Housewrap: It's All about Balance

Walking past a remodeling job recently, I looked at the structure, which was covered in housewrap with some siding already installed and said to myself: “There’s no seam tape and no window flashing. The housewrap has been X cut and peeled back around rough openings. The application is substantially useless. When the mold hits, any warranty will likely be void.”

I can’t count how often I see housewrap installed improperly. Some contractors simply staple it up, overlap the seams 6 inches and start siding—which is wrong. Housewrap, also known as weather-resistant barrier, is designed to work as a system comprising the housewrap, seam tape and flashing so it performs well in four key areas: 1) moisture resistance, also called water holdout, 2) moisture vapor transmission, 3) surfactant resistance and 4) strength or tear resistance.

Housewrap resists weather, but the weather doesn’t know to avoid seams or rough openings. Weather assaults the entire wall, and the housewrap must be installed so it can balance resistance to air infiltration (wind) and water (mostly liquid that gets behind the siding). It also must allow diffuse water vapor to escape from inside the structure.

Why the need for high performance and such attention to detail during installation? The average house produces 3 to 6 gallons of water vapor each day, or 2,190 gallons per year. The moisture is often water vapor, diffused or driven through the walls by temperature differentials between the conditioned space and the outside air. But there’s water coming from the outside, too. Moisture in the form of humidity or liquid water as rain inevitably gets behind the siding.

If you don’t install housewrap properly, it will admit and trap water as a liquid or vapor inside the wall, and the water creates ideal conditions for rot and mold. By not sealing the seams or flashing rough openings, water from outside likely will get into the wall.

Don’t expect the sheathing underneath the housewrap to magically stop moisture. The sheathing will have an effect but, according to the Air Barrier Association of America, a typical 2,500-square-foot home has more than 1 1/2 miles of cracks and crevices, and water or moisture will find its way into those openings.

It’s about balance. The best housewrap balances its ability to hold water out and allow the escape of water vapor from within. If the housewrap has low water holdout characteristics and resulting high vapor permeability (called perm), it will likely admit too much water. On the other hand, if the housewrap does not allow enough water vapor to leave from the inside of the wall (low perm), that trapped water vapor may condense within the wall, potentially leading to rot and mold. A housewrap with an exaggerated capability for water holdout achieves that performance at the expense of its ability to allow diffuse water vapor and vice versa. When evaluating your housewrap, keep these things in mind:

1) Moisture resistance and moisture vapor transmission: Most building science research states that a well-balanced housewrap should have a perm rating between 10 and 20.

2) Surfactant resistance: A surfactant is a compound that lowers a liquid’s surface tension and degrades the water holdout capabilities. Your housewrap should have superior resistance to them. Look for manufacturers’ claims in their literature and on their labeling.

3) Strength: The strength of a housewrap is determined by its resistance to tearing. With building cycle times running more than 120 days, remember that a housewrap is temporary siding subjected to the weather, especially wind and sun. Look for a housewrap that is engineered for tear resistance.

In addition, look for a manufacturer’s statement about UV resistance. Finally, ask how the housewrap enhances the thermal envelope. Independent studies by DOE and Energy Star show housewrap, when properly installed, flashed and sealed as a system, can lower energy bills between 25 and 40 percent—all the more reason to take the manufacturer’s installation recommendations seriously.

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