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re: a strawbale doghouse
7 mar 1999
the bubblewall version might have two concentric plastic film
half-cylinders with the gap between them filled with bags of leaves
except for the lower south wall:         
                                   up
                              .  .   .  .           [use courier font]
                         .llll leaf bags llll.
                     .e.lllll.   .   .    .lllll.         ---
                  .c  v2. -->    h is a      .llll.   l are leaves
                .a    .  h  -->    vent        .llll.       in bags 
               .p   .|  |f|        hole         .llll.            
    south    .s   .  |  |o|  4'  v are           .llll.    8'
            .n   . bw|ww|a|        automatic      .llll.  
           .u   .    |  |m|        foundation      .llll.  h are holes
          .s  h <--  |  v3 <--     vents            .tire.   
      ----------0'------~2'------------------------16'-------
 
                   bw is a bubblewall. ww is a waterwall.

tiny cold soap bubbles are almost as good as fiberglass insulation.
this technique dates back to an old swedish patent on insulating double
glazed shop windows at night by filling the cavity with foam. j. groh
at u arizona and otho wells at u new hampshire built greenhouses this
way, and the most recent version (now in its 4th year of operation)
may be a 12,000 ft^2 tomato greenhouse in alberta designed by 
richard nelson of thermatics in montreal.

the bubblewall might look like this from above
                             north
               |             32'?              |
           ---  ------------------------------- 
               |ppppppppppppppppppppppppppppppp|  p is a 2" pvc pipe
           ~2' |                               |    with some holes 
           ---  -------------------------------     in it, sitting in
                                                    a shallow trough of
and like this from the south                        2% firefighting foam
                                                    solution, 200x less
          ---   ^                                   than the foam volume.
               |||
	  ~2'  |s|                                s is a photoelectric
          ---  | |-----------------------------     foam height sensor.
	       |       soap bubble foam        |
     west  4'  |                               |  east
	       |                               |
	       |ppppppppppppppppppppppppppppppp<--v is a shop vac that 
          ---   -------------------------------     can blow air into the
                                                    pipe to make foam.

the photosensor might turn on the shop vac whenever the foam in the
transparent tube at the west end falls below a certain level... 

the bubblewall might be a long bag made from a layer of plastic film
sealed at the top, that looks like this from the east

                                    ..
                                   1..1
                                   x..x
                                   3..x
                                   . .
                                  .  .
                                 .   .
                                .    .
                               .     .
                              .      .
                             .       . ~4'
                            .        .
                           .         .
                          .          .
                         .           .
                        .            .
                       .             .
                      .....      .....
                      |foam.pipe.foam| 
                      |board....board|
                      |--------------| 
                      |   foamboard  |
                       --------------   
                             ~2'
the greenhouse bows on 4' centers might be doubled 1x3s bent to an
8' radius with 1x3 spacer blocks every 2' and deck screws to hold
those curved sandwiches together. the foundation might be 2 east-west
rows of old tires on 4' centers, laid flat on the ground between the
inner and outer films, with an east-west 2x4 bolted through the treads
on each side, and the bow bottoms screwed to the 2x4s. this would make
the gap between the films about 2' near the ground.

home depot sells the $11 leslie-locke afv-1b foundation vents. you can
adjust the minimum soft temperature threshold where the bimetallic spring
opens the louvers with a screwdriver, or take the spring off and reverse
it if you'd like the louvers to close (v3) vs open on temperature rise.

v3 opens when the room is cool to heat the room from the waterwall.
v2 opens when the sunspace is warm to heat the room if needed, ie
when v3 is also open. 

a long version of this structure would have a conductance of about
8pift^2/r48 = 0.5 btu/h-f per linear foot, so keeping it 70 f over a
30 f day requires 24h(70f-30f)0.5 = 480 btu/linear foot. a square foot
of sunspace that gains 900 btu/ft^2/day and loses 6h(100f-30f)1ft^2/r0.8
= 500 btu in january in phila gains 400 btu/day, net, so we need about
1.2 ft^2 of sunspace per linear foot of greenhouse.  

if we make the sunspace cavity about 4' tall, it will receive about
3,600 btu/day, and supply 480 to the structure on an average day. this
leaves 3,120 btu = 6h(t-30f)4ft^2/r0.8 and an average daytime sunspace
temperature t = 134 f. the waterwall might be close to this temperature.

keeping the structure warm for 5 30 f cloudy days in a row with water
cooling from 120 to 80 f means the closet needs 5x480/(120f-80f) = 60
pounds of water per linear foot, about 1 cubic foot of water in a wall
3" thick and 4' tall, or a horizontal 55 gallon drum, or some sealed
containers holding 8 gallons of water stacked up in a wedge that is
wider on the south base, with a vertical foamboard wall on the north. 

the waterwall needs to supply 20 btu/h per linear foot to the structure
at night and on cloudy days. an 8x16" = 0.9 ft^2 foundation vent with
a 4' chimney might allow 16.6x0.9square_root(4'x(80-70f)) = 93 cfm of
airflow and about 93(80f-70f) = 930 btu/h of heatflow, according to
one empirical formula, so one of those v3s every 32' might work. 

the sunspace needs to supply 20 btu/h to the structure during the day,
and that foundation vent might give 16.6x0.9square_root(4'x(134f-70f))
= 236 cfm of airflow and 236(134f-70f) = 15,104 btu/h of heatflow, so
a single v2 sunspace vent might work. 

nick




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