structure becomes a problem. Will straw bale on both levels be a strong enough support?Also, is there a way to "fire/ moisture" proof the straw bale interior?

I was picturing just the basement with one story above that when I suggested the strawbale upper portion. Strawbale building rarely goes above one story. My idea was that the basement would provide the foundation for the upper story. The strawbale would ideally be exterior, since this is where you need the insulation. Stonework would be on the inside. But if you want a basement plus two stories, then the first story should probably also be stone, although any exposed stone building needs some form of insulation to make it comfortable. This can be accomplished with an interior core of insulation sandwiched between two layers of stone...a very time-consuming and likely expensive project, especially for a house as large as you want. Anyway you look at it, you would be embarking on a MAJOR project. Strawbales are surprisingly fireproof, because of lack of oxygen in the bales, but then they can be made even more fireproof by plastering them with an earthen plaster.

Q: Would there be sufficient insulation if I were to use a solid stone wall (exterior), then straw-hay (middle layer for insulation) and the light-weight concrete for the inside finishing touch?

A: You might be able to do what you describe, but I have a few concerns: 1) The wall would be very thick. 2) You need to make sure that the strawbales can breathe, so an air gap between them and the stones would be a good idea. 3) Lightweight concrete does not provide as good thermal mass as stone, or even earthen plaster would...and you want plenty of thermal mass on the inside. An ideal hybrid wall system would be strawbale on the outside for insulation, and stone on the inside for mass.

Or, another approach would be to create a double stone wall with insulation in between. Even though commercial rigid insulation is not as green a material as straw, if I were designing your wall, I would think about embedding several inches of this material between two layers of stone, and end up with a beautiful and thermally efficient building.

Q: I have a load-bearing strawbale house with a stem wall made of gravel filled polypropylene bags. I'm planning on covering the bags with flag stone mortared together with lime mortar. My question is will the mortar wick moisture into the bales and should the rocks just sit on the ground or do I need to put something under them like gravel.

A:  The bags should do fine with mortared stone up against them. If the stone also makes contact with the bales, I suggest providing some means for air to circulate between the two, using wire mesh or something to keep them apart. The stonework should probably have some foundation other than just the soil, and a small rubble trench should suffice for this, to keep the soil from heaving when frozen.

Q: I intend to build a hybrid house; basically it would be an earthen cellar (used as such), dug into a hill, in front of which a small dwelling would be constructed. The front part (south facing, front wall exposed) would be build of stone in the lower part and glass in the upper part. The whole structure would be covered with a green roof, used for gathering rain water. My main concern is proper insulation and construction material for the external underground walls and floor as well as the roof. The dividing wall between the earthen cellar and the dwelling would be a massive straw bale wall with clay finishing. I live in an area with four distinct seasons, spring and fall being quite wet with relatively high temperature oscillations, winters cold with snowfall and summers hot with frequent showers and storms. Temperatures vary from -15C to +35C. I would like to avoid using conventional construction materials if at all possible.

A: Basically it sounds like a good design that you have in mind. I recommend using materials underground that cannot be adversely effected by moisture, such as masonry materials. One such material that is also insulating is volcanic stone, which can be used in a crushed form packed into earthbags or made into pumicecrete. Otherwise, you will probably need to use some other commercial insulation product to insulate stone walls, or whatever you decide to use. This can be as little as 1.5 - 2 inches of rigid insulation.

As for your roof, I think you will need to decide if you really want a green roof, or do water catchment, since the two cannot be done effectively together. With water catchment you need a roof that will not impart any toxins or shed organic material. Metal roofing and clay tiles have been used effectively for this. Such a roof can be fairly lightweight in structure, and insulated with bags of rice hulls, straw, etc. from below. A true green roof requires much more substantial support, and needs to be engineered to handle all of that weight. This usually means using very hefty wood or steel beams, or reinforced concrete.


Q: I need to find an insulating material for my stone wall basement. I don't want to cover the stone with dry wall. I would consider adobe or some other natural material that might provide some warmth.

A: Adobe would not make a good insulating material, so unless you left an air space of several inches between it and the rock wall, it would not do very much good. Of course doing this would diminish the interior size of the room noticeably. Most any insulating material, natural or otherwise, would need to be covered with something else to make it look good. One approach would be to put a relatively thin layer of insulation (such as blueboard) over the stones, attaching it as firmly as possible, and then put wire mesh over this and plaster the wall with an earthen plaster. This would give you the adobe look and feel without taking up as much space. The best approach would be to dig around the basement and insulate the wall from the outside with blueboard or something similar, although this may not be feasible. This way the interior natural stonework would remain the same, you would not lose any space, and the thermal mass of the stone would work to your advantage on the inside, being insulated from the ground.

Q: I live in Bali and I am building on the seaside in a quite hot and dry area. We have a lot of lava stones on the land and I would like to use them for the walls. But I am not sure about the insulation and sound proof qualities. My idea is to build the outside from the rocks and the inside of the wall with mud bricks. But then I think that the concrete for the rock wall destroys the good qualities of the mud wall. I do not want to use air concrete; that's why I want to have a really good thick wall as heat insulation.

A: (Kelly) If the lava stone is light weight and rather airy, then it should provide fairly good insulation. I once built a very well insulated house with crushed volcanic stone put into earthbags to build the walls. Using mud bricks on the inside is also a good idea, since these will help stabilize the interior temperature, and absorb any extra humidity you might experience on the seaside.


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.


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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.