Is it possible to describe all of the factors that influence heat and moisture movement through a wall during a single day? Perhaps. We could start by listing the outdoor conditions, including air temperature, relative humidity, wind speed, wind direction, the angle of the sun with respect to the wall (its altitude and azimuth), the cloud thickness, the precipitation rate, and the depth of snow on the ground. Needless to say, many of these factors change from minute to minute.
We could describe the indoor conditions, which include the air temperature (remembering, of course, that the air temperature near the floor may be different from air temperature near the ceiling), the relative humidity, and the mean radiant temperature of the surfaces in the room.
We could list the construction details of all of the many layers of the wall assembly, including the vapor permeance of each layer, the R-value of each layer, the air permeance of each layer, the moisture-storage characteristics of each layer, the location and size of the wall’s cracks and holes, the location and size of any windows, the leakiness of the window flashing, the SHGC and U-factors of the window glazing, the distance from the top of each window to the roof overhang, the width of the roof overhang, the depth of the rainscreen gap, the size of the ventilation openings at the base of the rainscreen gap, the size of the ventilation openings at the top of the rainscreen gap, and the orientation of the wall — that is, the cardinal direction it faces.
If we know all of this information, and more, it might be possible to determine how heat and moisture move through the wall — in other words, to describe the wall’s hygrothermal performance.
Then again, it might not.
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