A quick analysis of the water-vapor balance should be done for buildings where humidity control is a problem; that is, when there is a big difference between the humidity outside and the humidity maintained in the conditioned space. This is usually not a problem in comfort applications except in tight humidity control spaces or where an area is required to have very high or low humidity.
Water vapor migrates through a building envelope from the high to the low water-vapor pressure regions. The permeability of the structure determines the rate of mass transfer. Common vapor barriers are aluminum foil, galvanized iron, and aluminum or asphalt paint. The vapor barrier should be situated on the side of the structure with the higher vapor pressure so that water does not condense within the wall. The transmission of water vapor is measured in terms of latent heat gain, so it is a function of the moisture in terms of latent heat gain. It is also a function of the moisture content of the air on either side of the building envelope. Manufacturer’s data tables give this permeance in (Btu/h)(100 ft2) or (grains/lb) difference in latent heat. The latter is more convenient since the grains/lb can be read directly from a psychometric chart for the interior and exterior conditions.
Latent heat gain = (A/100) * (g2 – gt) * P
A = area, ft2
g2 = moisture content outside air,
grains/lb gt = moisture content inside air,
grains/lb P = permeance (grains/lb)(100 ft2)
This implies that the greater the moisture difference, the worse the problem. Some real problems might occur, for example, in low- humidity laboratories or in heated buildings in the winter (artificial space heating dries the space out) in humid climates.
The shading coefficient for drawn blinds or drapes is just as effective as that of costly insulated or tinted glass. If a space is not occupied much of the time and the shading devices can be closed, then an option is to install single-glazed windows and save the expensive glass for other locations.
A benefit of maintaining the vapor barrier secure is that it reduces the migration of humidity into the conditioned spaces, which lowers the latent load on the cooling coils.