I hope no one is assuming that if the air is at 70% relative humidity that building materials will also be at 70%.
Standard air has the ability to hold a certain amount of water vapor at a specific temperature. This amount is 100% relative humidity.
Air and other building materials do not share the same properties (or the ability to hold the same amount of moisture per unit), therefore the amount of moisture in 100% relative humidity air at 70° is not the same amount that represents 100% relative humidity of a 2 x 4 (35%).
I have some documentation around here somewhere about the ratios of air and other materials but they’re not at hand, sorry.
[size=2]Percentages between air and wood are measured on different scales; relative humidity% for air and moisture content % in wood.
One of the ratios off my top of my head is that 20% relative humidity of air is equivalent to 5% moisture content of wood that has a fiber saturation point of 35%.
Seasonal fluctuation of 5%-15% is considered normal in bare wood.