Water has a tendency to get into places where it doesn’t belong, including spaces and cavities in new construction projects. No matter how well a building has been insulated, no matter what kind of exterior has been placed around the building, moisture is a resilient contender. Since moisture can undermine the structural integrity of buildings, foster mold, and rot, and cause untold levels of damage, moisture management becomes essential. That is where housewrapping comes into play.
Here is everything you need to know about housewrap, including what housewrap is, what it does, and how to select a housewrap for your new construction project or home.
What Does Housewrap Do?
Housewrap is a kind of water-resistant barrier (WRB) and comes with the primary purpose of preventing moisture from entering the wall cavity, similar to a waterproof jacket. If moisture is allowed to build up in cavity walls or studs, mold and rot will set in and any insulation will lose R-value because of heat-conducting moisture. Contrary to belief, housewrapping is not meant to be a barrier against cold air, but with proper installation and the correct materials, WRBs can also become a reliable air barrier.
Plastic housewrapping came about in the 1970s, but before that other forms were used. Over the years, housewraps of various kinds have come about, and popularity has increased due to the ease of installation, ability to successfully block water from moving into the wall assembly, and durability.
3 Main Functions of Housewraps
- A vapor-permeable membrane that lets moisture trapped within the framing lumbar or insulation to escape.
- A secondary weather barrier that sits behind the siding and prevents rain and other moisture from permeating the sheathing.
- An air barrier that prevents air from getting into the house—an effective countermeasure against drafts and increased utility bills.
In order to be effective, housewrap needs to have two properties: water shedding and a higher moisture vapor transmission rate (MTVR). UV resistance is another factor to keep in mind because housewrap is often left exposed for long periods of time during construction. The standard MTVR is about 200 grams per 100 square-inches over 24 hours or greater. For example, the well-known Tyvek is an MTVR of 400.
Different Types of Housewrap
There have been different kinds of housewrap introduced to the housing development market over the years. Originally, housewraps were an asphalt-impregnated felt paper, called “tar paper,” which is still used today underneath shingles. The name “housewrap” became more appropriate with the release of plastic fabric types, such as Tyvek.
There are currently six main types of housewrapping:
- Asphalt felt/tar paper – required to use a Type 1 felt that meets ASTM D-226 standards.
- Polyolefin fabric – standard “plastic” housewrapping that is made of either woven polypropylene or polyethylene fibers. Some examples on the market today include HomeWrap, PinkWrap, Typar, R-Wrap, and GreenGuard.
- Liquid water-resistant barrier (Liquid WRB) – a tar-like liquid that is applied by a paint roller or spray, serving as an air barrier.
- Grade D building paper – similar to asphalt felt kraft paper. Commonly used underneath stucco siding.
- Rigid foam – when properly sealed, rigid foam insulation can serve as a WRB.
- WRB sheathing – made from oriented strandboard (OSB) and covered in a special coat of water-repellent materials to serve as a sealing for panel joints. Often secured with manufacturer’s tape.
Choosing The Correct Housewrap
As you can imagine, there are hundreds of housewrapping styles and makers to choose from. Whether you opt to go with a familiar brand like Tyvek or what to consider something new, here are tips to help you choose the right housewrap.
Remember, that housewrap is going to be the only barrier against inclement weather for several days to months, depending on the construction. Because of this, housewrap needs to be capable of withstanding damage that comes from more than just the weather. Consider the tensile strength (or “tear resistance”) of the product, UV resistance, and cold resistance. Ideally, UV resistance should provide 90-180 days coverage, even though housewraps should be covered within 30 days of application. Cold resistance is what prevents housewraps from cracking in below-freezing temperatures.
The most basic function of housewraps is to serve as a barrier against moisture. The International Code Council Evaluation Service (ICC-ES) allows for three testing standards: the boat test, water ponding, and hydrostatic pressure. Avoid the housewraps that have only undergone the boat test, since that solely considers humidity transfer. Water ponding and hydrostatic pressure tests are more stringent and can approximate how much water can be sheeted off the housewrap within a set period of time. The best housewrap is going to be the one that passes both water ponding and hydrostatic pressure tests with high numbers.
The higher the “perm number,” the more vapors will get through. For housewrapping, you want a permeance range of about 10-20 perms. However, it’s important to note that a higher perm rate doesn’t always mean better protection. Micro-perforations will allow for more water vapor but also increase the chance of leakage.
One of the widely accepted measurements of effectiveness would be the drainage since it’s the main job of housewrapping. A study done by Jonathan Smegal and John Straube of the Building Science Corporation found that a 1mm drainage gap drains “water at a rate considerably greater than rainwater is expected to penetrate behind cladding even in extreme conditions.” In other words, the 1mm gap provides a “measured drainage rate in excess of 1.1 liters per minute-meter, more than the extreme driving rain intensity of the worst climate in Canada.”
The four previous factors are important but don’t always give you the whole story of a specific housewrap type. You also want to consider things like weather the housewrap is resistant to surfactants, if it has features like creping, embossing, weaving, or filament to add gaps for drainage, if it perforated or not, or woven.
That wraps up our guide to housewraps. Hopefully, you should now understand the basics of housewraps, including what they do, what kinds exist, and how to select the right kind of housewrap. You should always consider housewrap with the same dedication and thoughtfulness as you would siding or shingles. While no one will see the housewrap after some time, the integrity of the building depends on it.
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