Thermal Characteristics of Rammed Earth

Clifton Schooley is a green building professional specializing in insulated rammed earth construction and architectural design in Canada and worldwide. His vision is to create environments that are beautiful, artistic, sustainable and have a positive social impact. Clifton believes that both design and construction of a building must be intimately connected to bring maximum benefit to both people and the environment. Beyond rammed earth his ambition is to become an eco-developer and influence society on a larger scale. For more information, visit:  www.rammedearth.info

Questions and Answers

Q: We are promoting uses of sediment and soils as part of our research, education and project-guidance on how to plan and manage large-volume materials. In our Iowa Initiative, particularly, we are looking at uses of uncontaminated lake sediment and "spent" soils, and want to work with the best people in the world to plan and manage home and building construction. One of our problems is different messages from different groups. The machinery companies for compressing blocks are hard to believe because of their own promotions. But experts we have found differ, too. For example some say that rammed earth homes do not cool down in the summer when temperatures have been low for a long time and then rise fairly quickly. Others talk of freeze-thaw in prolonged winters. Can you help us find objective information, as we move forward in many places, but with particular emphasis on model projects in Iowa and Massachusetts?

A: (Bruce McHenry) I am not a soil scientist, but you might try to do some basic testing on the soils from the lake. A shrink box is effective to judge the expansion of the the clay, and there is a three tube testing kit that allows you to judge the quantity of silt sand and clay. I am not sure of the name of the scientific testing kit, but I am sure it is available from a soil testing lab supply. I am confused about the the word spent soils? The pressed block manufacturers are somewhat vague about the techniques in soil selection and it seems to me that it is more a case of experience than an exact recipe.

The thermal characteristics of a rammed earth wall are not as good as adobe. The optimal wall thickness of adobe is 10" here in the southwest. With rammed earth being either 18" or 24" this could effect the thermal transmission of the material. It also would have to be integrated into the latitude/ number of days of sun/ altitude. As far as freeze thaw cycling, it is important to have the stem of the foundation well above the point of were sitting snow and moisture might rise into the wall. One last consideration would be thicker wall in cold climates take more energy to (warm up), but the transmission of the severe outside cold swings would be less than of conventional buildings. My suggestion would be to build a small to scale structure and do basic temperature measurements using an inexpensive Radio Shack remote monitoring thermometer.

Q : For the rammed earth walls (2.5 m high, tapered w/ bases over 1 meter thick), do you think it's possible to add a layer of styrofoam that has wire attached to it (to make the styrofoam composite w/ the earth wall) during the ramming construction process, or will that compromise the structural integrity of the wall?

A : I presume that you are talking about adding a layer of styrofoam "inside" the earthen wall...?? I have seen this done on a couple of occasions, but I really don't have an opinion one way or the other... What I do know is that it's difficult to keep the styrofoam in place when you are ramming, and it makes ramming quite a bit slower and more difficult. If I was going to use styrofoam in conjunction with a rammed earth wall, I'd prefer to place it on the outside of the wall. That way it can just sit just inside the forms while the wall is being compacted. Of course, the styrofoam will have to be plastered.

Q: I am an Architecture student at NDSU in Fargo, North Dakota. I am designing a Furniture Design school and I want to use rammed earth as my load bearing structure. What methods of insulation would possibly work in our climate range of -40 degrees F in the winters and 100 degrees in the summer? If some of the walls were below the ground, do you think that insulation would be necessary? The frost line around here is about 5 feet deep?

A: The North Dakota climate you write about is not a lot different than some Colorado mountain locations where rammed earth and other earthen houses have been built.  It would be possible to insulate a rammed earth house on the exterior with various types of foam board insulation.  However, a better method might be to berm up around the rammed earth with local soil so that the thermal mass of the rammed earth is thermally protected from the very cold winter weather.  This would allow the mass of the rammed earth to hold heat better and to provide it to the interior of the house when needed.

Rammed earth is not recommended as a foundation material for a number of reasons.  Rammed earth walls should be built on top of concrete footers or other very solid foundations. I'd suggest that you look into a technique called the Shallow Frost-Protected Foundation - which was developed in Scandinavia to provide frost protection to shallow on-grade foundations similar to the sort of foundations normally used by rammed earth walls.

Q: Would it be possible to incorporate a PEX/solar- hydronic heating system into the rammed earth wall using a solar bulk water storage system and a photovoltaic powered pump to circulate the warm water throughout the house just like in a poured floor?

A: (Paul Shippee) Well, what is your climate? You can embed pex into rammed earth, yes . I have pex in the plaster on straw bale wall, and other installers have put it between sheet rock framed wall and an interior adobe layer wall successfully.

What is the insulation in your rammed earth wall -this question depends on the climate question above? Earth is both a conductor of heat and a capacitor/insulator of heat (and cool). Also it might depend somewhat on your rammed earth recipe?

Why do you need the solar bulk water storage? Why not just dump all the solar heat straight into the wall? If it's deep or wide enough. When you have a large area, like a floor or many walls to store & distribute solar heat, then temps will be moderate enough, usually.

C: Our Climate is similar to Santa Fe , NM . Temps vary from 10F to 115F.  Summer monsoons and occasional winter snow. Our soil formula will contain a new soil stabilizer product we distribute for mine and forest service roads that makes the compacted road almost as hard as concrete and water-proof.  I will use the natural soil as the interior wall surface.

The bulk storage will be to provide pre-heated  water to the water heater and the radiant floor.  Also, it shall provide added heat to the green house in winter. I am working on a parabolic trough/steam system to generate electricity to the house, water well and grounds as well as the root cellar that will be dug into the hillside.

Q: After a few years of disillusionment in a conventional architecture practice I find myself back in graduate school trying to develop projects around earthen construction. One of the projects I'm working with (multi-family residential in St Louis, MO) proposes a rammed earth structure. Can straw bales be used directly against rammed earth on the outside of walls (in non-load bearing capacity) to boost R-value assuming they are finished with earthen plaster & lime wash? Have you heard of anyone using this combination? Also, assuming proper overhangs and grade separation for moisture issues.

A (Paul Shippee): Yes, this sounds like an excellent idea: thermal mass on the inside, super insulation on the outside. I have heard and seen interior adobe and other thermal materials - I don't see there'd be a problem with them close like you propose, as long as moisture intrusion (from outside or inside) and rodent access are addressed.

Q: I am designing a house in Estes Park, Colorado. I would like to use rammed earth for its thermal mass capabilities as the shell of the building. Also as an experiential entrance sequence, I added rammed earth walls that project outwards to the exterior and then become a load bearing wall for the second floor on the interior of the building. Since the wall goes inside and outside, does there need to be a thermal break in the wall at this point. Ideally the wall is one continuous surface from outside to inside, but if there needs to be a thermal break what material do you suggest for this case.

A: Since you are in Colorado I would recommend a thermal break in the wall. You could use 4" polyiso or at the minimum 2.6". If you are using structural glass up against the side of the wall then you would want to bring the insulation about 1" or so from the surface of the wall and make the insulation tapered, this helps for wall integrity at the thinnest point and makes sure of proper compaction in a tight spot. If energy is the primary concern over aesthetics then bring the insulation to the edge of the wall and cover it with a window or door buck or your exterior wall.

Q: I am planning to make a just a room (20 X 16 feet) using rammed earth. This is in Sri Lanka where the temperature remains pretty constant around 30 C. Basically it is to keep it as cool as possible. The room is underneath complete shade. I am hoping 12 inch thick walls to be sufficient for strength (only 8 feet tall). But will it be better to make it 18 inches thick for the purpose of keeping inside cool ?

A: I think it would be best to go with 18" walls for structural as well as heat considerations. A 12" wall may be sufficient for both heat and structural if the quality of the wall is good and the conditions are right (shading, the path of the sun etc.) but the extra wall width is a good idea.

Q: What options are available for insulating a rammed earth wall? How would they affect the "breathability" of the wall? Could you tell me where I could find more information about it? What do you think about the idea of building a pumicecrete wall on the outside of a rammed earth wall to increase the insulation value of the wall?

A: Suitable insulation needs to be able to withstand the pressure of compaction during the wall building process. You can use Polyiso, Rocksul or Polystyrene to insulate with. Insulation will not affect the breathability of the wall, that is
more determined by the sealer and amount of sealer applications. Pumicecrete will not meet building code for r-value requirements. It would add an extra stage to the wall building process in addition to needing plaster finishing. Covering up a rammed earth wall is a beautiful thing to cover up.