Handbook of air conditioning and refrigeration / Shan K

3.16 CHAPTER THREE
Moisture Transfer in Building Envelopes
Moisture transfer in building envelopes of a typical residential building can proceed along two
paths, as shown in Fig. 3.4:
1. Moisture migrates inside the building material mainly in the form of liquid if the relative humidity
of the ambient air is more than 50 percent. It will be transported to the indoor or outdoor air
by convective mass transfer. The driving potentials are the moisture content of the building material,
the vapor pressure gradient inside the building material, and the humidity ratio gradient between
the surface and the ambient air.
2. Air leakage and the associated water vapor infiltrates or exfiltrates through the cracks and
gaps around the windows, doors, fixtures, outlets, and between the joints. Air and moisture enter the
cavities and the airspace in the building envelope. If the sheathing is not airtight, air leakage and its
water vapor penetrate the perforated insulating board and discharge to the atmosphere, as shown in
Fig. 3.4. If the sheathing is a closed-cell, airtight insulating board, then the airstream may infiltrate
through gaps between the joints of the insulating board or through cracks between the window sill
and the external wall and discharge to the atmosphere.
The driving potential of the air leakage and the associated water vapor is the total pressure
differential between indoor and outdoor air across the building envelope due to wind effects, stack
effect, mechanical ventilation, or a combination of these. Moisture is moving in the vapor form.
In leaky buildings, the moisture transfer by means of air leakage is often far greater than the
moisture migration through solids. For better-sealed commercial buildings, moisture migration
through the building envelope may be important.
FIGURE 3.4 Moisture transfer in building envelope along two paths.