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Housewrap (or house wrap), also known by the genericized trademark homewrap (or home wrap), generally denotes a modern synthetic material used to protect buildings. Housewrap functions as a weather-resistant barrier , preventing rain or other forms of moisture from getting into the wall assembly while allowing water vapor to pass to the exterior.
Elastomeric coatings can provide a vapor barrier and water proofing with permeability ratings of .016 perm rating with 10 mils/min. of coating and can be applied on interior or exterior surfaces. Aluminum foil, 0.05 US perm (2.9 SI perm). Paper-backed aluminum.
Moisture vapor transmission rate (MVTR), also water vapor transmission rate (WVTR), is a measure of the passage of water vapor through a substance. It is a measure of the permeability for vapor barriers. There are many industries where moisture control is critical.
The plate is heated and water is channeled into the metal plate, simulating perspiration. The plate is then kept at a constant temperature. As water vapor passes through the plate and the fabric, it causes Evaporative Heat Loss and therefore more energy is needed to keep the plate at a constant temperature.
A variant of the metric perm is used in DIN Standard 53122, where permeance is also expressed in grams per square meter per day, but at a fixed, "standard" vapor-pressure difference of 17.918 mmHg. This unit is thus 17.918 times smaller than a metric perm, corresponding to about 0.084683 of a U.S. perm.
Cellophane is the most popular material for manufacturing cigar packaging; its permeability to water vapor makes cellophane a good product for this application as cigars must be allowed to "breathe" while wrapped and in storage. Cellophane sales have dwindled since the 1960s, due to alternative packaging options.
The term pervaporation is a portmanteau of the two steps of the process: (a) permeation through the membrane by the permeate, then (b) its evaporation into the vapor phase. This process is used by a number of industries for several different processes, including purification and analysis , due to its simplicity and in-line nature.
For example, at 23 °C and 95% relative humidity polyvinylidene chloride has an oxygen permeability of 0.6 cm 3 μm m −2 d −1 kPa −1 while low-density polyethylene under the same conditions has an oxygen permeability of 2000 cm 3 μm m −2 d −1 kPa −1, or a factor of over 3,000 times more permeable. [15]