Winter humidity

Lifehacker suggests that you Humidify Your Home for Increased Winter Comfort.

If it seems like your skin is dryer, you get zapped with static shocks more often, and you wake up frequently with a stuffy nose, you’re not just imagining things—you’re experiencing the effects of low humidity.

In an RV, winter humidity can be a control issue. The walls are often poorly insulated and there is a lot of wall surface compared to the inside air volume. One of the comments linked to Humidity and the Indoor Environment at the Minnesota Blue Flame Gas Association. That website has a lot of good information.

The basic problem is that cold air cannot hold much water. That means that when outside air with a very high relative humidity (that means it is hold about as much water as it can) is brought inside and heated, the humidity is significantly reduced as the amount of water in the air is the same but the warmer air can hold a lot more.

If the outside air temperature in winter is 0°F and the relative humidity is 75 percent, that same air inside your 70°F home will have a four percent relative humidity. That’s dry! The Sahara Desert has an average relative humidity of 25 percent.

One way to gauge indoor humidity is to “Drop three ice cubes into a glass, add water and stir. Wait three minutes. If moisture does not form on the outside of the glass, the air is too dry; you may need a humidifier. (Do not perform this test in the kitchen, because cooking vapors may produce inaccurate results.)”

But the problem in an RV is that many things you do add water. Just breathing can add a quarter cup per hour. Cooking for a family of four will add 5 pints a day. A shower can add a half pint. A catalytic heater or stove burner also adds significant amounts of water to the air. What that means is that you can get condensation on windows and walls in the RV and that can lead to mold and mildew and rot. That means poor air quality which can lead to musty smells, head or chest colds, stuffiness, headaches and other ills?

How do you balance the ‘too little’ and ‘too much’ humidity? They call it ACH or air changes per hour. This means ventilation that replaces inside air with outside air. Your RV probably needs more than one complete air change every hour. For a thirty foot RV, that means more than a thousand cubic feet of air every hour or 20 cubic feet per minute. For contrast, a high end vent fan on high will move nearly a thousand cubic feet of air per minute.

Generally, an RV will ‘leak’ 2% of the volume a vent fan can move so the problem is usually reducing air changes per hour unless you are preparing dinner or running a catalytic heater or showering. Balancing air exchange with inside humidity can conflict with the energy budget for keeping warm. Perhaps that is why Quartzite is popular in January and February – except that they are having significant rain problems so keeping warm and reasonably dry might be more of a challenge this year than usual.

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