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Cob: Thermal properties of cobShannon C. Dealy dealy at deatech.com
Mon Jun 12 12:11:39 PDT 2000
On Sun, 11 Jun 2000, W wrote: > Poor Insulator? > uh oh. I was under the impression that cob was second only to straw > bale in insulative properties. (with cob walls being the same 16-20 > inches thickness of bales)? [snip] Nope, while there has been quite a bit of debate over how much insulation cob provides, I know of no one who considers it to be a really good insulator. This does not mean however that it is necessarily a poor choice for any climate. Cob provides excellent thermal mass, and at least some insulation value (depending on your mix and who you believe a 16-20 inch thick wall is probably somewhere between R-8 and R-18. It is important to remember that in many if not most climates, the insulation value of cob will be far less important than the thermal mass properties for most of the year. Thermal mass acts as a stabilizer for temperature swings, if the outside temperature reaches 100 degrees F for one hour each day for a week or more, but the average temperature for each 24 hour period is only 75 degrees, then your cob walls will tend to hold the temperature of your house to something near that 75 degree average. This can of course be altered (for good or bad) by the use of large areas of glass facing the sun, trees for shading the house, open/closed vents or windows, etc.. These items will alter the average temperature in and around your house, but the cob will still hold your interior temperature somewhere near that average, rather than letting it go to the temperature extreme. The same holds true for cold climates - the cob will act to average the temperature swings, and by careful use of glass facing the sun, shade trees, barriers to block high winds, etc.. It is possible to build a cob house which will require little or no additional heating/cooling (depending on your personal temperature requirements) year round in a fairly wide range of climates. The area in which cob generally will have a problem with temperature extremes (that cannot be dealt with just by using passive solar designs) is places where it gets cold and stays that way for many days or weeks at a time. For climates like this, I would recommend a layer of good insulation (possibly straw bale) wrapped around the exterior of your cob building. This will make for very thick walls, but with the cob walls inside the insulation, your interior temperatures will be very stable, and in the event of a failure of your heating system, it could take several days before the interior temperature drops enough to be a significant problem, by which time you would hopefully have it fixed. Note: rereading the above, I may not have made it clear, while cob evens out the temperature swings, it is over a period of several days, not just the swing for one day, the more cob (or other thermal mass) in your structure, the longer the averaging period. Shannon C. Dealy | DeaTech Research Inc. dealy at deatech.com | - Custom Software Development - | Embedded Systems, Real-time, Device Drivers Phone: (800) 467-5820 | Networking, Scientific & Engineering Applications or: (541) 451-5177 | www.deatech.com
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