Characterize Building Envelope Response to Outside Climate

Multiple rainy, foggy, damp days followed by clearing overnight and rapid temperature drop starting Friday morning. The interior RH on the 3rd floor of this historic property built in 1728 tracked the outside RH closely until 4PM Friday afternoon with a dramatic change of 30% over 10 hours suggesting the possibility of air infiltration and exfiltration.

Most historic structures do not have insulation and vapor barriers commonly found in modern construction. Differentials in air pressure contribute to the rate of air transfer through diffusion or channel flow. Wind pressure on buildings is an important factor in energy loss and moisture transfer. The stack effect is a result of a difference in pressure at the top and bottom of a building. During the winter, cold heavy outside air creates pressure driving infiltration at the bottom and resulting exfiltration at the top. The reverse may occur in the summer with air-conditioning.

Interior relative humidity green, Outside temperature purple.

Outside relative humidity on Friday. Source: NWS.

A drop in outside temperature Friday morning through Saturday morning for a total change of almost 40 degrees in just over 24 hours. Throughout this four day period, the heating system kept the interior temperature within a narrow range above 65 degrees. The outside humidity Saturday morning continued higher at 70%.

Relative humidity is the ratio (expressed as a percentage) of the amount of moisture in the air to the maximum amount that the air can hold at that temperature. Because warm air can hold more water vapor than cool air, relative humidity falls when the temperature rises(denominator increases), assuming no moisture is added to the air.

N.B. Humid air is lighter than dry air because the molecular weight of nitrogen (28) and oxygen(32) is higher than the molecular weight of water vapor molecules(18) which displace an equal amount of nitrogen and oxygen molecules.

UPDATE

At 8AM on the 13th, the interior RH (green) declined to a very dry19% vs 42% outside RH. Individual room forced-air heating units (gold) may have contributed to further drying of the interior air at a rate greater than any infiltration, possibly combined with stack effect pressure differential. Exterior temperature (purple).