winter humidity, condensation on windows, etc. SOLAR.KnowledgePublications.com

winter humidity, condensation on windows, etc.
1 feb 1997
i've had a nice morning figuring out an approximate simple formula for the
dew point temperature for a surface in a room with farenheit temperature t
and relative humidity r: td = 9621/(9621/(t+460)-ln(r))-460, starting with 
the clausius-clapeyron equation--ln is the natural log function. this equation
says condensation will form on a 50 f surface in a 70 f room with 50% rh, on
a 37 f surface at 30% rh, etc, which limits the max winter humidity in a house
or the minimum temperature and efficiency of a single-glazed sunspace used as
a solar collector, assuming the glazing averages outdoor and sunspace temps.

to avoid condensation in a single-glazed sunspace with 30 f outdoor air and
70 f 50% rh house air, the air temp next to the sunspace glazing needs to be
ts, where (ts+30)/2 = 50 or ts = 70, which seems ok in january where i live.
with 20 f outdoor air, the sunspace needs to be 80 f, or the house humidity
needs to be lower. double glazing in a colder climate allows higher house
humidity, or a lower and more efficient solar collection temperature. 

a local college keeps their pool at 80 f, with 85 f air in a 120 x 120 ft
windowless masonry building with a flat roof with no wall or roof insulation
using steam from an oil-fired central plant about a half-mile away. the plant
has a huge chimney, and the largest wall faces south, and it's all windows
(the only building on campus like that) and the windows that open are mostly
open all year because it's so hot inside that building. heating the pool
probably takes about 400 gallons of oil per day in january, when oil trucks
make  daily  deliveries, for heating the pool and 46 other buildings. there's
a line of 8' vertical south windows in a triangular roof monitor running 100'
ew along the north side of the pool building for solar heat and daylighting
(a good idea), but they are now all insulated and boarded up on the inside
because "they leaked," according to the maintenance department.

there were bleachers below the windows for watching swim meets, and i imagine
the windows "leaked" condensation on people below, and nobody seems to have
thought of putting a gutter under the windows and running the water back into
the pool. or insulating the building, or putting a dehumidifier/pool heat pump
inside the building, or putting some $1/ft^2 greenhouses on the flat roof,
to solar heat the pool.

this institution of higher learning (they actually teach physics there :-)
uses over 400,000 gallons of oil per year, and their combined yearly electric
and oil bills amount to over 1 million dollars per year, and they have had
faculty wage freezes, despite the average faculty salary being less than
that of local elementary school teachers. they say they have been thinking
about cogeneration for years. so far, their brightest idea seems to be
switching the steam plant from oil to gas. 

arrrgh.

nick