Dana, thanks for the thoughtful reply. You've got my attention re the draft hood now, and I'll be talking to the guy who installed the water heater, who is a plumber, about that.
As far as the humidifier, that may be true in lower altitude, more humid states, but we're in Denver at around 5400 feet. We have digital clocks that have humidity sensors in them. When they get down to about 42%, we start getting dry skin and static problems, which can harm electronics. We like the humidity around 43-48% all year, but it doesn't stay there without the Aprilaire 360, in the laundry room wall, which has a sensor switch at the other end of the house. We've had it for years and it's pretty good. But we use it all year, as needed, otherwise even in the summer, it gets too dry in here. When it's 99 outside, like it was Sunday, the humidity outdoors can be 7%. This can almost be like parts of Arizona at times. But thanks for the concern.
Somehow I hear that argument a lot from people in CO, but since sealing the place up and putting the ventilation system under dehumidistat control works fine for folks in Santa Fe or Jackson Hole, it surely works in Denver.
First, the "relative" of "relative humidity" is the temperature. The
absolute humidity can be expressed either in dew point temperature (the temperature at which fog will form if you lower the temperature of the volume of air) or wet-bulb temperature (which is easier to measure directly.) The dew point of 7%RH 99F air is about 25F, which is also a pretty common wintertime outdoor dew point in Denver. The the outdoor TEMPERATURES in Denver in winter are colder, even if the outdoor dew point temps are pretty similar year-round.
The relative humidity of 25F dew point air at a temperature of 35F is 65% RH. Same air, same humidity, different temperature.
The relative humidity of 25F dew point air at a temperature of 75F (a typical summertime indoor temperature) is 15%, still drier than the optimum human healthy & comfortable zone. At 70F it's 18% RH, still too dry.
Over-ventilating with 25F dew point air will over-dry the indoors. But that doesn't mean you need a humidifier- it just means you need to lower the infiltration/ventilation rate:
Human activities such as breathing / bathing /cooking adds substantial moisture to the indoor air, and as long as that humidity isn't being purged faster than it's being introduced by the activities of the occupants, the indoor RH will rise. In Denver the outdoor air is dry enough that the indoor humidity can be controlled year-round by adjusting the ventilation rate, since the outdoor dew points are only rarely above the human-healthy range- higher ventilation rates always lower the indoor RH, lower ventilation rates always raise it. (That isn't true in the gulf-coast states, where summertime outdoor dew points soar, and mechanical dehumidification is necessary.)
The summertime outdoor dew point is irrelevant to the problem. The
indoor dew point relative to the
wintertime outdoor temperature is the problem.
Setting the humidity to 50% RH @ 75F in summer isn't a problem. Setting it to 40% or higher @ 70F in
winter becomes an issue. Moisture only collects in the sheathing when the temperature at the sheathing is below the dew point of the conditioned space air. The dew point of 40%RH/70F air is 45F. The
January/February mean outdoor temperature (and thus the sheathing temperature) in Denver is about 35F, fully 10F colder than the dew point of 40%RH/70F air. The moisture in the insulation's entrained air accumulates in the sheathing when that happens. If enough accumululates to support mold, in spring when the temperatures rise and the moisture is release you get mold growing inside the walls.
The dew point of 50%RH/75F air is 55F.
The mean outdoor temperature in Denver in summer is north of 70F, so if there is remaining residual moisture in the wall cavities the vapor pressure is in the other direction, drying toward the interior.
