|
|
re: windows sweating?
29 dec 1997
wrote:
>nicksanspam@ece.vill.edu says...
>> wrote:
>> >...there is no natural migration of humidity from high-humidity
>> >areas to low--humidity areas.
>> i beg to differ. it seems to me that humid and non-humid air desire to
>> mix by diffusion, at least. nature abhors differences in pressure and
>> temperature and molecular concentration.
>nope. this is a misconception.
let's see... page 9.8 of the 1990 energy crafted home spec says
water vapor can get into wall cavities in two primary ways: by diffusion
through materials that are permeable to water vapor, and by air
flowing into or through the wall.
diffusion works as follows: because water vapor is a gas, it is able
to pass through many materials that seem to be solid. the ability of
a material to allow water vapor to pass through is its permeability and
is measured by a perm rating... materials with high perm ratings allow
water vapor to move through relatively easily, while materials with
low perm ratings retard water vapor diffusion...
water vapor will diffuse through a high permeability material when
there is a vapor pressure driving force... generally towards the inside
of the wall assembly. in winter, it comes from the indoors...
ten years ago, diffusion was thought to be the primary means of moisture
migration into wall and ceiling cavities. tremendous emphasis was put on
vapor barriers as a way to retard this water vapor diffusion. today we
know that air leakage is actually far more significant a path for moisture
migration into wall and ceiling cavities. studies have shown that as much
as 100 times the moisture moves into the building shell by air movement
as it does by diffusion. a 10'x 10' wall section consisting of painted
drywall (no vapor barrier) will allow, in a heating season, about 2/3 of
a pint of water to diffuse through it in a typical new england climate,
while a 1/2" hole in the same wall can transmit as much as 50 pints of
water per season via air leakage...
page 20.12 of the 1993 ashrae handbook of fundamentals says
water vapor is a gas occupying the same space as the other gases that
collectively constitute air and, like them, acts independently. it can
migrate with or against airflow and follows its own physical laws of
migration... for any given temperature and degree of saturation, water
vapor in the air exerts a specific vapor pressure; it diffuses or
migrates from points of higher vapor pressure toward points of lower
vapor pressure, whether in air or within materials...
page 20.14 says
as a constituent part of the atmosphere, water vapor always migrates
with airflow. as an independent gas, it also migrates by diffusion
through air and materials according to its own pressure differentials.
moisture diffusion in free air is rapid, resulting in minimal vapor
pressure differentials between connected spaces and rapid flow to
condensing surfaces, such as cold glass or dehumidifier coils. as a
result, vapor pressure differentials in buildings and across the
building envelope usually are not as strong a driving force as air
convection. conversely, in industrial applications such as cold storage
or built-iun refrigerators, the primary driving force is diffusion.
therefore, the control of diffusion becomes more important with
improved convection and air leakage control in buildings.
it continues with an equation:
mv = m a theta dp, where mv is the mass of water in grains that flows,
m is the permeance of a material in gr/h-ft^2,
a is the area of the flow path in ft^2,
theta is time in hours, and
dp is the vapor pressure difference in "hg.
so, for instance, saturated 70 f air has a water vapor pressure of 0.73966
"hg from table 2 on page 6.4 of the ashrae hof (or exp(17.863-9621/(460+70))
= 0.748 "hg using the approximate clausius-clapeyron equation), so at 50%
relative humidity, the vapor pressure is 0.30983 "hg, and 30 f air at 50%
rh has a vapor pressure of 0.08232 "hg, and kraft paper or tyvek or typar
housewrap have m = 40 gr/h-ft^2, approximately, and there are 7,000 grains
in a pound, so a 10x10' wall with a house wrap but no vapor barrier and
70 f air on one side and 30 f air on the other, with no air leaks, might
pass 40(100)24(0.30983-0.08232)/7,000 = 3.1 pints of water per day. adding
a well-sealed 0.006" poly film vapor barrier with m = 0.06 lowers this to
less than a teaspoon per day.
>> warmer air is lighter than cooler air, so it tends to rise. humid air is
>> lighter than dryer air, so it tends to rise as well.
>actually, warmer air is less dense than cooler air.
and less dense than you? :-)
>the difference is signficant.
a pound of 70 f dry air occupies 13.349 ft^3, 8% more than a pound
of 30 f air that occupies 12.338 ft^3...
>> so warm humid air will have a powerful urge
>> to rise up through little holes in a house in the winter, no?
no response, although this seems like a no-brainer...
>it's been a while since you studied physics, right?
it's been about 30 years since my last physics course.
>do a bit of research. i did mine. i stand by everything in my response.
right... :-)
nick
nicholson l. pine system design and consulting
pine associates, ltd. (610) 489-0545
821 collegeville road fax: (610) 489-7057
collegeville, pa 19426 email: nick@ece.vill.eedeeyou
computer simulation and modeling. high performance, low cost, solar heating and
cogeneration system design. bsee, msee. senior member, ieee. registered us
patent agent. solar closet paper: http://leia.ursinus.edu/~physics/solar.html
web site: http://www.ece.vill.edu/~nick
|
|
