The Sweet Spot for Building Wrap Permeability

Codewatcher Typar

This first in a four-part blog series on how to construct buildings to manage moisture addresses finding the elusive “sweet spot” for building wrap permeability.

Water is an elusive foe for builders. It can find its way into a building envelope in a variety of ways. According to Bijan Mansouri, Technical Manager, TYPAR Construction Products, a key area of concern is flashing. “Bad flashing installation at punched openings such as windows and doors as well as balcony and deck interfaces are the places where water typically finds its way into a home.”

One of the most challenging situations to manage moisture is in areas with annual rainfall of 20 inches or more and a reservoir cladding like fiber cement, brick, or stone is specified and where air conditioning is often used¾which is most of the United States. When these types of claddings get wet and then are warmed, the vapor pressure of the stored water increases, driving it both inward and outward from the cladding material. Where that moisture ends up, and how quickly, is a function of how permeable the adjacent materials are within the assembly.

In addition to water seeping through these openings, moisture can be siphoned into the wall when a wind event causes negative pressure within the wall assembly. And, in regions with high humidity and extreme temperatures, warmer indoor air can cause condensation on colder outside surfaces leading to vapor diffusion.

Find the Balance

Codewatcher Typar moisture illustration

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.

The key is balance, explains Mansouri. “You don’t want to be too vapor open so that the inward vapor drive is slowed, but not too vapor closed so that the wall can still dry out.”

Typically, inwardly driven moisture vapor is managed by separating the cladding from the rest of the assembly with a capillary break, which can be a gap or a sheathing material that sheds water or does not absorb or pass water. Impermeable sheathing, such as extruded polystyrene (XPS), is one option for halting inward vapor drive. In these types of assemblies, the inwardly driven moisture condenses on the surface of the XPS sheathing and drains downward.

But in situations where a reservoir cladding is paired with a highly permeable sheathing like gypsum board (which can be as high as 50 perms) or a moisture-retentive material like oriented strand board (OSB), an air gap may not be enough to slow down inward moisture intrusion. In these applications, an added weather-resistant barrier (WRB)—commonly referred to as a building or house wrap—is needed to reduce unwanted moisture intrusion.

Sometimes, finding that balance seems to be more art than science. “The permeance for the inward drive is not easily optimized because different claddings store different amounts of water vapor and different airspaces between the claddings and building papers and sheathings affect the inward flow,” says Mansouri. “It’s much easier to know the exterior flow; more difficult to know the interior flow.”

In the recent paper “Inward Drive – Outward Drying”  building scientist Joseph Lstiburek identifies the “sweet spot” for the permeance of this WRB layer as between 10 and 20 perms. As Lstiburek explains, if the perms are too high, the moisture driven out of the back side of the reservoir cladding 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.

Use a Proven Solution

Fortunately building wrap technology, particularly using a “systems” approach, can help builders hit the sweet spot every time.

“A building envelope that works properly and consists of components that are chemically compatible in extreme weather conditions is critical in moisture management,” says Mansouri. “The TYPAR Weather Protection System offers that.”

As the core component of the TYPAR Weather Protection System, 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. This optimal breathability reduces the risk of standing water in wall cavities, which can potentially lead to mold growth and degradation of indoor air quality. For an added layer of performance, TYPAR BuildingWrap combines tear strength, UV stability, and the ability to withstand performance-robbing surfactants like oils, tannins, and soaps that can compromise an ordinary housewrap’s ability to block bulk moisture penetration.

With the ability to function as a type 1 air barrier, the TYPAR Weather Protection System eliminates drafts in exterior wall and helps reduce energy costs over time, while providing a more comfortable indoor environment for homeowners. By properly installing the complete system of TYPAR building wraps, flashings, and construction tape, installers get the backing of a lifetime limited warranty.

Check into how TYPAR BuildingWrap can help you manage moisture in your homes today visit