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Importance of Vapor Permeability in Housewrap

Did you know that the average, single-family home produces 3-6 gallons of moisture every day? Normal activities such as cooking, showering, and doing laundry generate moisture that can dissipate throughout the home and potentially into wall cavities.

This is yet another area where a high-performance housewrap can play a critical role in helping keep exterior walls dry. The hidden science of a housewrap lies within the ability for the material to allow free passage of water vapor so that wall cavities and framing members can dry to the outside of the building, thereby reducing the threat of mold and rot. The rate at which the moisture vapor moves through an object is measured in perms, and is often referred to as permeability. Unlike bulk water holdout, which refers to water in its liquid form, vapor permeability concerns water in its gas form.

Current building codes require a housewrap to match or exceed Grade D building paper’s rating of about 5.0 perms. To meet this requirement, perm ratings for commercially available housewraps range from 7 to 59 perms.

Materials with higher perm ratings essentially speed the escape of trapped moisture vapor. But higher ratings do not necessarily equal better housewraps, as methods of achieving a high perm rating can vary. For example, some low-tech housewraps achieve their high perm ratings with mechanically punched perforations in the membrane. These perforations increase the passage of water vapor, but they also make the housewrap more susceptible to bulk water intrusion. Conversely, non-perforated housewraps offer even greater moisture vapor transmission (higher perms) than their perforated counterparts and are more effective at preventing the movement of bulk water.


In his recent paper, “Inward Drive – Outward Drying,” building scientist Joseph Lstiburek identifies the “sweet spot” for the permeance of the housewrap layer as between 10 and 20 perms. Too high, he writes, and the moisture driven out of the back side of reservoir cladding (i.e., vinyl, stucco, etc.) into the air space will blow through the layer, through the permeable sheathing and into the wall cavity. Too low, and the outward drying potential of the cavity is compromised. Thankfully, advances in building wrap technology are adapting to meet this need.

A research study conducted by the University of Massachusetts, in which various housewrap products were subject to a set of laboratory conditions to identify how they compared from a permeability standpoint, found TYPAR BuildingWrap to be one of the best housewraps on the market—for both resisting water infiltration and permitting the evaporation of water vapor.

The water holdout of TYPAR BuildingWrap is balanced, achieving a perm rating of 11.7. This optimal perm rating ensures that while water is prevented from entering the wall cavity, ideal levels of moisture vapor are allowed to escape. For more information, visit http://www.typar.com/system/superior-performance.html.


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