Monday, September 29, 2014
Water vapor moves from the warm
side of a building to the cold side. We must design our insulation
system to
allow trapped water vapor to get out and keep exterior water vapor from getting
in.
Materials that retard the flow of water vapor
are called impermeable. Materials which allow water vapor to pass are called
permeable. Naturally there are degrees of permeability and all materials and
systems have varying degrees of permeability. Most building codes require a
vapor retarder whose permeability is 1.0 or less.
Vapor barriers and
air barriers are two different things. The purpose of air barriers is to have a
structure whose holes, tears, openings, and penetration are closed up or as
much as or possible because with differences in air pressure, air can move
easily through these “openings” and carry moisture through these openings. An
air barrier is not a vapor retarder.
In practicality it
is impossible to eliminate all penetrations, so it becomes a matter of
controlling as much as common sense will allow. So, HVAC systems can be designed
in concert with your air and vapor barriers to control ventilation,
dehumidification, and air pressure. Your local climate and four seasons affect
this as well.
