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re: waterbed insulation
12 nov 2000
joelncaryn  wrote:

>the *real* energy hog in my house is the waterbed...

how about human heat? i can sleep comfortably when a thermometer near
my naked body in the airspace under the covers reads 85 f...

a person makes about 300 btu/h of heat, ie an average 100 btu/h, spending
8 hours/day in bed, or less, since heat from exhaled air isn't captured
that way. (we need to average over 24 hours since a waterbed can store lots
of heat compared to a conventional bed.) a 5'x7'x1' thick waterbed with a
us r12 cover (2 2" quilts or 2 sleeping bags laid flat?) and r10 sides (2"
styrofoam inside the wood sides) and a floor with r-value rf has a thermal
conductance g = 35ft^2/r12+26ft^2/r10+35ft^2/rf = 5.5+35/rf btu/h-f. to
keep it 85 f in a 70 f room, we need 100 btu/h = (85f-70f)g, so rf = 30.4,
roughly 6" of styrofoam (2 people only need r4.5.)

with 300 gallons (about 2400 pounds) of water and g = 5.5+35/30 = 6.67,
rc = 2400btu/f/6.67btu/h-f = 360 hours, so the water temp doesn't change
much over a day. with 300 btu/h of continuous heat, the temperature would
eventually climb to 70+300/6.67 = 115 f. if the water were tmax at dawn,
it would drop to tmin = 70+(tmax-70)exp(-16/360)) by dusk and climb to
tmax = tmin+(115-tmin)(1-exp(-8/360)) at dawn, which makes tmin = 84.67
and tmax = 85.34 f. 

we might put styrofoam under the bed, on top of the drawers, if any (it's
good for 30 psi in compression, 68x more than the 64 psf waterbed load.) 

but maybe we'd like more storage space. joe pasma, pe, of premier building
systems (800) 469-8870 wrote structural insulated panel roof loading tech
bulletin no. 19, which says their r30 8" osb/foam/osb glued sandwich with
a 2x8 perimeter board can support 288 pounds per square foot (vs a mere
64 psf waterbed) with a 4' span. they conducted full-scale destructive
transverse load tests at an independent code recognized testing lab, and
the ultimate failure load was 864 psf, like a stack of 14 waterbeds. 

we might make something like this with 3 4'x8'x2" sheets of styrofoam
(r30, about $60) and 2 7/16" sheets of osb ($4.95 each at home depot)
and 3 8' ripped 2x8s and some drywall screws, and support it with
4 stacks of hollow 16x16x8" tall concrete blocks:

     -------------------------------
    |                               |
    |                               |
    |       b       4'      b       |
    |                               |
    |       2'                      |  4'
    |                               |
    |       b               b       |
    |                               |
    |                               |
     -------------------------------
                   8'

      | ~~~~~~~~~~~~~~~~~~~~~~~~~ |      -----
      |         waterbed          |
     -------------------------------
    |            8" sip             |
     -------------------------------     ~32"
            b               b            
            b   2' high     b      
            b  crawlspace   b      
  --------------------------------------------  concrete floor

we might hang decorative curtains over the crawlspace. 

alternatively, 4 55- or 30-gallon drums would make a taller crawlspace
with more stability, and 8 might make a waterbed loft. filling them with
water would add desirable thermal mass to the house. they should last
20 years, with a quart of oil to slow down the rusting.

joe pasma says a waterbed sip supported on 4 steel drums is no problem.
he suggests laying a plywood circle inside the 1/2" tall rim at the top
of the drum to increase the bearing surface...

nick




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