UNDERSTANDING MOISTURE TOLERANCE
Water vapor is able to penetrate plaster and wood readily. When the vapor comes in contact with materials within those walls having a temperature below the dew point of the vapor, they form moisture or frost within the walls. This moisture tends to accumulate over long periods of time without being noticed, which in time can cause building damage.
To prevent condensation, a large space is needed between the outer walls and any insulation which permits vapor flow. Reducing the space or the temperature converts vapor to moisture which is then retained. Using a separate vapor barrier or insulation that is also a vapor barrier are alternative methods to deal with this problem.
Aluminized Mylar™-like laminate is impervious to water vapor and with the trapped air space Thermal 3Ht Insulation is immune to vapor condensation.