Cob - R of dirt / Earthships

[M J Epko]

At 01:26 PM 12/2/97 -0600, you wrote:
>Did 18 inches of dirt with polystyrene have much effect, then?

	It was 97.6% as effective as 9.8 feet of dirt. In our climate, anyway,
next-state neighbor: specifically, the Minneapolis/St Paul, Minnesota
climate. 9.8 feet of dirt was slightly more effective than 3.9 inches of
polystyrene and 18 inches of dirt.

	Which is in no way intended to decry the goodness of earth. I've only
addressed thermal resistance, and not mass, which is trickier. I'm still
trying to figure out both.

	Another study in that book pitted a precast concrete roof that had 4
inches of high-density polystyrene and 18 inches of dirt versus a precast
roof that had 4.6 inches of polystyrene and no earth. Using January for the
control month, the diurnal extremes averaged from a low of 5F (-15C) to a
high of 15F (-9.4C) for 15 days; then a cold front came through for 5 days
with the average high and low being -5F (-20.6C) and 5F (-15C), after which
the previous average highs and lows resumed. 

	The response of the no-earth roof was almost immediate when the cold front
hit, and again when it left. The roof with earth plus foam didn't respond
to the cold front for a full day after it hit (the trade-off being that it
took a full day after the cold front left to stop losing heat, because the
dirt was colder than the air until that point).

	The roof with the earth (plus foam) needed 8% less energy to cope with the
heat-loss effects of the cold front... but more significantly, the no-earth
roof, because of its low thermal mass, required a whopping 196% more
heating energy *during* the cold snap to maintain an even temperature. So
the net heating energy requirements were close to the same (only 8%
different), but the energy requirements for the no-earth roof were
concentrated. The mass effect of the earth moderated the swings. It didn't
eliminate the swing, but did reduce it overall by 15%. (The heating energy
requirements weren't reduced by that amount because additional energy is
required to heat the mass back up. That's a complaint some people have
against masonry heaters: they're not *immediate*.)

	Which is why my thoughts are (for this climate!) to build a SB house with
a cob interior. Insulated mass, the best of both cob and SB. I like the
look of SB, but am intrigued by the sculptability of cob.

>How are those jim-dandy earthships insulated, if auxiliary methods are 
>used to supplement the earth sheltering?

	A quick dig into Volume 1 has the (brilliant, IMO) Mr Reynolds saying, "At
a four foot depth, the temperature is usually between 55 and 60F..." which
contrasts against that Other Book I keep referencing, which shows that (up
here) at *ten* feet the temperature swings between 40 and 60F. For better
or worse, New Mexico ain't Minnesota or Wisconsin. But he does address
this: for Cold Climates, he says, "For extreme cold, the depths, widths and
heights of the spaces should be decreased to increase the mass relative to
the air volume... the building should be submerged into earth as much as
possible."

	Still quoting Earthship Vol 1, "Since most heat would be lost through the
roof of the structure, it should have a minimum [that word is underlined]
R-60. 8 inches of foam insulation board, plus the added insulative
qualities of the deck and roofing materials, will be sufficient." It's the
same dog.



   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      M J Epko        duckchow at ix.netcom.com
      almost Wyoming, north of Nebraska, USA
               by way of New Mexico
      (not soon enough) - for now, Minnesota
   ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

   The tongue of man is a twisty thing, 
   there are plenty of words there of every kind.
                               - Homer, The Iliad