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The Work of Art and The Art of Work
Kiko Denzer on Art
Cob in maritime and cold climatesJohn Schinnerer jschinnerer at seattle.usweb.com
Wed Oct 22 10:07:44 PDT 1997
------ =_NextPart_000_01BCDED2.57B04700 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: quoted-printable Aloha, Thanks for the detailed info on thermal testing of earthen walls. It's = good to know some folks are working on that sort of thing. It's = important to consider how thermal mass and insulation differ and what = the consequences of those differences are. Trying to compare them = directly is like comparing papayas and starfruit (or apples and = potatoes, or whatever...). Insulation values are a product of the = insulation manufacturing industry, and have therefore become far more = important than they deserve to be, especially since conventional = construction does not involve any thermal mass either. Here's a shot at = an explanation... Regardless of what you build out of, you will still have to heat your = home during the chilly season, correct? If you build with a thermal = mass material, it will absorb some portion of the heat over time and = re-radiate it over time when you stop heating. This is the same effect = as when the sun is heating the house, except the heat source is whatever = you are using inside your dwelling. It is also the same effect as when = you heat a cast-iron pan - it heats up slowly, and stays hot for a long = time after you turn off the heat. If you build with very little thermal = mass (i.e. straw bale, light clay, stick-framed, etc.) then the dwelling = will absorb almost no heat and will therefore have almost no heat to = re-radiate when you stop heating. This is more like, say, an aluminum = pan - heats up quick, cools off quick (especially compared to cast = iron). With lots of insulation but little thermal mass, it stays warm inside as = long as cold from outside doesn't enter (i.e. opening a door, window, = etc. or just ordinary ventilation needs). There is very little stored = heat. With lots of thermal mass and little insulation, it will stay = warm as long as the thermal mass is still warm enough to keep it that = way - the mass will cool down slowly as it releases stored heat to the = air. Occasional blasts of cold air (going out to play in the snow, = etc.) will have less of an immediate effect on the inside temperature, = because the heat-storing capacity of the earthen walls are much, much = greater than the heat storage capacity of the air. Wood stoves/fires, warm thermal mass walls, piles of hot rocks, the sun, = etc. are all radiant heat sources, which are apparently the most = 'comfortable' to humans (makes sense, since the sun heats us that = way...the original passive solar!). The 'industrial' version of this, = which is (I've read) the most efficient way to heat a conventional = house, is in-floor hot-water systems. =20 Radiant heating, such as thermal mass walls (and floors!) re-radiating = stored heat (from passive solar gain in sunny times and from whatever = heat source one uses in chilly times) makes humans feel more comfortable = at lower temperatures than, for example, forced air heating, which only = warms the air and also creates air currents that cool us at the same = time we are trying to heat us...! You may notice that a lot of the cob = designs run a stove pipe through quite a bit of cob thermal mass (to = absorb more heat) before finally giving up the remaining heat to the = outside air. Hopefully this helps illustrate the different basic natures of thermal = mass and insulation...the study that "adjusted" R-values for adobe by = accounting for solar exposure, color, etc, is interesting. However, I = feel they are trying to 'convert' apples into oranges, driven by the = dominance of 'R-values' in the building industry. What is really needed = is a wider understanding of what thermal mass is and how it works. I = reckon those old adobe builders knew... Cob could be combined with insulative materials. The insulative = materials would ideally be at or near the outside of the cob wall, = putting the thermal mass "inside." I have seen several designs where = the north wall is built of straw bale and the rest of the structure is = cob, thus insulating against heat loss on the coldest exposure and still = having lots of thermal mass. Cheers, John Schinnerer -----Original Message----- From: David DeFauw [SMTP:davidde at hevanet.com] Sent: Monday, October 13, 1997 5:37 AM To: coblist at deatech.com Subject: Cob in maritime and cold climates 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' =3D .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.=20 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 ------ =_NextPart_000_01BCDED2.57B04700 MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit A non-text attachment was scrubbed... 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