No matter how tightly a building is constructed, no matter how well it’s insulated, no matter what type of exterior cladding is chosen and how expertly it’s installed, moisture will always find a way into the building enclosure. Moisture infiltration can undermine structural integrity, cause exterior surfaces to deteriorate, and shorten life of paints and stains. It can also foster mold and rot that not only cause structural damage but also pose serious health hazards. Ridding the wall assembly of liquid water and water vapor as quickly as possible (before it can damage structural components) is what moisture management is all about.
There are many moisture management products currently on the market and choices are expanding to include traditional felt paper, rainscreen systems, sealants, and self-adhered flashing membranes. That’s where housewraps enter the scene. Installed behind siding, they prevent wind-driven rain and water from reaching the sheathing and framing, and reduce air infiltration, helping to decrease heating and cooling costs. In addition to resisting initial moisture infiltration, housewraps go a step farther by helping to remove trapped moisture vapor from the building enclosure.
Selecting the Right Housewrap
Housewraps are now available in dozens of varieties. So how do you choose? One good place to start is a set of performance criteria recognized by the International Code Council Evaluation Service (ICC-ES), which evaluates building products and provides opinions on code compliance. For housewraps, the ICC-ES evaluates a variety of key characteristics including:
Water Resistance: ICC-ES allows for three different testing standards, including The “Boat Test,” the least stringent standard; “Water Ponding,” a measure of a housewrap’s resistance to a “pond” of 1-inch water over two hours and “Hydrostatic Pressure,” the most stringent test, exerting significant pressures on a housewrap sample through a column of water (55 cm or higher).
Durability: A housewrap must be able to withstand the handling and application process without compromising its water-resistance performance. UV resistance and resistance to cold temperatures are also evaluated by ICC-ES.
Air Resistance: An air barrier is a continuous system that stops the unintended movement of air across the building enclosure. Most housewraps are tested for their resistance to air movement, but the ICC-ES standard does not account for seams and penetrations. Therefore, air resistance is not a critical product selection criterion since real-world performance depends very much on installation conditions.
Vapor Permeability: Permeability measures the amount of vapor transmission that a housewrap will allow over a period of time, minimizing the potential for accumulation of moisture vapor. The higher the “perm number,” the more vapor permeable the material. For a product to be considered a housewrap (and not a vapor retarder), the permeance rating must be higher than 5 U.S. Perms.
Drainage: Drainage is widely accepted as one of the most effective measures for reducing moisture damage due to rain penetration. Drainage is a critical component in allowing the housewrap to do its job—particularly in keeping the walls dry.
In addition to these factors, other things like the cladding type, climate, geographic orientation and wall assembly are important to consider in relation to housewrap choice, as well as the amount of annual rainfall. As a rule of thumb, the Building Enclosure Moisture Management Institute recommends that “any area receiving more than 20 inches of annual rainfall should incorporate enhanced drainage techniques in the wall system, especially if using an absorptive cladding material.” Information about the geographic orientation, overhang, and altitude of the wall further help in understanding the potential for wind pressure, wetting, and drying, and therefore what level of moisture management to incorporate into a wall.
A New Generation of Housewrap
Moving forward, housewraps increasingly will be judged by how effectively they provide positive drainage of water from the wall. Compared to standard housewraps, the newest drainable housewraps—especially those that provide a minimum of a 1 millimeter gap—can be as much as 100 times more effective at removing bulk water from the wall.
Impact of Building Codes
Among the factors driving the need for better moisture management solutions are the continued growth and standardization of requirements such as the International Residential Code and the International Building Code, which requires a means of draining water that enters the wall assembly. The IRC now mandates the use of water-resistive barriers, stating that, “The exterior wall envelope shall be constructed in a manner that prevents the accumulation of water within the wall assembly by providing a water-resistant barrier behind the exterior veneer... and a means of draining to the exterior water that enters the assembly (R703.1.1 Water Resistance).”
Importance of Installation
No housewrap on the market will do its intended job if it isn’t carefully and properly installed. The time between installation of the housewrap and the addition of exterior cladding should be kept to a minimum. Housewrap can deteriorate if it is exposed to the elements for longer than its intended UV rating, not to mention tears and holes that damage the material.
Another key decision in installing any housewrap is whether or not to use seaming tape and cap fasteners, which largely depends on whether it is being used primarily to reduce air infiltration or to resist water entry. In a well-constructed wall assembly, there is little evidence that a layer of housewrap will significantly tighten the building against air infiltration.
Wrapping it up
Ultimately, selection of the right housewrap requires an understanding of the product’s key attributes, including water resistance, durability, UV resistance, surfactant resistance, vapor permeability and drainage. Architects, builders and buyers must understand and appreciate the differences between perforated and non-perforated, and woven vs. non-woven housewrap products. It is also important that they continue to research and understand the differences between traditional housewraps and the next-generation drainable products that will provide enhanced moisture protection at a cost-effective price.