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re: underground house...
20 jan 2000

>>                iiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiii  p     l  t
>>                     g  l s   pfd ->     -> |     i  o  t  e   
>>                     g  o u    i            |      i  l  i  a  y
>>    g is glazing     g  w n    i            |       i  y  r  f   
>>    i is insulation  g    s    i            |        i     e     p
>>    pfd is a one-way g  m p    i ^  70 f    | 70 f    i  f     b   
>>      plastic film   g  a a    i |  room    | earth*   i  i  n  a  a
>>      damper         g  s c    i            |           i  l  e  g
>>                     g  s e   pfd <-     <- |            i  m  t  s  r
>> ------------------------------------------------------------------
>>         * or maybe steel drums filled with water or soil...

             or maybe stacks of tires loosely packed with dirt and
             covered with plastic trash bags and twine so the tires
             don't smell too much, with a few strings to the roof or
             tomato stakes inside the stacks to help them stand up...

             or maybe just junk, eg engine blocks, which have about
             the same heat capacity as water by volume.

             (what holds up the roof? it might be lightweight, with 2x6s on
             4' centers, sloping slightly upwards to the south, with 2"x4"
             welded wire fencing beneath, and a layer of poly film over that,
             and some bags of dry leaves over that, and typar over that.)

the sunspace glazing might be a single layer of clear 10-year polycarbonate,
which comes in 4' wide rolls and costs about $1.50/ft^2.

or it might be two layers of cloudy 4-year cloudy polyethylene film which
comes in rolls up to 40' wide and costs about 5 cents/ft^2. this could be
inflated with air during the day and tiny cold soap bubbles at night in the
winter (vv in summer) to eliminate the need for the south overhang and
north sunspace wall and dampers.

>the one-way damper is a hinged door that lets hot air
>in during the day, but automatically closes
>using just gravity when all the
>heat is out at night.

it's a dry cleaner bag hung from the top of an opening (maybe 2% of the
solar glazing area) with some staples through a piece of duct tape along
the top edge so the staples don't tear through the bag. the opening also
has some chicken wire or window screen over it to keep the bag from being
sucked through it in the wrong direction. this is known as "the 7 cent
solution," and it was invented in 1973 by doug kelbaugh in princeton. 
these things need to be inspected every week or two to make sure they
haven't gotten stuck, folded, torn, etc.

another alternative is some leslie-locke afv-1b 8x16" automatic foundation
vents, which home depot sells for about $12. they have a coiled bimetallic
spring that opens some louvers on temperature rise. the (soft) temperature
threshold can be adjusted by rotating the spring with a screwdriver, and
you can remove the spring and reinstall it backwards if you would like
the louvers to close on temperature rise. 

another alternative with more precise temperature control is some large
foamboard dampers with honeywell's $50 6161b1000 damper motors (with the
$15 optional limit switch kit) that use only 2 watts when (rarely) moving,
with a $16 5e266 dpst line voltage thermostat from grainger. 

another alternative is heavier and more durable plastic films for one-way
dampers, combined with a couple of grainger's $36 4tm67 4550 cfm 165w
20" window box fans with thermostats.

with enough room temp thermal mass and heat transfer surface, we can use
rarely-operated manual temperature controls. a pa friend with a good solar
underground house gets ready for winter by closing some windows around
thanksgiving to increase the average house temp from 70 to 72 f over
about 2 weeks.  

let's see: a 32' cubical house like this with r24 insulation all round and
no windows or air infiltration has a thermal conductance of 6x32'x32'/r24
= 256 btu/h-f, so it needs 24h(70-30)256 = 246k btu to stay warm on an
average jan day in phila.

if it begins a cloudy week at 70 f and ends at 68 f, it needs 7x246kbtu/2f 
= 1720k btu/f of thermal capacitance. a 2' diameter x 6" tall tire packed
with 1.6 ft^3 of soil at 30 btu/f-ft^3 stores 47 btu/f with pi ft^2 of heat
transfer surface, if the tread is exposed to house air, so we might use
1720k/47 = 18,301 old tires for this project, 286 pickup truck loads at
64 tires per load. the tire disposer might pay $1 each to get rid of them.
but they would occupy 37k ft^3 of living space, and the house only contains
33k ft^3.

we might make the house bigger, since the cube's volume grows faster than 
the exterior surface. a 48' cube needs 3871k btu for a cloudy week, ie
41k tires with a volume of 82k ft^3, leaving 3,500 ft^2 of living space.
this does not seem promising...


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