Cooking (dinner, family of 4)1.2 (1.5 with gas cooktop) SourceEstimated Amount of Water (pints) 75504.41.870 65503.01.470 5-minute shower.5 Temperature (ËšF)Starting Relative Humidity (%)Starting water content of apartment air (pints)Add this much water to the apartment air (pints)Resulting Relative Humidity (%) 70503.61.670 5 – 7 house plantsAbout 1 per day Evaporation – new construction materials10+ per day Unvented kerosene space heater7.6 per gallon of kerosene burned 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) Indoor line-drying of clothes4 – 6 per load Washing dishes (dinner, family of 4).7 Respiration/perspiration.4 per hour 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) 1 cord green firewood stored indoors over 6-month period400 – 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 hygrometer from Radio Shack for about $20 to keep track of interior RH.