In the past, when energy costs were relatively low, the building materials normally utilized and the building procedures normally practiced were such that moisture vapor could readily escape building wall cavities through holes and permeable materials. As energy costs increased, more attention was paid to methods and means for insulating and waterproofing the building envelope, resulting in less air and moisture being transmitted through the building structure. Rigid foamed plastics were developed for use as sheathing boards which provide high thermal resistance. In virtually all cases, however, such materials are substantially impermeable to water vapor, typically having a water vapor transmission rating of 1 perm or less, which by industry standards is considered to be impermeable.
Sheathing boards of this type can be a problem in areas where the transmission of moisture would be beneficial. For example, depending on the temperature and relative humidity of the indoor and outside air, moisture in the form of condensation may develop in the wall cavity. In another common situation, wet lumber used to construct the building contributes to moisture in the wall cavity. In either case, the moisture needs an avenue of escape. If it cannot escape, it can decrease the effectiveness of the insulation in the cavity and contribute to mold and mildew and to the rotting of the building components.
Although it is desirable to have a wall structure capable of breathing or transmitting water vapor, it is also desirable for the purpose of energy conservation to continue to use sheathing boards that have low thermal conductivity, such as rigid foam boards. Because such foam material does not have high water vapor transmission properties, it would appear that its use in a building wall structure inherently prevents the wall structure from being permeable to water vapor.
It is an object of the invention to provide a rigid foam board that permits the passage of moisture without significantly diminishing the thermal insulating capacity of the wall structure. It is also an object of the invention to provide a simple, economical, efficient method of manufacturing such a board.