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Cob in maritime and cold climatesDavid DeFauw davidde at hevanet.comMon Oct 13 07:36:54 CDT 1997
Hi everyone in cob world, Thank you Shannon for setting up this list. Shannon and I attended the same workshop in the basics of cob, two years ago in Cottage Grove, Oregon, from Michael Smith, Linda Smiley, and Ianto Evans. That was a wonderful experience that I would recommend to anyone. Yet, I still have some questions about whether cob or other earthen buildings are appropriate in cool and cloudy places such as my own Oregon, or Washington, Britain, Ireland, France, and other colder climates. I would truly appreciate some feedback, especially our friends in England that have lived in cob houses, or anyone that has access to research on the thermal properties of cob. Let's look at some of the advantages of building with cob: 1. It is a wonderful medium to work with. The feeling of using mother earth to make a non toxic home is wonderful. The process feels spiritual rather than industrial or like a construction site. 2. Earth for cob is available on site in most places. It takes almost no petro-energy to manufacture. 3. It has proven to be a truly lasting material even in rainy Wales. 4. It has a large amount of thermal mass which evens out daily temperature fluctuations. 5. Very beautiful houses can be made from it. The main disadvantage is its lack of isulative value. In the archives (9608) of this coblist is a message from Simon Randell. He states that he had just finished testing the thermal properties of a cob cottage in the UK. His results found a U' value of 1.0 W/m2degC at a density of 1860 kg/m3 over a duration of 29 days. An architect friend translated this out of metric for me as U' = .176 Btu/ft2/hr/F, which is the same as an R-value of 5.667 for the wall. This sounds quite bad when compared to conventional construction, where walls are about R-20, and straw bale walls, that are about R-40. But this is not the whole story. The book "Adobe and Rammed Earth Buildings" by Paul McHenry has a discussion of the thermal properties of earthen walls. It showed a U' value for a 14 inch adobe wall at.203 Btu/ft2/hr/F, which is slightly less insulative than the cob wall tested by Simon Randell. Yet, McHenry goes on to state, that there was a broad feeling that adobe houses are quite comfortable and energy efficient, and therefore further tests were required. The tests were done at the U of New Mexico to determine the effective U' value for earthen walls taking into account the properties of thermal mass and solar gain. Those tests found that the thermal mass and solar radiation had striking effects on the performance of the walls. Thermal mass was found not to increase the thermal efficiency of the walls, but still added to a houses comfort by moderating interior temperatures. Thus, if the the outdoor temperature ranges from a high of 90F, and a low of 50F, as might be common in New Mexico, after several days the wall would obtain a steady state temperature of a comfortable 70F, or the average daily temperature. Additionally, the tests showed that earthen walls act as solar collectors. The sun heats the wall, especially if it is a dark color, and this mitigates some of the thermal loss during winter. Based on these tests, the state of New Mexico adopted an energy code that took into account whether a wall was dark or light in color, and whether the wall faced North, South, East, or West. Thus a south facing, dark colored, 14 inch adobe wall, has a effective U' value of .040 which is the same as R-25. This information is all from McHenry's book. The conclusion, at least for New Mexico, is that earthen buildings are comfortable because they even out large temperature fluctuations and take advantage of solar gain. It does not seem that this would be true in a place like Oregon. In January and February our typical day has a high of 44F and a low of 40F with a pea soup cloudiness and drizzle. In such a cloudy environment, it seems the solar advantage of earthen walls would be small. Additionally, after a few months of 40F degree weather, the wall, it seems, could become very cold. Forty-two degree walls: think refrigerator. Yet, I have heard that in equally soggy places, such as England, cob houses are comfortable and highly valued. I know they are beautiful. Does it really cost less to heat such a building? Who has lived in one? What are the heating costs? How thick are your walls? Do you heat your home to the 68F that is common in the USA? I would appreciate your feedback. I would rather figure this out before I build something than freeze, and wish I had built out of straw bales with the cob on the interior. Hope the numbers I have quoted are not overwhelming. Thank You, David DeFauw
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