Not just a fad, ‘cause it’s been going on so longDear Nick,
I recently read the article “Moisture Management in Exterior Wall Assemblies,” (Walls & Ceilings, October 2002) written by Dave Olson, product manager for Fortifiber Building Products. On behalf of DuPont, we would like to offer clarification to W&C readers on differences between Tyvek and other plastic housewraps that are not addressed in the article.
Olson compares the performance of plastic housewraps with asphalt saturated kraft building paper, but does not identify the type of plastic housewraps he is comparing. Identification of products is necessary to make the comparisons accurate and meaningful since many of the comparisons do not apply to all products in this category. The discussion is even more misleading to Tyvek spunbonded olefin products, which are uniquely engineered, and offer superior performance characteristics and test results compared to other plastic housewraps, as well as asphalt saturated kraft building paper. Tyvek is made from a breathable nonwoven web of polyethylene fibers. It does not absorb water, rot, or lose water repellency with continuous exposure to water. The continuous structure of the spunbonded microfibers, unique only to Tyvek, offers a highly effective combination of durability, water and air hold out properties, all achieved while offering maximum breathability. Other plastic housewraps depend on either film perforation, a woven structure, or delicate microporous films laminated to a structural backing to achieve breathability. While these manufacturing techniques may be effective for improved breathability, other properties such as water hold out or strength for example, may be compromised.
We disagree with the article’s statement that “because building paper has the ability to intercept moisture and release it over time, that it is far less susceptible to moisture overload than other types of weather resistive barriers such as plastic housewraps.” Tyvek actually holds out bulk water at levels well above black paper, regardless of the moisture load, thus offering added protection to the wall system. Furthermore, the structural integrity of black paper is compromised with time as it absorbs moisture. In fact, a study titled “Comparative Analysis of Residential Construction in Seattle, Wash., and Vancouver, B.C.,” CMHC, January 1999, states “if the weather barrier is building paper, it relies on impregnated chemicals for water repellence. These chemicals are slowly dissolved and expose the paper, an organic material susceptible to loss of structure and rot.”
The strength of Tyvek is unaffected by the presence of water. Additionally, since Tyvek does not absorb water, it will not rot and is not a food source for mold.
Olson correctly summarizes the importance of designing wall systems that provide additional means for moisture to escape, and he emphasizes the importance of using a weather barrier that allows drying to occur through moisture vapor transmission. He then refers to a recent study that indicates that the MVT of building paper responds to changes in relative humidity in the most favorable way possible, becoming higher in conditions of high humidity. A variable permeability may have utility for an interior vapor barrier application but weather membranes should maintain high breathability in order to promote dry wall systems. High breathability combined with a high resistance to bulk water is essential for proper moisture management in wall systems. While the average pore size associated with the fibrous structure of Tyvek is small enough to prevent bulk water intrusion, it is large enough to allow smaller molecules of moisture vapor to pass through, leading to a product with extremely high breathability or MVT, regardless of the relative humidity. In fact, Tonyon, et al., stated that a combination of good gross water holdout and consistent, relatively high vapor permeance likely increased drying in a study conducted in EIFS wall assemblies, where the drying of sheathing was compared when using Tyvek compared to an asphalt-based product in an article titled “Water Management and Moisture Transport in Direct-Applied and EIFS Assemblies,” Journal of Testing and Evaluation, Vol. 27, No. 3, May 1999.
Olson also comments on the tendency of building paper to form channels when used behind traditional stucco wall assemblies, leading to the formation of a natural drainage plane. In a study titled “Field Observations and Laboratory Tests of Water Migration in Walls with Shiplap Hardboard Siding,” by George A. Tsongas, Dennis P. Govan, and Jack McGillis, it was found that “once building paper got wet from a leak, the building paper wrinkled with the major wrinkles lying in a horizontal direction such that the water was preferentially channeled horizontally.” Although these tests were not conducted in a similar stucco system, the data does not support the drainage hypothesis presented by Olson. Tyvek StuccoWrap is designed with a vertically creped structure and offers significantly higher drainage compared to asphalt saturated kraft building paper when used in EIFS.
Builders have been wrapping their homes with Tyvek for more than 20 years. Its use of Tyvek is not a “new fad” as Olson states of plastic housewraps. DuPont Tyvek has proven over the past two decades that it is a great secondary weather barrier.
Catherine M. Wilson
Western Region Manager