The Impact of Housewrap
in Green Building
What to Look for in A High-Performance Housewrap
- by Michael Clark/Fiberweb Inc.
The exponential growth of the green movement and the development of green building standards over the past decade has builders from all regions of the country scrambling to make their projects more sustainable. One of the best ways to do so is by selecting building materials with the most sustainable attributes, such as high-performance housewrap. It's one of the most practical and cost-effective components of any sustainable home building strategy.
Today's housewraps are separated into two types – non-perforated and perforated. Perforated housewraps are either woven or laminated polypropylene films that have small microscopic punched holes for better breathability.
Non-perforated housewraps are constructed of either spun-bounded polyethylene or made from fiber-mesh reinforced polyolefin. Non-perforated housewraps use a higher level of micro-porosity to achieve breathability, which typically gives them superior bulk water holdout and, in some cases, superior air resistance. The structure of these materials allows moist, humid air to escape from the inside of the building, while preventing outside precipitation from entering the home.
When installed properly, housewrap acts as a secondary layer of defense from external elements for a home or building, performing several equally important functions. Most notably, it keeps water from coming into contact with the structural sheathing and framing during construction and after the building is complete.
If wood framing is exposed to moisture for extended periods of time, it can rot, resulting in structural damage such as settling or shifting. In addition, when wood is wet, it can become a food source for mold growth, which significantly degrades indoor air quality.
Some housewraps are also designed to function as an air barrier, to stop the movement of hot and cold air through the wall cavity. The right housewrap can even help reduce utility costs and increase indoor air quality by minimizing air infiltration and drafts.
Of course, housewrap products will differ in size, strength, and price. Dealing with the vast array of housewrap options and the strict requirements for installation can often prove challenging to builders. Some builders are not aware of the full range of choices they can offer, and they may also be unclear on how to install a particular product properly. Many others are simply unaware of this product category's full range of capabilities.
When comparing housewrap products, there are five performance factors to consider:
Rate of Permeance
Permeance is one of the most important factors to look for in a housewrap, as it measures a material's ability to transfer water vapor. This is rated in perms: the number of grains of water vapor that pass through a square foot of material per hour. So, the higher the perm rating, the more permeable the material.
Current building codes require a housewrap to match or exceed Grade D building paper, which has a perm rating of about 5.0. To meet this requirement, perm ratings for housewraps range from 6.7 to 59.0 perms.
There are good arguments for lower-perm barriers, notably in climates with high, solar-driven vapor and with absorbent exterior cladding materials, such as stucco and brick. However, there are just as many arguments for maximum permeance.
Manufacturers such as Fiberweb, makers of the popular Typar brand housewrap, fall into the optimal moisture vapor transmission range of 5 to 20 perms. With an 11.7 perm rating, Typar HouseWrap ensures that while water is prevented from entering the wall cavity, ideal levels of moisture vapor are allowed to escape.
In fact, the National Association of Home Builders (NAHB) Research Center designated Typar HouseWrap products with its Green Approved Products seal of approval. Through green approved labeling, the Research Center reviews and approves third-party evidence that building products meet specific scoring criteria for homes seeking certification to the National Green Building Standard, the consensus-developed standard recently approved by ANSI.
Surfactants like wood tannins, cedar oils, and turpentine – which are found in some cladding materials – can decrease the water holdout of a housewrap. Once these surfactants reduce the surface tension of the water and allow it to flow more easily through the pore structure of the housewrap, the sheathing is no longer protected from wind-driven rain and water damage.
Selecting a housewrap with superior surfactant resistance minimizes the risk of degradation caused by contact with these components, making them an excellent choice for installation under wood siding and stucco.
Ultraviolet stability is also vital to the performance of a housewrap before it's covered by the cladding. Even limited exposure to ultraviolet rays can cause a housewrap to deteriorate. Because of this, some technologically savvy manufacturers have begun to incorporate UV inhibitors in the coating and fibers of their housewrap products.
Builders should also seek out a housewrap featuring exceptional tear-resistant strength to ensure air and water holdout during construction. Many housewraps develop tears from exposure to strong winds or from the rigors of the job site, rendering the housewrap ineffective.
Ability to Serve As An Air Barrier
Some housewraps may also achieve Air Barrier Material classification by reducing air permeance when evaluated in accordance with ASTM E 2178, which is the standard test method for air permeance of building materials. Housewrap materials that qualify as air barrier materials must then be installed in strict accordance with the manufacturer's instructions in order to perform as required. A properly installed housewrap system that includes proper flashing and sealing around penetrations will vastly improve the overall thermal efficiency and performance of the building's wall system.
As an air barrier, housewraps reduce convective wind washing against sheathings and air infiltration into stud wall cavities. As part of an air barrier assembly, a housewrap can reduce drafts, increase comfort and reduce energy use by decreasing the amount of non-conditioned air entering and exiting conditioned wall cavities. Surprisingly, the average 2,500 square-foot house has more than a half mile of cracks and crevices that are open to wind and wind-driven rain.
The use of housewrap contributes to the requirements for the ENERGY STAR Qualified New Home program as an important component in the air sealing and insulating category. Utilizing this program can lead to a reduction in heating and cooling costs by as much as 20 percent. Various housewrap products can also contribute to the US Green Building Council's LEED certification points in both LEED — New Construction (commercial) and the LEED for Homes (residential).
Easy to Handle and Install
Weather-resistive barriers require thorough, comprehensive integration with other building envelope elements to retain system integrity. Flashing and other components, including windows, doors, attached decks and band joists, can present difficulty. Therefore, installation knowledge and techniques are key to maximizing performance of any housewrap. Because of this, some manufacturers provide educational support in classes and in the field to ensure proper installation.
A Complete System Option With Applicable Warranty
Compatibility of flashing materials with the housewrap should also be taken into serious consideration. For example, sealants with high solvent or plasticizer content can damage bitumen flashing products. Because of this, some manufacturers have developed a whole system approach – that includes compatible tapes for seaming, and adhesive flashings for openings – which vastly improves the air and moisture resistance of the barrier. When installed properly, in many cases these systems are backed by a manufacturer's warranty.
Today, many builders and contractors are aware that addressing moisture and air management through careful product specification and smart building processes can drastically reduce potential moisture problems. A high-performing housewrap can help improve building durability, indoor air quality and decrease maintenance costs by reducing the risk of moisture intrusion. In addition, by acting as an air barrier that prevents hot and cold air movement through the wall cavity, housewrap also helps reduce utility costs and increases comfort. Therefore, it's more important than ever that the right housewrap be chosen for the job.
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