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re: sod on a shed/pole roof--will it work???
20 dec 2002
coyotefred  wrote:

>> >this upcoming year i'm planning to build a small (16' wide x 12' deep)
>> >passive solar 'cabin'...

a rarely-used alaskan drumstove could extend a delightful drumshack motif.

>...i was just figuring on around 20% glazing for direct gain based
>on the floor area, and this front wall would be more than enough for
>that glazing (about 38 sq ft).  i'd really like to keep it simple with
>a direct gain system at this point.

some people call these "direct loss systems." 

>> >...finished with cement stucco).
>> 
>> will it stick?
 
>most folks put up some chicken wire.  one question i didn't raise was
>how to attached the blueboard to the earthbag wall (which i know
>several folks have done with their earthbag homes).

crazy glue and velcro?

>one person suggested using some stainless steel wire laid in between the
>layers of earthbags, which could then be poked through the blueboard and
>twisted to not only secure the blueboard to the bags, but to attached
>your chicken wire for the stucco as well... any thoughts?

you might put a horizontal board (deadman) on the ground under the lower
drum of a drumshack wall, another between the upper and lower drums, and
another on top of the upper drum, and screw the blueboard to the deadmen. 
the 2 drums would be 6' tall. you could screw some 8' vertical 2x4s to the
deadmen to support the roof, and put a few dramatic clerestory windows
between the insulation (eg bags of leaves) in the upper 2' perimeter gap.
the 104x55x8 = 45,760 pounds of water in the drums could act as a ballast
foundation and keep the cabin from blowing away in great winds. you could
squeeze more bags of leaves in the triangular spaces between the drums
and the blueboard for additional insulation.

drums are hard to transport, even empty. they don't nest, and take up lots
of room. you might use 1,430 recycled 4 gallon ropak 13" tall x 11" square
plastic tubs with tight-fitting lids instead. i got about 800 from an exotic
bird-food manufacturer who stacked them 22-high in his barn while they were
still full of dried cherries. screwing blueboard to waterwalls would be easy. 

>> >...two large (4' x 5') windows for the direct solar gain...

>> ah. dimensions... 40 ft^2 of r2 south windows with 80% solar transmission? 
>> and the weather? would 1000 btu/ft^2 fall on a south wall on an average 30 f
>> january day, as in phila? you might check out "ohm's law for heatflow."
>
>i'm not familiar with this.

btu/h = (t1-t2)a/r, where the ts are (f) temps, a is a surface in ft^2,
and r is the us r-value stamped on the blueboard, eg r10 for a 2" board. 
january is the hardest month for solar house heating in north platte, ne,
with 1280 btu/h-ft^2 of sun on a south wall on an average 21.6 f day with
an average daily high of 34.6. 

>i'm not too concerned with whether this
>ends up being the perfect thermal unit.

well then, it won't.

>even if it works marginally well i'll be satisfied.

you probably won't be satisfied either...

>...i wasn't aware of these 'barrel shacks.'

very exciting. just thinking about them makes me want to build one.

>> a 55 gallon drum is about 3' tall and 2' in diameter, with about 25 ft^2
>> of surface and 450 btu/f of thermal mass if filled with water...

the 1982 sunset solar remodeling book has some "thermal furniture" plans...

    the ubiquitous water-filled drums characteristic of pioneering solar
    designs have never gained much favor with the decor-conscious, but
    thermal furniture like the bench shown at right can provide water-
    based thermal storage without creating an eyesore...
 
architects say "hide it or feature it." if it's really big, paint it red.

>that is an intriguing thought (the 55 gallon drums).  would you
>actually use these as structural load-bearing exterior walls...

sure, esp the steel ones. the sunset book suggests rustproof paint inside
and a teaspoon of sodium sulfite per gallon to remove oxygen from the water.
charlie wing just recommends adding a quart of motor oil on top.

>you talking about placing them inside earthbag walls?

no. bag the earthbags.

>i'd also want to finish them on the interior...getting stucco to stick
>to these would be tough i would think...

drywall over deadmen, altho this would ruin the delightful drum motif.

>> >...i'd like to have a sod roof, both for aesthetic and functional
>> >reasons (slightly more insulation value and evaporative cooling benefit
>> >during the summer)...i'm a little concerned about whether the roof
>> >i've described would be sufficiently strong to hold a sod roof.
>> 
>> you  have  left out a few details. you may not need evaporative cooling if
>> your cabin has enough thermal mass and insulation and night ventilation.
>
>very true.

i'd make the roof plywood over 2x6s on 2' centers, with epdm over that.
a flat roof, with 6" fiberglass insulation between the rafters.

>> >i would also buttress the side and front walls a little (as earthbag
>> >builders suggest when you build a vertical, rather than domed wall). 
>> 
>> domed sounds better. maybe a simple timber dome covered with chicken wire,
>> then poly film, then a tire hairnet filled with soil for mass, then more
>> poly film, another tire hairnet filled with leaves for insulation, more
>> poly film, and another tire hairnet filled with peat moss to provide a home
>> for suitable volunteer roof vegetation. 
>> 
>> >'any ideas/thoughts/suggestions greatly appreciated!
>> 
>> how about a barrel shack with a shallow 12' wide roofpond made from a
>> single 20' wide piece of epdm rubber? you might add a solar pool cover
>> in cold weather.

a 2" pond would weigh about 12 psf, so 12' beams on 2' centers would support 
288 pounds. m = wl/8 = 288x12x12/8 = 5184 in-lb and s = m/1000 = 5.184 in^4
makes the beam depth d = sqrt(6x5.184/1.5) = 4.55". 

>> make the whole south wall a shallow poly film sunspace over blueboard...

with a shallow epdm pond in front. in north platte, a square foot of r1
sunspace glazing with 90% solar transmission would collect 0.9x1.3x1280
= 1498 btu and lose about 6h(80-28.1)1ft^2/r1 = 311 on an average january
day, for a net gain of 8x16x(1498-311) = 152k btu. with 52 drums stacked
2-high and 12'x16' id and 16x20' od and 896 ft^2 of exterior surface and
lots of thermal mass inside, 24h(65-21.6)896/r = 152k in january in north
platte makes r = 6.1, if we ignore the warmer roof and the warmer south
wall during the day.
 
with 1.5" (r7.5) blueboard walls and ceiling, rc = 7.5/896x104x450 = 392
hours. if the cabin were 75 f at the end of a long string of average days,
it would cool to 30+(75-21.6)e^(-5x24/392) = 69.3 f after 5 21.6 f cloudy
days in a row. 
 
the 25x104 = 2600 ft^2 of drum surface would have about 3900 btu/h-f of
conductance to room air. the drums need to store 18h/24hx152k/6h = 19k btu/h
of overnight heat from warm sunspace air on an average day, which would
raise the room air temp by 19k/2600 = 7.3 f. the drum water temp would
increase by 2.5 f. at night, we would have something like this:

               t
       1/3900  |  7.5/896
   75 ---www---*---www--- 21.6    i = (65-21.6)/0.00863 = 5031 btu/h, 
           -------->
               i               so t = 75-5031/3900 = 73.7 f.
 
>i'm intrigued by the barrel shack idea, but i'd like to stay away from
>the water roof business.  that's a little too ambitious for me for now.

just a perimeter 2x4 on edge, with the single 18'x20' piece of epdm rubber
draped over that, and a solar pool cover in the winter.

>and i'd like to stick with traditional glass glazing for the front wall,
>since there is a nice view to the south that i'd like to be able to take
>in from inside the structure :)

at night? how about a nice 8' tall x 16' long quarter-culindrical sunspace?
one layer of clear polycarbonate plastic stretched over double 1x3 bows on
2' centers would add 2/3 to the daytime floorspace... 

nick




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