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re: passive solar goof?
17 aug 1999
charles elias wrote:
>...i'd really appreciate some information on difference in heat gain
>for a window with low e glass versus untreated glass.
at the june ases conference in portland, me, physicist tim ellison gave a
paper on his "ten-day thermal time constant" house (a passive solar house
with a hydronic slab used for heat distribution and collection, with storage
in "an outdoor strawbale structure containing 3 3,000 gallon polyethylene
tanks" :-) he mentioned that "low-e hardcoat" (a technology that's apparently
becoming obsolete) is more efficient than more recent low-e coatings, and
he presented some data from various window manufacturers...
he suggested a weighted "figure of merit" based on the heat gain minus the
u-value, which makes sense to me, for a simple passive direct gain house
(vs other more efficient configurations, eg houses with low-thermal-mass
sunspaces.) on an average december day in pennsylvania, 1000 btu/ft^2 of
sun falls on a south wall, so a square foot of window gains 1000shgc btu
of sun per day and loses 24h(70-30)u-value = 960(u-value) over an average
december day, so the net heat gain is approximately proportional to the
shgc minus the u-value, with a weighting of about 1:1. colder cloudier
places would weight heat loss more and heat gain less, and so on...
glazing type shgc u-value s-u
clear single pane 0.85 1.11 -0.26
cardinal low e^2-145 0.32 0.27 +0.05
marvin low e^2 0.41 0.32 0.09
cardinal low e^2-171 0.41 0.24 0.17
marvin high r tripane 0.34 0.14 0.2
clear hm tc88 1/4" g 1.5" ig 0.47 0.18 0.29
ppg clr sungate e-500 (3) 1.5/0.5 w ar 0.47 0.18 0.29
cardinal low e^2-178 0.58 0.26 0.32
clear hm tc88 1/8" g 1.5" ig 0.50 0.18 0.32
pilkington e-adv 1/4" ar ib 0.66 0.28 0.38
3/4" ig hardcoat low e w ar 0.71 0.29 0.42
3/4" ib low-e hardcoat w ar (marvin) 0.71 0.29 0.42
in this application, the most efficient windows are at the bottom of the list.
prices vary, of course, and tim said (and i've found) that the manufacturers
are not always aware of the best combinations, and the reps and salespeople
often don't realize that these particular combinations are available.
i've yet to find anyone at marvin (800-346-5044) who can identify or verify
the performance of the last window on the list...
nick
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