Insulation Materials for Earthbag Buildings
Questions answered by Kelly Hart

Q: How much insulation value do you get from earthbags filled with soil?

A: Soil is generally a very good thermal mass material and very poor at insulating, except that, because earthbag walls are usually quite thick they do provide some insulation from the outside temperature. The problem is that once the bagged material gets warm or cold it likes to stay that way for a long time, which can mean uncomfortable temperatures inside. In climates with extremes of temperature (either hot or cold) it is best to fill the bags with an insulating material.

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: 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: How can pumice or mixed pumice and earth be stabilized without large cement coverings or lots of wood?

A: This is where the earthbags come in very handy, because they can hold the loose pumice in place, and then a thin earthen plaster can be used to protect the bag material. It is possible to make pumicecrete that will support itself, but this does use cement.

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.

C: It seems like with papercrete you did not  have to worry about humidity in Colorado. Instead you had to tackle another beast in cold weather, that we don't have to worry about in Hawaii. Looks like papercrete provides a cozy feeling with good insulation in the winter.

A: Yes, papercrete provides very good insulation, but it may not be the best material to use in Hawaii because of the humidity, which can promote mold forming on it. Actually, even in Colorado, I am not sure that I would use papercrete again, because it does not hold up to the weather as well as cement stucco would, especially on the outside of a dome.

Q: We recently bought 5 acres of land in East Texas, which we plan to build our Earthbag home on. We were wondering if it would be possible to build the home with soil from on-site in the bags and then somehow attach the fiberglass type insulation to the outside. We live in a very hot and humid area, so we are very worried about the insulation and about the potential for mold. Another idea we had was the bags of cellulose they sell at Home Depot. It would be a more green alternative and doesn't seem to be too expensive, but we're not sure how much we would need for a 1,000 sf home? Would it be mixed in the bags with soil or would the entire bag need to be filled with cellulose?

I have seen where you mentioned rice hulls on the FAQ section but have been unable to find anyone who sells this in our area. Any ideas you have would be greatly appreciated! We really feel like the
Earthbag is a very doable method for us, and we love the way they look, but the mold and humidity problems really frighten us. And the cost of cooling the home is very important, as well as the price of the insulation. We are looking for the most budget friendly methods that we can do ourselves, as we are doing everything with cash and don't have much of it!!!

A: Conventional fiberglass batt insulation requires a framed void to attach it to, which is what a typically wood-framed structure provides. It would be hard to make it work with an earthbag wall without adding some sort of framework to attach it to. Cellulose insulation is a somewhat greener alternative, but it also requires some sort of framed space to support it. I am afraid that bags of cellulose would probably compress over time and lose much of their insulation value. Mixing this with soil would not likely add significantly to the insulation factor. Another possible fill material is perlite or vermiculite, but this is likely to be more expensive.

Or, you could make the home with earthbags filled with soil, and then use a layer of rigid insulation on the outside of the wall before plastering...or plaster the whole thing with a thick layer of lightweight concrete...There are many possibilities.

As for mold and humidity, the best thing is to keep the walls breathable, and have a good roof with eaves (if this is the style of building you are doing). If it's a dome, then it may be necessary to have a moisture barrier on the outside, and provide lots of good ventilation within.

Q: My colleagues and I are evaluating earthbags as a way to insulate existing potato storage warehouses. A typical building would be framed by reinforced concrete columns, in-filled either with reinforced concrete panels or blocks of cut natural stone.  My idea is to build a vertical earthbag wall outside the existing wall. The existing roof would be extended outward to protect the new wall. I plan to fill the rice-bags with scoria which is readily available. In general, I will build the wall according to Dr. Geiger's article, "Step by Step Earthbag Construction". Should the earthbag wall be butted against the existing wall, or should I leave an air space?

A: The potato storage buildings that I am familiar with from Idaho are generally substantially underground, with earth-covered roofs over pit storage areas. It sounds like these are not even bermed with earth. No air space is necessary; it will be easier and more secure to butt the bags against the existing wall.

If butted against the existing wall, should I attach the earthbags to it? If so, how?

A few ties to anchor the bag wall, especially near the top, would assure that it will stay in place over time. This could be done with loops of wire running through the masonry wall and around the bags, perhaps at 5 foot intervals.

How tall can I make a vertical earthbag wall?

There is really no limit, especially if anchored to the wall as described above.

Q: Is there a proportion of earth to pumice that allows earthbag construction without additional wood, steel, or bamboo reinforcement? What specific heat and thermal conductivity would this mix have?

A: You can certainly mix pumice with earth, and the folks who wrote "Building with Earthbags" advocate this, but I don't really see any advantage to doing so. This severely limits the insulating qualities of the pumice. Either of these techniques, however, require very little wood, steel or bamboo reinforcement. Usually just some barbed wire placed between the courses of earthbags is all that is needed to make very durable walls.

I do not have technical number for heat and thermal conductivity for these systems. My empirical evaluation of pure pumice is about R-2/inch for the insulative value, and the more earth that is added, the less this number would be.

Q: What do you think of using styrofoam (currently not recyclable in Seattle) as fill or partial fill in earth bags?  I would think the insulation factor would be better and the material could be used for something other than land fill.  I don't know what the structural effect would be of using such a light weight material.

A: Using styrofoam in earthbags would certainly provide good insulation, but it would not provide a load-bearing wall. If you used something like a post-and-beam construction for the framework of a house, this could be in-filled with bags of styrofoam.

Q: If we put 4'' of styrofoam on the outside of the wall, would we need to bother plastering the bags directly, or would we just tack the foam against the bags, cover everything in chicken wire and and plaster over the foam?

A: It is common practice to lay cement-based stucco over insulation panels that have a stucco mesh attached to help with adherence. This should also work with stabilized earthen plasters. You would want to make sure that the panels are well attached to the bags, with wire or twine, or possibly large staples if they hold well enough.

Q: Would it be okay to add rice hulls to the earthbag to add insulation value to the mix or would it cause trouble?

A: Rice hulls do indeed provide a great deal of insulation when used in bags, but I'm not sure about mixing them with other materials, such as soil...this would likely diminish their insulative value considerably.

Q: My husband and I are buying land in Northern Arkansas. They have hot summers that are super humid, and cool winters with barley any snow, but it does get chilly. We are going to be living a travel trailer untill we can get the land ready for homesteading. But we need buildings for animals...nothing huge, just for goats. It must have an angled shed or flat roof. I have looked at several pictures of homes that look like there are beams and then another layer of earth bags put on them and plastered or stuccoed. Is that correct?

A: It is possible to design an insulated shed roof by providing rafters for support, covering this with a heavy wire mesh, and then adding very light-weight bags filled with rice hulls or such for insulation. To then shed the water this would then need to be covered by plastic sheeting that would be protected with plaster (probably with more wire mesh in it). This is all pretty elaborate. For an animal shed I would suggest just using inexpensive corrugated metal for the roof, and then suspending insulating bags beneath it if you want the insulation. See http://earthbagbuilding.com/articles/ceilings.htm

Also for insulation is it possible to cement or stucco the wall, then use cotton blue jean insulation with a layer of chicken wire over it, then stucco that too? That way it's a little warmer?

It might be possible to do this, but I would be concerned that the cotton material would get damp in the process and then remain so, eventually rotting and providing poor insulation. A better solution to make an insulated earthbag wall is to fill the bags with insulating material in the first place. You can use rice hulls, crushed volcanic stone, perlite, vermiculite...

Comment: We're looking at building a tipi with these bags full of rice hulls at Pine Ridge. Here's a picture. This is what the Riceland 50 lb bags are like. They are very much like straw bales, except that the sandbag wrapper makes them even stronger. The size is approx 11 x 20 x 30.

Response: This should certainly simplify the building process since you won't even need to fill the bags! How much do they charge for those bags of rice hulls? Are the bags polypropylene?

C: The bags are a very strong polypropylene, and very well packed, like a straw bale. They are somewhat slick, so a netting would be required for the stucco and plaster. The bags are $6 each, fob Jonesboro Arkansas. They come 16 to a pallet, 48 pallets, 768 blocks to a truckload.

Q: Regarding rice hull homes, does it matter if the hulls are whole or ground? Why? Some processing plants only have ground hulls available.

A: I would think that the hulls would provide better insulation if they are left intact rather than ground, because then they will create more effective air pockets. Also, ground or powdered hulls might tend to hold more moisture, which could lead to rotting and less effective insulation.

Q: I've been searching for some natural alternatives to ceiling insulation. I came across your article about
filling earthbags with rice hulls or vermiculite and am strongly considering using it (or some variation thereof) in my structure. I'll probably be using 2x10 rafters and could get about 9 inches of insulation in there so while still not optimal, it would be nice to be in the R-25 ballpark. I have a couple questions for you however, mainly in terms of air infiltration/circulation in the rafter bays. Using the earthbags will create lots of joints where the bags meet and which will allow air to travel through unless sealed somehow. It seems to me that all the effort and work of insulation will just be compromised by the fact that there is room for air to move, thus reducing the effective insulating value of the roof by a huge amount. Do you have a method for packing in the bags and making sure the gaps are filled as well? What are your thoughts about vermiculite in the ceiling since warm, moist air goes straight up, (I'll probably try to use a reed mat or fabric ceiling covering which will be vapor permeable and vermiculite holds twice it's weight in water? I would think that perlite would be more desirable, since it doesn't absorb water, right? Also, do you know of any good sources of rice hulls up this way?

A: You are probably right that the spaces between bags will allow more heat loss that other areas, but I think that this can be minimized by pushing the bags tightly together. Think of a bunch of pillows being pushed together to form a solid thick blanket; earthbags also have this degree of flexibility, so you can really smoosh them together to form a uniform insulation barrier.

Both rice hulls and vermiculite have been used effectively for many years as loose insulating material in residential situations. Any tendency to absorb moisture will eventually be offset by their ability to release that moisture, as long as the wall or ceiling has some degree of breathability.

Most rice processing in the US is in the southern states, but I know that rice is also grown in California, so that might be your closest source. Some of the processing plants are now marketing bagged rice hulls for mulch and other purposes, so it may be more widely available as time goes on.

Q: Could one mix a 70/30 sand clay mix with rice hulls to provide insulation and not degrade the structural integrity of a building?

A: Rice hulls are supposed to be very good at insulating, but this is primarily because of the all of the trapped air that occurs when they are confined to a bag and plastered over. I'm afraid that if they are mixed with soil, the primary benefit would be diminished considerably because the soil would fill the voids around the hulls, and provide a solid with more the characteristics of thermal mass. It is like the straw in cob or adobe doesn't really provide much worthwhile insulation.

You would be better off using the hulls themselves and then plastering both sides with a stiff plaster with mesh imbedded to create a kind of "structural insulated panel". Other natural insulating materials of mineral origin are crushed volcanic rock and perlite, if either of these are available in your locality.

Q: We are building our earthbag home in Patagonia. We have the perfect soil mix on lour land, so will get a backhoe to level it and dig it up, and then we will use it for our bags. I have decided that I want a light framed wood /tin roof (wood and corrugated tin being the most common building materials produced here in Chile). So, I am wondering if the wood (plywood) and tin of the roof, and paper vapour barrier combined with the the air space between the bags and the roof - a six meter dome with a two tiered roof that falls down to three meters - extends out to at least a meter at bottom - will provide enough insulation for the walls which will be thick not to cool us down in the height of winter? I understand about thermal mass and we're building it into the plans, but what about when there is not enough sun? We cannot get straw here for insulation, pumice is large and we're trying to think of other alternatives. Any ideas?

A: The metal/wood/air space roof over your earthbag dome will provide a little bit of insulation, but not very much. I know that it can get fairly cold in Patagonia, so I would try to get more insulation between your roof and the bags. Is is possible to have the pumice crushed? This can be bagged, like I did with my house, and you could actually build the upper part of your dome this way. Other natural insulation materials that might be available are rice hulls, perlite, vermiculite, wool, cotton, or even papercrete.

Q: I'm considering using slipstraw to insulate the outside of my planned first 8.7' dome here in the UK. Apparently it has been tested to achieve an R value of 1.58 per inch. The benefit of slipstraw to cover a dome as I see it is the fact that you can simply slap it on, not having to worry about trying to strap straw bales to the dome surface. Do you think there would be much cold bridging due to the clay in the mix ? What would happen if it got wet ? Would you advise covering it with a waterproof membrane in our wet climate ? I guess I'd have to wait until it completely dried out before I added a waterproof membrane ? Do you think diluted PVA glue mixed with straw may be a better option?

A: Slipstraw does provide good insulation because it is mostly loose straw with just enough clay added to bind it together. I have never heard of it being used to insulate a dome, and probably for good reason. As you suggest, it would need to be protected from moisture, and this is very difficult with an exposed dome. A moisture barrier that would protect it would also hold in that moisture if it ever did leak, leading to rot and failure. Light straw-clay walls need to be breathable and under roof eaves to work well as exterior walls. The use of PVA would not likely improve the situation.


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