source of humidity

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?
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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)

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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
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