Q: I am concerned about the moisture conditions inside a home. I asked one the experts at the university here and he said that you could calculate the interior relative humidity (RH) from knowing the outside temperature and RH. He did a few calculations for my location (central and northern Midwest) and his numbers seem awfully dry in winter. Do you have any comments?
A: Your university friend is not correct in his statements about accurately calculating the indoor RH for a home. You can indeed do the calculation, but the calculated interior RH in the wintertime is actually lower than actual RH. (For example, consider that the outside temperature is -20 F. Your calculation for a home heated to 70 F will give you an RH under 3 percent RH; the wood is trying to dry to under 2 percent MC; these dry conditions in a home do not happen.)
The actual RH is higher because of the addition of moisture from cooking, plants, bathroom showers, and other living activities. These sources of moisture, especially in a modern, tight home, are significant. In addition, in cold climates the home owner often will also have humidifier that adds moisture during the wintertime. Also, with today's furnaces, fireplaces and wood stoves using outside air for combustion and not the air in the home, RHs are also much higher inside the home.
The calculation values are closer in a home with lots of air leaks or with a wood stove where there is plenty of make-up air for combustion taken from the home's environment.
But let's consider a manufacturing plant. What sources of moisture are there? If there are none, then the calculations of interior RH using the outside temperature and RH are quite accurate. Can you imagine how a piece of lumber, dried to 7 percent MC will react when exposed to humidity under 3 percent RH? Shrinkage and end cracking will be common. It is therefore essential to see that manufacturing facilities are humidified slightly to avoid the extremely dry conditions. Of course, never humidify higher than the customer's home or office. This usually means that a manufacturing facility should be at about 38 percent RH in the wintertime, using humidifiers as necessary to achieve this value. This will create and maintain approximately 7 percent MC in wood; this condition is therefore called 7 percent EMC, where the "e" stands for equilibrium.
(Note: The calculation is done by following an adiabatic heating line on a hydrometric chart. Such a chart is available in the US Forest Service's text Drying Hardwood Lumber, figure 2.5, as well as in many heating and cooling engineering texts. Nyle Corp in Bangor, Me., has free copies. Using this chart, I can see that anytime that the outside conditions are under 30 F in the early morning hours, the air inside an unhumidified plant will be 5 percent EMC or less.)
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