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re: a roofpond house (updated aug 6)
9 aug 1996
rich lobrovich   wrote:

>nick thanks for the great article on roof ponds i have not seen that much 
>effort put into a subject for a long time.

glad you liked it, rich... now if we could just  build  one :-)

>from what i remember many years ago , the research was based on
>cooling the house by water on the roof; absorbing the heat from the 
>interior space during the day.

that sounds like harold hay's work, with motorized insulating shutters
over the pond. this would be less ambitious, and use a little more energy
for cooling, with permanent insulation under the pond, pumping some water
up through the pond at night for cooling, and using a fan coil unit (like
an auto radiator) inside the house. 

>at night the primary heat of rejection 
>for the roof pond was radiation to the dark body of space.

i suppose one might lose more heat that way than by evaporation, but it
seems unlikely to me that the pond could get much cooler than the wet bulb
temp of the surrounding air, when condensation would begin.
 
>please comment on this for general discussion.

ok. here are some crude thoughts on how to actually make a roof pond,
perhaps on top of a strawbale house...

one way to make a reliably leak-free roof pond might be to use two layers
of epdm rubber instead of one, with a spacer between that lets water flow,
eg a layer of greenhouse shadecloth. and build the pools in 8' wide strip
compartments, with a 2x4 to make a standing seam screwed to the edge of one
piece of osb from below, and an aluminum cap strip to hold the two pieces of
rubber on to the 2x4, with a 2x4 or 2x6 rafter underneath as well, screwed
to both pieces of osb, and 3 1/2" of insulation with a vapor barrier under 
that, perhaps like this:

       | <--         8'              --> |           the rubber comes in 10' 
     al                                al            wide rolls. i don't like
    rrrrr                            rrrrr           wasting 2' of the width 
    rrrrr                            rrrrr           this way...
    rrrrr         w a t e r          rrrrr
    rrrrr                            rrrrr
  rrrr2rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr2rrrrrrr
     rxr      s h a d e c l o t h     rxr                
 rrrrr4rrrrrrrrrrrrrrrrrrrrrrrrrrrrrrrr4rrrrrrrrrr      
------- ---------------------osb-------- --------------
       2   fiberglas   2                2                 
       x   insulation  x                x             
       4               4                4               
               polyethylene film                                 
-------------------------------------------------------
    flat 1 x 3s on 2' centers, perpendicular to 2x4s
-------------------------------------------------------

then make all the underlayers of rubber drain to one monitoring point,
and if a leak is ever detected, figure out which pond leaked, drain it,
unscrew it, lift it up, inspect it and patch or replace it. 

there is a nice inexpensive prefab/prestressed concrete section that could
probably hold up strawbales on top for roof insulation, under a pond...

                8'?
 ------------------------------------ 
|                          2"?       |      24' long?
 ---------------    -----------------
                |  |
		|  |  3"?
		 --

this is probably stronger than needed for most house roofs. perhaps it
should be cast in place, and thinner.

the main drain might just be an overflow onto a sheet of rubber over more
osb, a continuation of the roof over the north side, down to a cistern made
of more epdm along the north side of the house. that overhang might cover
the cistern, and form the north weather wall of the house, like this: 

                 main drain -->
           .............
 sunspace. .           ..
        .  .   house   . .
       .   .           .  .
 . ref.    .           .   .
 .pool..................   .......
                       ..... <--cistern, 10' wide x 4' deep x 32' long?
                                         about 10k gallons...

the underlayers might terminate in lips that come over the house wall
in the sunspace, so if there's a leak we can see it running down and
dripping off an underlayer of rubber, and tell which section is leaking.
the north edge of the underlayers would go up 2"...

the insulation under the roof needs a vapor barrier, and some way to breathe
to the outside a little along the south wall, and a tiny air gap between it
and the osb.

i suppose strawbale walls might be built like this: 

   no plaster on this surface
        cwcwcwcwcwcwcwcw
        w ------------ c
	c.            .w
	w.            .c
	c.            .w
	w.            .
	 .            .
          ------------ 

ie, drape 2" chicken wire over 10% (?) of the bales, before plastering.
the inside plaster might be less permeable to water than the outside.
more stearic acid or bitumin in it?

and a roof pond might look like this exploded view:

        .                    water                  . epdm
                  .                         .
                          .        .

        .                                           . cement
                  .                         .
                          .        .

        .                                           . burlap
                  .                         .
                          .        .
        .                                           . chicken wire
                  .                         .
                          .        .
       ---                                         ---
      | 2 |                                       | 2 |
      | x |                                       | x |
      | 4 |                                       | 4 |
       ---                                         ---
        .                                           . straw
                  .                         .
                          .        .
        .                                           . poly film
                  .                         .
                          .        .
        .                                           . chicken wire
                  .                         .
                          .        .

or more simply:


        .                    water                  . epdm
                  .                         .
                          .        .

        .                                           . cement
                  .                         .
                          .        .

                      6" of loose straw 
        .                                           . chicken wire
                  .                         .
                          .        .
        .                                           . poly film, painted white?
                  .                         .
                          .        .
       ---                                         ---
      | 2 |                                       | 2 |
      | x |                                       | x |
      | 4 |                                       | 4 |
       ---                                         ---

this version would not have as much insulation around the 2x4's, but
that wouldn't matter much if they were 10' apart... we'd probably want
some compressive 10' bracing between the 2x4s until the cement sets,
to keep them from sliding together... 

how about this for a foundation on grade?

             --------------------------------------
				   sill            .
	unvented crawl space	   cinder <--4'--> . <--airtight skirt
	over vapor barrier         block           .
        ------------------------------------------------------------
                                     ^
				     |
                                     the ground should not freeze
                                     under here, if it is kept dry.

this would work even better with an 8' cantilever, or a strawbale
inside the skirt...

nick




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