Moisture Sources, Relative Humidity, and Mold
A little water goes a long way
We hear a lot about how moisture can be an indoor pollutant in tight houses. But just how much moisture can be a problem; how does boiling a pot of water compare to a 15-minute shower? This keeps some of us mold worrywarts up at night, so I thought it would be a good idea to run some numbers.
In the table below, look at how little water it takes to raise the relative humidity (RH) from a reasonable 50% interior RH to a mold-marginal 70%:
MOISTURE CONTENT: 800-square-foot apartment (6,400 cubic feet of interior space)
|Temperature (˚F)||Starting Relative Humidity (%)||Starting water content of apartment air (pints)||Add this much water to the apartment air (pints)||Resulting Relative Humidity (%)|
Source: Nathan Yost, 3-D Building Solutions
An interior RH of 70% will make just about everyone uncomfortable (see “Comfort Comes with Green Building"). It also means that cooler surfaces in the home can approach the dew point, a condition that mold and dust mites just love.
Moisture sources in the home
The table above tells us that adding less than a quart of water (2 pints) to this apartment’s air can be a problem; but just how much water is that? Time for table number 2:
Common Moisture Sources in the Home (family of 4)
|Source||Estimated Amount of Water (pints)|
|Indoor line-drying of clothes||4 – 6 per load|
|5 – 7 house plants||About 1 per day|
|Washing dishes (dinner, family of 4)||.7|
|Cooking (dinner, family of 4)||1.2 (1.5 with gas cooktop)|
|Respiration/perspiration||.4 per hour|
|Unvented kerosene space heater||7.6 per gallon of kerosene burned|
|Evaporation – new construction materials||10+ per day|
|1 cord green firewood stored indoors over 6-month period||400 - 800|
Source: Minnesota Extension Service, University of Minnesota
Yikes! For a family of four, cooking dinner, eating, and then washing the dishes adds enough moisture to raise RH to levels that spell T-R-O-U-B-L-E. Turns out that there are lots of ways to dump what ends up to be quite a lot of water into the air. Admittedly, this is a rather small dwelling unit, and there is no air exchange accounted for in the first table.
But that is really the point. Every home, but especially an energy-efficient airtight one, needs dedicated air exchange, particularly in high-moisture areas like the kitchen and baths. So keep your eye on moisture sources in the home, ventilate for moisture control, and spring for a hygrometerA device that measures relative humidity of air. Mechanical hygrometers that rely on a coil of thin metal are not terribly accurate; electronic hygrometers available at most electronic or hardware stores are usually accurate to about plus or minus 2 - 3%. from Radio Shack for about $20 to keep track of interior RH.
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