Q: In designing a hybrid passive solar home for myself, your idea of using scoria in earthbags fits my particular needs* perfectly in ONE of the exterior walls (*a material unaffected by moisture yet insulating, thick for aesthetics, inexpensive, natural & easy). My concern though is its R-value. From what I have been able to find, it is only .6 to .8 per inch....which, is less than half of what I'm looking for. However, you have mentioned an efficiency in your earthbag/papercrete home which would indicate something very different. Do you think this may be due more to the continuous layers of papercrete than the crushed volcanic rock itself? If that is the case, I will have to rethink my use of scoria in earthbags as I am wanting to use earthen plaster inside and out (for various reasons) yet still have a highly insulative material within that is unaffected by moisture.

A: This is an interesting question, and one that I cannot give you definitive answer for. I don't know of any tests that have been done on the R-value of earthbags filled with scoria, although it sounds like you found some data somewhere about scoria.

Our earthbag/papercrete home in the high mountains of Colorado did perform extremely well in terms of insulating value, there is no doubt. After a sunny day, the house would stay comfortably warm inside even without extra heat, even if the outside temperature plummeted into the teens. I, and others, have compared this performance to that expected with strawbales, which are rated at around R-40.

Papercrete is said to have an R-value of about 2 per inch, and my walls had an average of maybe 5 inches of papercrete, which would account for about R-10. The other say 15" of bagged scoria would then account for the rest, which works out to R-2 again.

So my empirical guess is that this is a better value to work with than the lower figure...but then I have no proof. Certainly the fact that the bags were well-sealed with the plaster is very important. Also, the size and cleanliness of the aggregate might be a factor. I used about 3/4" aggregate, with very little fines, so there is a LOT of trapped air in there, which is very good insulation.

C: R-values for lightweight aggregate are readily available (for example, cinders = .59/inch, pumice = .86/inch). I have not seen any test criteria though. However, if one was to round cinders up to an R-1/inch (a pretty big margin of 40% more), a 15" bag would only give an R-15 rating. With your 5" of papercrete, you'd then have a total of R-25 or thereabouts. While much lower than "R-40", an R-25 still coincides with your comparison with strawbale.......IF you adhere to the findings of Oak Ridge National Laboratory which found two-string bales laid flat equaled R-26.

That said, it would seem your performance comparison between your scoria/earthbag home to that of straw might be correct but perhaps your estimation on the insulative value of scoria alone to be overestimated. I'm not doubting your experience/claims but rather trying to piece this puzzle together. Any thoughts?

A: I think that the test data for a solid piece of scoria or pumice might be quite different from an earthbag filled with loose scoria gravel that has been well-sealed with plaster. All of that trapped air in there can make a big difference...but I am not an expert, this is just my gut feeling.

Also, there is quite a bit of variation in the quality and density of strawbales, so the R-26 value for that may be undervalued in general. The only way to know for sure would be to run a scientific test with the earthbags, and even then, the results might vary depending on many variables, such as type and thickness of plaster, etc.

C: (Owen Geiger): I've been quoting R-30 for bales.  The old books (and tests) claimed R-40, but most people are in agreement now with R-30.

Q: Have you ever heard of anyone mixing perlite or vermiculite with the scoria in the earthbag or, mixing perlite or vermiculite into plasters?Knowing I won't have the extra insulative benefits you had with papercrete, I'm trying to think of ways that might boost the thermal efficiency of scoria filled earthbags even more in my project, if for no other reason than peace of mind. Or, perhaps I could do as you did and use papercrete over the bags then come back and apply a finish coat of earth plaster over that.

A: Perlite and vermiculite are common aggregates in lightweight concrete formulas, although I have not heard of them being used with earthen plasters, this is certainly a possibility. One could conceivably even fill the bags entirely with these materials. Another natural material that has excellent insulative value (reported R-3/inch) is rice hulls, which can be purchased for very little, if you happen to be near a processing plant. Papercrete does make an good substrate for other plasters, since it is so dimensionally stable.

Q: I couldn't find the picture and details of how you used scoria behind your dome. What were the dimensions? I'm thinking of using scoria for my zero energy house, both in the bags and between the walls and earth berm.  This seems like the most effective design: passive solar, scoria filled earthbags, earth berming, super insulation in the roof.  The current design is semi-circular or a curve close to semi-circular, with a glass wall and planter in the front. My goal is to achieve zero net energy usage and extremely low embodied energy. 

A: Basically, I filled many of the lower bags with sand in that north wall for more thermal mass. I allowed about a foot and a half gap between this wall and the natural earth bank, which I then filled with scoria after having lined it with the 6-mil plastic. (I can't remember now whether I used one of two layers of plastic; I know I used two over the pantry dome.) The bags above the natural ground level were filled with scoria, so I then lifted the plastic apron up to the height of just above where the eventual berm would be and embedded it in the next course of bags. Eventually that whole back wall was substantially bermed. I have long been convinced that really the only chance for a "zero-energy" house would be either substantially bermed or underground. Paul Shippee came pretty close to it back in the 70's with his SunEarth house ( http://dreamgreenhomes.com/plans/sunearth.htm ), and this was earth-sheltered. You could not call this house low-embodied energy however, since it is mostly concrete.

Q: What is the R-value of perlite or vermiculite?

A: I think that an average of 2.8/inch for perlite and 2.3/inch for vermiculite would be appropriate, given some on-line research that I have done.

Q: I was looking at making a earthbag home in TX and was wondering if you thought it would work to use a double wall on all exterior areas and single bag (14" wide) on interior with all load bearing into a foundation that is backfilled with concrete on top of fill+rice hull for insulating factor and place a standard roof on the rectilinear structure. Pretty much build a "normal" looking home with bags single story.

A: If I understand your description correctly, I think what you propose would work. I designed something similar with a double wall (the interior filled with soil and the exterior filled with crushed volcanic stone). You can see this at http://dreamgreenhomes.com/plans/bagginsburrow.htm  . As long as the load-bearing portion is mineral rather than the rice hulls, this should work.

Q: I am interested in using rice hulls in bags as an alternative to fiberglass in the earthship type home that I am building. I need some direction on how to proceed. See my house at www.flickr.com/photos/whatisupwiththestanleys

A: I've never tried using rice hulls in bags myself, but I understand that they make excellent insulation. First you need to find a source for the rice hulls, which should be quite inexpensive if you can find them locally enough. I've heard that there are rice processing plants in Texas and Florida, and perhaps some other southern states. I would not recommend using bags filled with rice hulls in bermed situations, where the bags can't breathe well...

C: I live in Arkansas which happens to be the largest producer of rice in the United States.  I can get a truck load brought to my land for $1200 which is about a 2 - 3K savings over fiberglass or cellulose.  My second choice was the recycled blue jean batts which are 2X the price of fiberglass.  So, rice hulls it is.  I spoke to the lady who built the rice hull house.  She was not in favor of putting the hulls in bags.  She liked the idea of sealing off the ceiling and then blowing them in like cellulose.  So, for now that is our plan.

Q: I was thinking of using paper as insulation placed in bags. Would this work?

A: Dry shredded paper makes great insulation, and is sold commercially (usually with borax added to keep pests away) as "cellulose insulation". This could be put in bags and stacked for building, but it would compress a great deal, so the only realistic way to build with it would be as in-fill in a post and beam structure, or some similar arrangement where the bags don't bear any weight.