Subject: a roofpond house (updated Aug 6)

From: [email-masked] (Nick Pine)
Date: 6 Aug 1996 09:38:59 -0400

Medford, OR, doesn't have a lot of sun in the winter.
Here are some NREL climatic average data... 

          24 hour   daytime  nightime  average sun on  
          avg temp  high     low       s wall  horiz surface

December      38 F  44 F     31 F      550     390 Btu/ft^2/day
July          73    91       55        1020    2450

In July, the average windspeed is 6 mph, and the average humidity ratio
of the air is W = 0.0076 #water/#dry air. The elevation is 1299', and 
the air pressure is 14.1 psia, vs 14.7 at sea level.

A 2-story 2000 ft^2 house in Medford with 3000 ft^2 of walls and ceiling with
an average US R-value of 10 might need 3000 ft^2/R10 = 300 Btu/hr-F, ie
(91 F - 75 F) x 300 Btu/hr-F = 4800 Btu/hour of cooling when the house is 75 F
inside and it's 91 F outside in July. Over the 4600 heating degree day winter,
it might need 24 hours x 4600 DD x 300 = 33 million Btu/year of heat, say 300
gallons of oil per year, at a cost of $300, or $900 in propane? Could we use
a sunspace and solar closet for space heat and domestic hot water? 

Some R1 sunspace glazing, eg a single layer of polyester film, with something
white on the ground in front of it, will receive about 550x4/3 = 733 Btu/ft^2
of solar heat, on an average December day, and lose about 6 hours x (80F - 44F)
x 1 ft^2/R1 = 216 Btu, for a net gain of 517 Btu/day. The house needs about 
24 hours (68 F - 38 F) 300 Btu/hr-F = 216K Btu to stay warm on that average
day, ie 216K/517 = 418 ft^2 of shallow sunspace glazing, eg a lean-to sunspace
16' tall and 32' long. (Moving the house to El Paso, Texas, would reduce that
sunspace glazing area to 16' tall x 8' long.)

Storing space heat for 5 cloudy days would require 5x216K/25K = 43 55 gallon
plastic drums full of 130 F water, each 2' diameter x 3' tall. We might use
50 of them in 5 layers, each about 10' wide x 4' deep in a solar closet, or
stack fewer drums on top of a concrete septic tank that is used for high
temperature sewage treatment, and partition the store into two sections, a
lower temp one for space heating and water preheating, and a higher temp one
for second-stage water heating.  

Spraying water on the roof would keep it close to the summer wet bulb temp
of 50 F, reducing the heat load for the house by an amount that depends on
the roof area and the amount of insulation between the house and the roof.
It takes about 1000 Btu to evaporate a pound of water, so 5 pounds of water
per hour would cool it by 5,000 Btu/hour. Spraying more water and collecting
it in a gutter and recycling it through a water-air heat exchanger in the 
house would cool the house more. Magic Aire (a division of United Electric
in Galveston, TX) makes a nice all-copper 2' x 2' SHW 2347 duct heat exchanger
that costs about $150 and transfers 45K Btu/hour between 125 F water and 68 F
air at 1400 cfm, with a 0.1" H20 pressure drop. This would make about 16K
Btu/hour of cooling with a 20 F air/water delta T, equivalent to about 3
window ACs, but with less dehumidification ability. Could the house be
dehumidified with a dessicant sunspace floor (a thin layer of reinforced
concrete, with some foam underneath?), vented to the outside during the
afternoon and the inside at dawn? We probably don't need this in Medford.

It would help if the house had lots of thermal mass inside so we could
ventilate it at night and button it up during the day, for good cooling.
Perhaps it should be made of rocks, with a rubble trench foundation and
a tiny amount of mortar, or none at all, and a poly film vapor barrier
on the outside, with a large overhang and some strawbales or a layer of
paper mache over that, and paint over that. The rocks near the ceiling
might store solar heat in the winter, with a ceiling fan to bring down
some heat on a cloudy day.

Maybe it should be a dome, or something. Here's a quote from D. C. Beard's
_Shelters, Shacks and Shanties_, first published in 1914: "The principal
difference between a white man's architecture and the Indian's lies in
the fact that the white man, with brick, stone, or frame house in mind,
is possessed of a desire to build perpendicular walls--walls which are
hard to thatch and difficult to cover with turf..." 

A flat roof with a 2" deep roof pond might need less materials and electrical 
cooling power, with winter heating and fire protection advantages, and make
a natural gravity-pressurized rainwater supply for the house. A 1000 ft^2 roof
would collect an average of 67 gallons of water per day with a 40" annual
rainfall and a 2" pond would store 1250 gallons of water. A 50' long x 12'
wide x 4' deep pond made from a single piece of EPDM rubber under a north
overhang of the house would increase rainwater collection to over 100 gallons
per day and make a total cooling and water supply storage of over 20,000
gallons. Richard Komp's house in Maine has a 1000 gallon cistern and a
roof/rainwater supply. One does have to conserve water, this way. Could we
evaporate and recondense greywater with a shallow pond inside the sunspace?
It would be nice to do the condensing inside the house somehow in winter.

With an average 38 F December temperature and an average 390 Btu/ft^2/day of
sun that falls on a horizontal surface, we might float an R1 pool cover on the
roof in winter, so the pond water at temperature T might lose 24 hours x (T-38)
x 1 ft^2/R1 on an average day, so T = 38 + 390/34 = 54 F, not counting the
heat that leaks up from the house through the roof. This would reduce the
heating load through the roof. (Perhaps this should be a one story house?)
Say we designed the roof to hold up 20 pounds per square foot (with 16' 2x6
or 24' 2x10 rafters on 2' centers?) 10 pounds for the 2" deep pond, and 10
more pounds to allow 2' of snow on the roof. (Which might be helpful extra
insulation, during a very cold spell. It would take about 1660 Btu/ft^2 to
freeze the roof pond, starting at T = 54 F, eg about 1660/38 = 44 sunless
hours at Medford's record low temp of -6 F.)

If the snow on the roof got too deep (we might use a microswitch activated by
roof deflection to sense this) we could pump some groundwater up through the
roof pond to melt it. How much 55 F groundwater would we need to keep up with
a very heavy snowfall, eg 3" per hour? I think that's equivalent to about
0.3" of ice per hour, ie 1.6 pounds of ice, or 230 Btu/hr-ft^2, since it takes
144 Btu to melt a pound of ice, at 32 F. For a 1000 ft^2 roof, that's 230K 
Btu/hour. Each pound of water supplies 55-32 = 23 Btu, so 230K/23 = 10K pounds
of water per hour could keep up with that snowstorm, ie 21 gallons per minute,
a big pump, so... we might want to set the switch to turn on with 1' of snow,
in anticipation. And keep a big post handy to prop up the roof inside, once
in a great while :-), especially if the roof starts to sag in the middle,
making the pond deeper and increasing the load in the middle... But it seems
to me all this would rarely be required, especially since the house would be
providing some heat through the roof too.

The National Park Service visitor's center building in Mt. Ranier, WA (contact
Victoria Jacabson or Eric Watkinshaw at (360) 569-2211) has a flat roof, even
though they get 20'/year of snow there. They use groundwater for snow melting.
If we have to do this seldom enough we might use stored solar heat or even
electric resistance heat... 

In summer, without the roof pond cover, we might pump some 70 F water through
the roof pond at night, when the average low is 55 F, in July. With a rough
surface thermal conductivity of 2 + 6 mph/2 = 5 Btu/hr-ft^2-F from the wind,
a 1000 ft^2 pond might lose 4 hours (70 F - 55 F) 1000 ft^2 x 5 = 300K Btu
per day, vs the 38K Btu/day needed for cooling the house for 8 hours a day.
That's good, but it gets better...

At sea level, the ratio of evaporative to convective pond cooling is about
1000(Pp-Pa)/(Tp-Ta)), independent of windspeed, where Pp and Pa are vapor
pressures of water in the pond and air in inches of mercury, and Tp and Ta
are Farenheit pond and air temperatures. The vapor pressure of water in 70 F
air is 0.74 "Hg (Clausius-Clapeyron: ln(Pw) = 17.863-9621/Tabs), and air with
a 0.0076 humidity ratio, W, has Pa = 29.92/(1+0.62198/W) = 0.36 "Hg. Medford's
elevation increases this ratio by another factor of 14.7/14.1, so the pond
might lose 25 times more heat by evaporation than by convection on an average
July night. A total of over 8 million Btu/day, 214 times the cooling required,
if the heat exchanger were perfect.

Nick

PS: Here's a system that looks balanced, with a 60 F roof pond that loses 186K
Btu on an average July night when the outdoor temp is 55 F, and collects 190K 
Btu from the 70 F house with that Magic Aire duct heat exchanger (or a used
auto radiator with its electric fan?) to the 70 F house, about 5 times as much
cooling as the house needs when it's 91 F outside for 8 hours a day. Adding
another heat exchanger and/or making the pond surface rough, somehow (ping
pong balls, so U = 2 + V/2 in line 100?) would let the house be even cooler.

10 'a program to help maximize roofpond cooling for a Medford house...
20 PALOC=14.1'local air pressure (psia)
30 TA=55'average minimum night air temperature (F)
40 H=4'duration of night air temp (hours)
50 W=.0076'humidity ratio of ambient air (#water/#dry air)
60 V=6'average windspeed (mph)
70 TH=70'house air temp setpoint (F)
80 TP=60'roof pond operating temp (F)
90 A=1000'roof pond area (ft^2)
100 U=1.5+V/5'air film conductance of smooth pond surface (Btu/hr-F)
110 QCP=H*(TP-TA)*A*U'convective heat loss from pond surface (Btu/day)
120 PA=29.921/(1+.62198/W)'vapor pressure of water in ambient air ("Hg)
130 'Clausius-Clapeyron equation: ln(Pw) = C-K/Tabs,
140 '         where Pw is the vapor pressure of water in inches of mercury
150 '         in saturated air at absolute temperature Tabs (R)
160 C = 17.86253'constants
170 K = 9620.979
180 TZ=459.67'Rankine base temp (F)
190 TABP=TP+TZ'absolute pond temp (R)
200 PP=EXP(C-K/TABP)'vapor pressure of sat air at Tp ("Hg)
210 ELF=14.7/PALOC'correction for local elevation
220 ER=1000*(PP-PA)*ELF/(TP-TA)'ratio of evap. to convective pond heat loss
230 QTP=QCP+ER*QCP'total heat loss from roof pond (Btu/day)
240 QHE=45000!'rated heat exchanger output (Btu/hr)
250 DTR=125-68'delta T for rated output (F)
260 UHE=QHE/DTR'effective thermal conductivity of heat exchanger (Btu/hr-F)
270 QH=24*(TH-TP)*UHE'heat exchanger output (Btu/day)
280 PRINT QTP,QH

1858676       189473.7

PPS: Our solar closet paper is at http://leia.ursinus.edu/~physics/solar.html

---

From: Rich Lobrovich <[email-masked]>
Date: 9 Aug 1996 07:53:48 GMT

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

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

Please comment on this for general discussion.

Thanks Rich Lob

---

From: Rich Lobrovich <[email-masked]>
Date: 9 Aug 1996 07:53:45 GMT

Thanks Rich Lob

---

From: [email-masked] (Nick Pine)
Date: 9 Aug 1996 10:12:28 -0400

Rich Lobrovich  <[email-masked]> wrote:

>effort put into a subject for a long time.

Glad you liked it, Rich... Now if we could just  build  one :-)

>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 

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