where does humidity come from?

we're renovating an upstairs appartment. the appartment downstairs is
occupied, the one upstairs is not.

on some windows upstairs I see condensation. where does this moisture come
from?
can the humidity come through the ceiling (lathe and plaster) of the bottom
appartment?

Humidity isn't an "it" but a measurement more correctly referred to as
"relative humidity." (And, if I still had my old Scholastic Book Club copy
of "Oliver Becomes A Weatherman" (fifth grade and about a hundred years
ago), I could tell you just how it is measured. All I remember is that in
the book Oliver compares the actual temperature (dry bulb) to the
temperature of a wet bulb thermometer. I think he put a piece of cloth over
the bulb of a thermometer, dampened the cloth and then swung the thermometer
around on a string (like a Biblical sling shot). The concept is that, the
dryer the air, the more moister will evaporate from the cloth, cooling the
bulb of the thermometer. When there is less moister in the air, more water
evaporates from the cloth and vice versa. So, low moisture in the air ("low
relative humidity) means the temperature difference between the dry bulb and
wet bulb thermometers will be greater than when there is more moisture
("higher relativity".) No doubt there is a formula somewhere. Relative
humidity is essentially the amount of water that the air is holding as water
vapor compared to ("relative to") the maximum amount of water it could
possibly hold as water vapor at the present temperature. So 70% relative
humidity just means that the air is currently holding 70% of the total water
vapor it could possible hold at its present temperature. Relative humidity
is always at the current temperature because warmer air can hold more water
than cooler air, in an absolute sense. (If a given amount of air could hold
a pound of water at 60 degrees, it could hold more than a pound of water at
70% and less than a pound of water at 50%.) Therefore, as you raise or lower
a rooms temperature, the relative humidity changes, too, but in the opposite
direction: Raise the temperature, lower the relative humidity.

Of course, you didn't really want to know any of that, but that's what you
get at 1 am. What you really want to know is where the heck the water vapor
is coming from that appears as condensation on the windows in the upper
apartment. Little mositure would be moving through lathe & plaster. (Ain't
that fun stuff for a renovation?) More likely, water vapor would come from
the lower apartment via a stairway and, if they are not properly vented, the
hot water heater and the furnace/boiler if they use natural gas or oil.

breathing - try breathing on a cold window
cooking - the water driven off when boiling as well as basically raising the
temperature of anything containing any water at all
showers/bathing - that's why we have exhaust fans in bathrooms with
showers/baths.
combustion - including a gas stove, water heater (which should be vented, of
course, but might not be exausting properly), etc.

First thing to check out is the temperature differential between the two
apartments. If you can independently control the temperature of the two
apartments, you most likely have the unoccupied one at a lower temperature.
The relative humidity of the warmer air from the lower apartment will rise
as it enters cooler areas of the house, including the cooler upstairs
apartment. At a higher relative humidity, it will more easily give up water
vapor to condense on the cold windows upstairs. In addition, the surface
temperature of the upstairs windows will be somewhat lower than those
downstairs, also leading to more condensation. You are also increasing the
amount of moisture upstairs when you have people working up there. (And, do
you have any non-electric unvented heating equipment being used upstairs
during the renovation?)

Most likely, when you raise the temperature upstairs for normal occupancy,
you won't have much difference in condensation between the two apartments.
If you still have problems with condensation, then you need to raise the
temperature of the inside surface of the windows (double-pane windows, etc.)
and/or lower the relative humidity indoors, mostly be making sure that
kitchens and baths have appropriate venting.

Bill
(who wishes he had more time for fun stuff like renovating instead of doing
web design at 1 am)

"j j" wrote in message
. ..
where does humidity come from?

we're renovating an upstairs appartment. the appartment downstairs is
occupied, the one upstairs is not.

on some windows upstairs I see condensation. where does this moisture come
from?
can the humidity come through the ceiling (lathe and plaster) of the
bottom
appartment?

On Tue, 2 Dec 2003 22:25:02 -0500, "j j" wrote:

where does humidity come from?

we're renovating an upstairs appartment. the appartment downstairs is
occupied, the one upstairs is not.

on some windows upstairs I see condensation. where does this moisture come
from?
can the humidity come through the ceiling (lathe and plaster) of the bottom
appartment?

Yes it can.

And thru the walls and ceiling and also from the breathing and body
of any visitors to the site, and from all the drains, etc. .

However, knowing that doesn't mean that you don't spend some time
checking for things like leaks from rains.


PJ

..