Thermal Characteristics of Rammed Earth
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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.