That’s a Wrap

Although the term “housewraps” is not brand new, there is still a fair amount of misunderstanding surrounding the word.

“Up until recently, you had building paper or nothing behind the cladding until housewraps and synthetic materials were introduced, but a wrap or a weather barrier was not required until about 2006 at least in residential codes,” says Alan Hubbell, residential marketing manager, Wilmington, Del.-based DuPont Building Innovations.

“That may be what you’re seeing in terms of the use of the word housewrap. The codes are driving the use of some kind of water-management system, and the codes are much more prescriptive,” he says.

“The International Residential Code [IRC] requires you have a weather-resistant barrier behind all exterior cladding and in some cases they require two layers, such as behind stucco,” says building science consultant, Steve Easley, Steve Easley & Associates Inc., Danville, Calif. “The reason for this is all exterior cladding leaks. It’s not a matter of if they leak; it’s a matter of when they leak. Typically, you see leaks happen at roof and wall intersections, around doors and windows, as well as at penetrations, such as dryer vents.”

“We talk about managing bulk or liquid water because obviously you want to keep that out of your wall system and also manage air leakage,” Hubbell says. “Moisture as vapor, of course, can also be carried by air, so by managing air, you’re also managing and reducing vapor transmission through the wall.”

Confusion About Barriers

“I think there’s a lot of confusion out there with respect to vapor barriers. In some cases you have to choose one type of vapor barrier to meet building codes, but if you use a certain cladding or a certain percentage of cladding, then you can choose a different type of vapor barrier; it’s not easy to decipher,” Hubbell says, noting DuPont is developing an application called Code Sense to help contractors and builders understand building- and energy-code requirements for air, water and thermal management in wood-frame walls, as well as provide instructions about how to meet these requirements for the specific code, climate zone and cladding type selected.

“The point is,” Hubbell adds, “not all housewraps are equal with respect to permeability, and although they may meet the minimum requirements for a water-resistive barrier, they may not meet the requirements for an air barrier because of the way they achieve permeability.”

Is building science clearly understood? “The answer is probably ‘no,’” Hubbell says. “You have to put your thinking cap on; you have to be very clear about what you want to know and about what you want to accomplish and then talk to somebody reputable. There is a lot of misinformation out there.”


The misapplication or incorrect installation of housewrap products potentially can lead to more problems than they solve. “These products, especially the wraps, are a small part of the spending on construction,” Hubbell says. “They probably don’t get the mind share from contractors they deserve. If done improperly, housewraps can be responsible for a large percentage of problems and callbacks.”

Hubbell adds that although his company’s specialists devote a good deal of time and attention to the housewraps themselves, they also spend time on the flashing products, which tie the windows, doors and other penetrations to the wraps.

The point where housewraps interface penetrations is particularly vulnerable to leakage problems. Hubbell advises contractors to think of the wall as they would a roof. “A wall is just a roof in a vertical configuration,” he says. “For example, you wouldn’t reverse shingle a roof, yet when people get to the wall they might consider it. But if you treat the wall like the roof, then you should be in pretty good shape at least in respect to water management.”

He adds: “We promote a drainage system as opposed to a barrier system where you just try to block the water. With the drainage system, if water does get behind the window or door, there is a way for it to get back out.”

Downside of Tight Homes

The downside is that by reducing energy and air coming across a wall system, you reduce the amount of energy available to help dry the home. “What you’re trying to do is reduce the outside energy coming in or the inside energy going out, so there’s less energy available to help dry the wall system. That means you better get your wall system and your water-management system done right. In tighter homes, water management takes on even greater importance,” Hubbell says.

Making homes more efficient and comfortable is not just a simple matter of saying “let’s put more insulation in and seal everything up tighter than ever,” Hubbell notes, adding, “exterior insulation or leveling board may create problems because they’re relatively impermeable and act like an unintended vapor barrier. That’s why it’s so important to understand the building science behind the wall system and how things really work.”

There are three main problem areas where moisture in the form of water can get into buildings, according to Easley:

  • Roof and wall intersections
  • Around doors and windows
  • Other penetrations, such as dryer vents

Like Hubbell, Easley sees the importance of permeability and the ability of the housewrap to let materials dry. “Drying is the real issue because it only takes seconds for building materials to get wet, but it can take days or weeks for hygroscopic building materials, like framing and OSB sheathing, to dry out. Once the water gets in, you want to dry out the wall system as quickly as possible,” Easley says.

Job One

Deflecting water is job one, according to Easley. Avoid situations that trap water, such as valleys that confine water and bring it into the building envelope. One such situation is a 1-story roof meeting a 2-story wall. “Water that runs off a 2,000-square-foot roof in 1-inch rainfall can easily deposit 1,200 gallons of water, so if you don’t deflect that water away it can easily be guided into the building envelope,” he says. “At roof-wall intersections it’s essential to use a good-quality seamless kick-out flashing.”

Job two is if water does get into the wall system, drain it as quickly as possible. “The faster you can drain the water away, the less it gets absorbed into hygroscopic building materials,” Easley says. Some manufacturers have come up with “drainwraps” that have a surface with deposits of plastic or other means of creating a texture so there is a drainage space between the wrap and the smooth undersurface of the cladding product.

Job three must take place when building materials get wet: Dry them as quickly as possible. “These are the three things you need to do, and tying it all together is really important. You have to think of it as a system,” Easley says.

Improper installation can cause many problems, as Hubbell points out. Easley agrees: “When contractors install windows, they’ll just put flashing around the building wrap to the window, but what you need to do at the top of the windows is cut the housewrap and fold it; then put the head flashing from the nail fin of the window up onto sheathing, so if water gets behind the building wrap, it runs down and hits that head flashing and runs to daylight. It doesn’t get between the window and the framing and into the house.”

The standard remodelers should follow is ASTM E2112 (see “Three Steps to Integrating Replacement Windows and Doors into the Building Envelope,” page 78), Easley recommends. “That’s the national window flashing standard upon which most window manufacturers base their installation instructions. Housewrap manufacturers also cite ASTM E2112.”

Misused Terms

Easley, too, encounters confusion about the terms surrounding housewraps, weather-resistant barriers and similar labels. “Often people misuse the term vapor barrier when they really mean weather-resistant barrier. The way the IRC explains these materials is that a vapor retarder is typically put on the warm side of the insulation,” he says.

“Traditionally, in colder climates vapor barriers are required because the thought was if you were to put a lot of insulation into the wall cavity, then as warm moist air infiltrated the wall cavity it could condense on cold surfaces. The thought was ‘we better put up a vapor retarder because it will prevent the flow of moisture as a vapor.’ And it’s true, they do; the reason they haven’t worked very well is because typically there are not very good air barriers on interior walls,” Easley says.

“Most of the moisture that gets into the wall cavity piggybacks on air currents. When you have air that exfiltrates carrying moisture with it and it comes in contact with a cold surface, it condenses,” he adds. “The concept behind a vapor retarder is to keep warm moist air out of the wall cavity; unfortunately, unless there is a very good air barrier, it doesn’t work.”

Further explaining the science, Easley says there are several ways water and moisture can move. First, water moves as a liquid; second, it moves as a vapor on air currents; and third, it can move by diffusion. Diffusion is the process of a building material picking up moisture from a higher relative humidity environment and then drying to a dryer or lower relative humidity. Moisture always moves to a drier environment, Easley notes.

“Diffusion is critical to drying building materials,” Easley continues. “For example, if you have wet OSB and you have a vapor retarder over the top of it, it’s not going to dry. That’s why code specifications for exterior weather-resistant barriers require they have drying potential.”

Two Different Things

Summing up, Easley says weather-resistant barriers and vapor barriers are two very different things.

“A weather-resistant barrier is designed to keep air and water out. If you install the system right, it acts as a barrier in terms of water but also blocks airflow so you don’t have as much infiltration and exfiltration. On the other hand, a vapor retarder is designed to control moisture flow by diffusion. It’s supposed to stop moisture-laden air from diffusing through drywall and getting into the wall cavity. The reality is not that much moisture moves by diffusion under normal circumstances,” he says, “unless you have a really high humidity home with lots of plants or aquariums.”

The difference between new construction and remodeling is worth noting, particularly when it comes to installing new windows, Easley says. “I think one of the biggest mistakes window installers make when retrofitting windows is they don’t take the windows down to the original sheathing and reinstall them. A lot of leaks happen as a result of just cutting the windows back and putting a new window in the opening. Wherever possible, I think it’s important to strip the window back to the original sheathing and then flash it normally.

“I also think it’s important for the average remodeler to try to air seal from the inside to prevent moisture-laden air from exfiltrating around the openings, especially in colder climates,” he says.