Cob in maritime and cold climates

[John Schinnerer]

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.  

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' = .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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