It is in the building code-- all homes should have a weather-resistant envelope covering the entire exterior. The principal component is a sheet-covering, which must be in place before the siding, stucco or brick veneer goes on. Its purpose is to keep water from entering the building through the walls. This skin or membrane is a water-resistant barrier, or WRB. We are accustomed to seeing paper on the roof deck before shingles are attached, and it is becoming more and more common to see WRB covering the walls before the siding or brick goes on. Unfortunately, we also find that this barrier is often being installed incorrectly.
The weather barrier is made up of several individual materials: housewrap or building paper (the WRB), flashings, sealants and tapes. When installed properly, these materials combine to protect the building from rain-induced moisture damage. If the WRB is sealed to block air flow it also contributes significantly to improving the home’s energy efficiency.
The WRB can be integral to the sheathing or sheet insulation, but it is usually installed as a separate layer of housewrap or building paper. It blocks the passage of water and air, but allows some water vapor to pass through. The wrap’s perm rating is a measure of how permeable it is to water vapor. A high perm rating means a lot of vapor will pass through; a low perm means most of the water vapor is blocked.
In south Louisiana, the primary vapor drive through the wall for most of the year is from the hot, humid outside to the cool, dry (air-conditioned) interior. The lower the perm rating of the wrap, the less vapor will be driven into the walls of your home. So, a perm rating close to 5 would be a good choice. Building code requires a minimum perm rating of 5. In this climate, where vapor drive is to the interior, there should be not be a low-perm material such as vinyl wallpaper on the inside; moisture that does get into the walls – primarily by condensation -- must be allowed to dry to the interior.
Choosing the System
There are two basic types of weather barriers for walls: housewraps (which are synthetic, plastic material) and building papers.
The housewraps come in rolls 3-feet to 10-feet wide, while building papers are usually 40-inch wide rolls. Lengths of rolls vary from 100 to 2,000 feet long.
There are several products of each type on the market. Use only products that meet code requirements for holding out liquid water and for allowing vapor to pass through (perm greater than five). Labels on the products or product data sheets show the materials have been tested and meet industry and code standards.
Note: Surfactants, soap and some other substances sometimes found in stucco, cultured stone and wood products can reduce the surface tension of water allowing the water molecules to pass through the otherwise water-resistant barrier. This can be addressed by sealing wood and by adding an intervening layer (a double layer) of WRB. The WRB can be exposed to surfactants (soap) during pressure washing. Check with the manufacturer to see if certain product lines are recommended for use with specific types of cladding (brick, stucco, wood or cement siding).
In addition to the housewrap or building paper you will need sealants or caulks, sealing tape, flashing tapes and rigid flashings. It is critically important that these components be compatible, and the simplest way to ensure that compatibility is to purchase the components from a single manufacturer. This will usually qualify you for a longer warranty and will also take the guesswork out of selecting materials that will work well together.
The important issue with compatibility is adhesion—how well the sealants and tapes stick to each other and to the other components in the system. Study the literature from the WRB manufacturer. You will want this system to last for years; there is no substitute for quality, and, in this case, for compatibility. This is not the place to look for cost savings.
Flashings
Flashings are needed wherever the housewrap is cut –for window or door installation, where service lines penetrate the walls, or by accident. Flashings come in two basic types, flexible and rigid. Rigid flashings are typically made of metal or hard plastic and must be formed on a machine. The most common flexible flashings are seen around doors and windows. A similar product is often used on roofs to keep water out.
We can break flexible flashings down further into self adhesive and mechanically attached (staple on). For our hot humid climate with frequent exposure to high winds, the self-adhesive flashings work best. They are manufactured in 4 inches, 6 inches and wider widths, up to 18 inches. The rolls are typically 75 feet long.
Sealants
Use sealants you can confirm are compatible with the other components of the system. When this is not possible, due to availability, the information below may help you find the right one.
Four Types of Sealants:
Silicones do not shrink, do not take paint well and do not stick to wet surfaces. Best use is wet areas, but must be applied to dry surfaces.
Polyurethanes cure by moisture in the air as do silicones. They have good adhesion, are paintable when cured, and are common in some weather-resistance systems.
Acrylics or water-based, cure by evaporation, are very paintable and easy to clean up. They must be allowed to cure before being exposed to rain.
Solvent based cure by evaporation of the solvent. Both acrylics and solvent based sealants shrink depending on the amount of solids. If there are 70 percent solids then the cured size will be 70 percent of the amount applied out of the tube. These have good adhesion, are paintable and stick to most surfaces. Be aware of the potential of noxious fumes. Best used outside.
Installation is the key to successful performance
Once the housewrap and other components have been selected, purchased and brought to site, it is important to follow code and the manufacturer’s installation instructions.
Begin wrapping the building at a corner, starting around the corner a minimum of 12 inches and overlapping the lowest part of the wall framing by 2 inches. Continue covering the building overlapping horizontally by at least 2 inches and vertically at least 6 inches. These are code minimums; more does not hurt and some manufacturers require it.
All manufacturers call for cap fasteners similar to the ones we see on roofing underlayment. These caps can be applied by hand nailing or in a “gun.” This is especially important if the siding or brick is not going up right away. Wind will tear the barrier and leave portions of the building unprotected from rain if the wrap is secured with only small staples like the ones shown in the left of the photo.
Some additional things to consider when installing the weather-resistant barriers:
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Where there is the possibility of exposure to high winds and driving rain additional measures should be taken to protect the building from exposure to excessive moisture. The simplest way is to increase the code-required overlaps. Code requires 2 inches horizontal overlap and 6 inches vertical. Increasing this to 6 inches horizontal and 12 inches vertical will reduce the possibility of wind-driven rain getting past the WRB. Taping these overlaps will also reduce the possibility of unwanted moisture.
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Flashing material comes on rolls that are packaged with a release paper on the sticky side. It is important to not remove the release paper until you are ready to use it. These flashings will adhere well to most clean surfaces. It is important again to make sure there is good adhesion.
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WRB on Steroids: In our very rainy climate, the housewrap or building paper will be most effective if there is a gap between it and the cladding. This gap, which is typical with brick veneer, allows water stopped by the housewrap to run freely down the wall and to the exterior. It can be created by placing furring strips on the wall after the WRB is applied. The siding is then attached to the furring strips creating a space for any water that gets behind the siding to drain to the exterior. This space can also help in reducing the exposure to surfactants that can reduce the effectiveness of the barrier.
Once the building is covered, all of the doors and windows are installed, and the service penetrations are sealed it is time to do a complete inspection making sure everything is lapped shingle style and sealed and to make sure that any tears are patched.
Now you are ready to install the siding, brick or stucco.
Audio clip: What's the difference between a weather-barrier, and air barrier and a vapor barrier?