re: cinder-block vacation home
28 sep 2001
bill bonde wrote:
>>>go with polysteel... an r-50 heat factor.
>>would you have any evidence for this article of faith?
>>steel is 400x more heat-conductive than wood...
bill bonde wrote:
>so the 'steel' in polysteel is a metaphor for strength and the material
>is some plastic?
no. the american polysteel site shows an expanded steel mesh running
perpendicular to the wall that periodically connects the inside surface
to the outside surface. this is a thermal short circuit ("bridge"), but
it may not be serious, if the steel strands are sufficiently thin and
sparse. i didn't see any thermal performance measurements that include
the heat loss of this bridge, and they don't give enough information
to calculate that.
they say the wall contains an average 4.8" of eps with an r-value of
4.17 per inch, and an "average r-value of 20." maybe. r20 vs r50...
they also say the "effective r-value is 26-30." the fudge begins...
the capital polysteel site claims "the strongest most energy-efficient
building system available..." and further claims "the total effective
r-value of polysteel walls actually exceeds r-50 in many parts of the
country." the polysteel business site calls their walls "super energy
efficient" and has photos of fistfuls of dollar bills...
i'm suspicious of these "dynamic r-values."
consider an r20 wall in minneapolis... here are nrel's long-term average
daily min, 24 hour average, and average daily max temps for each month:
jan feb mar apr may jun jul aug sep oct nov dec
2.8 9.2 22.7 36.2 47.6 57.6 63.1 60.3 50.3 38.8 25.2 10.2 f
11.8 17.9 31.0 46.4 58.5 68.2 73.6 70.5 60.5 48.8 33.2 17.9
20.7 26.6 39.2 56.5 69.4 78.8 84.0 80.7 70.7 58.8 41.0 25.5
let's say some mythical massless house has 100 btu/h of r20 walls with no
thermal mass or internal heat gain or air leaks. if it's 68 f for 24 hours
per day, adding mass to the external walls does not decrease the heating
bill from october through april, when the outdoor temp is always less than
68 f. the massless house needs more heat when it's 2.8 outdoors and less
when it's 20.7, but adding thermal mass to the walls doesn't change that:
with lots of wall mass, the furnace needs to supply the same amount of
heat over 24 hours a day to the equivalent of a constant outdoor temp of
11.8. from october through april, each house needs
30dx24h(7x68-48.8-33.2-17.9-11.8-17.9-31.0-46.4)100 = 13,968k btu.
so the concrete doesn't do any good at all thermally, over this period.
it just lowers the r-value and raises the price of the wall.
in may, the mythical massless house needs 12h(68-47.6)100 = 24,480 btu/day
of heat and 12h(69.4-68)100 = 1,680 btu of cooling (assuming the occupants
are unable to live in a 69.4 f room), a total of 26k btu/day. add lots of
thermal mass, and the house only needs 24h(68-58.5)100 = 22.8k of heat;
26,080/22,800 = 1.14, so we might say the "dynamic r-value" is 1.14xr20
= r22.8 vs r20 in may, at best. or we might say the mythical house needed
14,750k btu from october through may, and the massy one needed 14,652k, ie
the massless house needs 12h(68-57.6+78.8-68)100 = 25,440 btu/day in june,
vs 24h(68.2-68)100 = 480, for a dynamic r-value of 25,440/480xr20 = r1060.
wow!!! then again, the massless version needs 15,513,600 btu from october
through june, and the massy one needs 14,666,400, ie 5.5% less.
in july and august and september, the houses need 752,400 and 403,200,
734,400 and 180,000, and 734,400 and 540,000 btu, bringing the yearly
totals to 17,734,800 and 15,789,600 btu, if i did that right, for a
"seasonal dynamic r-value" of 17,734,800/15,789,600xr20 = r22.5 vs r20.
but there's no such thing as a massless house, or a wall with infinite
thermal mass, so the difference would be less in the real world, no?
and a house with exposed internal mass and resistive exterior walls
might store lots of coolth with night ventilation from may through
september in minneapolis, given the daily min temps, with no need
for air conditioning.
may jun jul aug sep
47.6 57.6 63.1 60.3 50.3 average daily min (f)
58.5 68.2 73.6 70.5 60.5 24-hour average
69.4 78.8 84.0 80.7 70.7 average daily max