Referred to in the building code as a “water-resistive barrier,” the main goal of a sheathing wrap or housewrap to keep liquid water out of the structural part of a building. At the same time, the sheathing wrap must allow water vapor to pass through so the framing and sheathing can dry to the exterior if gets wet. Fortunately a number of materials, including traditional asphalt felt (tar paper) have this ability to stop water but remain “permeable” to water vapor.
Types of Sheathing Wrap
All sheathing wraps fall into three basic types: asphalt felt, Grade D building paper, and synthetic housewrap.
- Asphalt felt (tar paper) was the the traditional choice of builders for many years and still has some advantages over newer alternatives.
- Grade D building paper is used primarily under stucco in the western United States and is essentially a lighter-weight version of asphalt felt.
- Housewrap refers to the wide variety of synthetic sheathing wraps widely used today. Originally over-sold as energy-savers, their main function is the same as other sheathing wraps – to keep out liquid water out of the structure, but let water vapor pass through to allow wet buildings to dry out.
Comparing one sheathing wrap to another is difficult since there is no single test standard for all products, and even where manufacturers follow the same standard, test conditions may vary dramatically from one company to the next. More important may be the material’s ease of installation and durability over time. Some of the synthetic products look great on the day of installation, but degrade quickly when subjected to prolonged wetting or exposure to UV.
In my experience, traditional 15lb. asphalt felt often outlasts the synthetic housewraps, especially where it remains wet for extended periods. Under prolonged wetting, plastic housewrap tends to deteriorate. Another advantage of traditional asphalt felt is that it can absorb excess moisture and release it later when drying condition improve.
Performance Characteristics of Sheathing Wrap
In order to be accepted by code as a “water-resistive barrier,” a housewrap must meet specific measures for permeance to water vapor, resistance to liquid water, and air infiltration.
Permeance. Permeance ratings measure the rate at which water vapor (as opposed to liquid water) passes through a material. In general, a sheathing wrap should have a permeance of at least 5 perms, similar to dry asphalt felt. High permeance allows wall assemblies to dry out should they get wet. Plastic housewraps range from around 5 to over 50 perms. Since common sheathing products like ply¬wood and oriented-strand board (OSB) have permeance ratings of less than one, the sheathing is more likely to interfere with drying than the sheathing wrap.
Water Resistance. There are several ways to measure the ability to stop liquid water from passing through. In general, products with high permeabil¬ity, such as perforated housewraps, do less well on water resistance tests. In practice, however, all approved sheathing wraps do a good of shedding liquid water in an actual building.
Air Infiltration. Many sheathing wrap suppliers tout their products’ ability to block air infiltration, often citing proprietary test results. Some follow ASTM E283, in which an 8-foot-square wall section is tested before and after installation of the sheathing wrap. However, since the manufacturer is free to specify the type of wall assembly, one test is not comparable to another, and none simulates real job-site conditions with seams and holes in the sheathing wrap.
If a house already has a reasonably tight wall assembly, there is little evidence that a layer of housewrap will significantly tighten the building. In general, air-sealing efforts are better spent on the building’s interior, using caulks and gaskets or a continuous polyethylene air/vapor barrier.
Choosing a Sheathing Wrap
In comparing sheathing wraps (also called sheathing paper and housewrap), you can look at a lot of technical specs regarding permeance, water resistance, and air infiltration. Unfortunately, these tests don’t tell us too much about real world performance. Also companies tend to use the test protocol that favors their product over their competitors The good news is that, installed carefully, any of the sheathing wraps can perform well as an effective barrier to water penetration. Other important factors to consider are how long will they last under adverse conditions (for example, when wet, hot, or exposed to UV for extended periods) and how quickly they will let a wet wall dry out.
The three main choices are traditional asphalt felt, Grade D building paper, and the newer plastic housewraps. The optimal product will depend upon the siding choice, building details, and cli¬mate. With any sheathing wrap material, however, the key to good performance is to carefully lap the material to shed water. This job has been made easier by the introduction of a number of peel-and-stick membranes for use around windows, doors, and other trouble spots. General performance characteristics of sheathing wraps are summarized in the Table below.
What’s a Perm, Anyway? Although numbers can be misleading, it’s useful to understand the basics of permeance. Permeance ratings, measured in perms, indicate the rate at which water vapor will pass through a material when the humidity is higher on one side. This indicates how quickly water can dry out if blocked by this material.(Technically speaking, one perm = one grain of water vapor passing through 1 square foot per hour per inch difference in vapor-pressure).
In general, a sheathing wrap should have a perm rating of at least 5 to enable walls to dry out in a reasonable period of time should they get wet. Traditional sheathing wraps such as asphalt felt and Grade D building paper (used under stucco) both measure about 5 perms. Plastic housewraps range from around 5 to over 50 perms.
By comparison, common sheathing materials like OSB and plywood have perm ratings of less than one, so they would slow down drying. Other sheathing materials that can trap water in a wall are
- Foil-faced panels: Vapor impermeable
- Extruded foam insulation (XPS), such Styrofoam and Foamular: perm rating of 1 for a 1 in-thick material; 0.5 for 2-in. thick panels.
- Expanded foam insulation (EPS), often called “beadboard”: per rating of about 3 for -1-in. panels, 1.5 for 2-in.
Asphalt Felt. The old standby, often called “tar paper,” allows drying with a perm rating of of approximately 5 (more when wet) and blocks water effectively on a vertical wall, making it an effective sheathing wrap .
Asphalt felt has other desirable qualities as well. If water leaks into a wall, the asphalt felt will tend to soak up the excess water (as will lumber and wood products like plywood), helping to store the water until drying conditions improve. Also, once wet, the permeability asphalt felt actually rises from 5 to as much as 60, further promoting rapid drying. This is the same characteristic engineered into some of the hi-tech “smart” sheathing wraps. In contrast, plastic housewrap does not absorb water and tends to trap liquid water (although it will permit slow drying by letting water vapor pass through.
While plastic housewraps now dominate the market, some contractors have returned to asphalt felt as a weather barrier on sidewalls. In addition to its good water-management characteristics described above, some people find that felt paper is easier to install because of its rigidity and narrow 3-foot roll width and easier to properly tie into flashings. Felt, however, will tear if not handled carefully, and tends to get brittle and deteriorate from UV if left exposed to sunlight for
extended periods. Otherwise, asphalt felt re¬mains a good option for modern homes.
However, remember that what is sold as #15 felt today is typically half the weight of traditional 15-lb. rag felt, which weighed – guess what –15 pounds! To get something comparable today you need to buy unrated #30 felt which weighs 15 to 20 lbs/square, or special-order ASTM-rated #Type 1 felt paper which weighs 11.5 lbs/square (ASTM D226). For certain critical applications, you may want to use ASTM-rated Type 2 felt, which weights 26 to 27 lbs./square. The modern felt papers are made from recycled cardboard and sawdust rather than cotton rags, but have similar properties.
Grade D Building Paper. Grade D building paper is an asphalt-impregnated reinforced paper, similar to the “kraft” paper backing found on fiberglass insulation. Unlike asphalt felt, it is made from new wood pulp, rather than recycled material. Grade D paper is commonly use is under stucco in the western United States. The vapor permeance of Grade D paper is similar to asphalt felt and it blocks liquid water effectively in tests for anywhere from 10 to 60 minutes.
However, in the real world Grade D paper tends to deteriorate when wet for extended periods. To protect against leakage in traditional three-coat stucco, contractors, often use two layers of 30-minute paper. Because the paper layers tends to wrinkle when installed, the two layers form a small air space creating a protective rain screen.
Plastic Housewrap. There are a wide array of plastic housewraps on the market. Most are nonwoven fabrics made from polypropylene or polyethylene. Some are engineered to allow water vapor to pass through the material, while others have small perforations to let water vapor through. Because there are several testing stan¬dards for plastic housewrap that yield very different results, it is difficult to make apples-to-apples comparisons. However, several field studies and published test data suggest the following:
Permeance to water vapor. In general, non-perforated housewraps (Tyvek, Amowrap, and R-Wrap) are considerably more permeable to water vapor, ranging from 48 to 60 perms, than perforated housewraps. Typar, a popular non-perforated material, is rated at 9-15 perms.
Water resistance. All sheathing wraps, including asphalt felt paper, will effectively shed water on vertical surfaces. Pooled water, however, will leak through most perforated plastic housewraps over time, while the nonperforated materials will contain liquid water indefinitely.
Damage from extractives. Researchers have reported that extractives leaching out of redwood and cedar siding can cause plastic housewrap to lose its water repellency and to deteriorate. Back-priming the wood siding or leaving an air space behind the siding will help prevent this. Stucco will also degrade plastic housewrap and is rarely installed over it. Details that trap water against the housewrap, such as poorly designed window flashings, promote rapid deterioration of the housewrap.
Energy efficiency. When first introduced, plastic housewraps were heavily promoted as an energy-conservation product, based on some questionable data showing that they reduced air leakage rates when installed with great care on a very leaky house. In real world applications using standard building techniques, there is little evidence that housewrap will significantly improve a new building’s energy performance. It is possible to use housewrap as an effective exterior air barrier, but this would require extensive taping and sealing, and proper detailing at all penetrations and transitions to other materials.
Recommendations. Given their high perm ratings and ability to block liquid water, the non-perforated housewraps are a good choice for most building applications, with the caveat that careful detailing, especially around windows, is required to avoid premature deterioration of the material. Traditional asphalt felt is also a good option.
Many contractors find the 10-ft-wide rolls of lightweight plastic housewrap more convenient to install than heavy 3-ft-wide rolls of asphalt felt. Also plastic housewrap remains flexi¬ble in the cold and is less likely to tear than felt paper. Some find the narrow width and greater rigidity of asphalt felt an advantage. Either material can perform well if installed correctly.
Draining Housewraps. With growing interest in rain screens, manufacturers have responded to the need for an air space and drainage plane with a variety of plastic housewrap products that are either wrinkled or corrugated to provide an integrated air space. These include products intended primarily for stucco, such as DuPont StuccoWrap, and others developed for use under siding. These include Pactiv Raindrop 3D, Barricade Drainage Wrap and Barricade WeatherTrek, Benjamin Obdyke’s Hydrogap, and Valeron Vortec.
The small air space created by these products generally range from 1-2 mm (one mm = about 4/100 inch). Although these materials may allow for a capillary break and some drainage (assuming they are not crushed by the siding installation) it is unlikely that they will provide any measurable airflow to promote drying. Where better drainage and ventilation are needed, consider using a plastic drainage map, such as Benjamin Obdyke’s Home Slicker HP, which creates a 6-10 mm gap.
Sheathing Wrap Installation
The main purpose of the sheathing wrap, whether building felt or plastic housewrap, is to prevent water leakage into the building structure. It is critical, therefore, to cover the entire shell from roof to foundation, including gable ends and band joists, and always to lap upper layers over lower layers, shingle-style, to shed water. It is also critical to correctly weave the sheath¬ing wrap into all wall flashings, paying special attention to the flashings around windows, doors, and other penetrations in the shell.
Shingle Principle. While the technical ratings of sheathing wraps vary significantly, how they are installed is far more important than the specific product used. The key is to always lap the joints in sheathing wrap so water always flows away from the building and on top of the layer below, shingle style.
Equally important is to properly detail all areas where sheathing wrap meets flashings, especially around door and window openings. The same Shingle Principle applies: Materials should always be lapped to shed water over the top of the layer below – always directing the water away from the building – never into the structure.
Code Requirements. The building code (International Residential Code) requires that all new homes have a “water-resistive barrier” which in practical terms means a waterproof barrier behind the exterior cladding. According to the code, the barrier should consist of a weather-resistant sheathing paper lapped at least 2 inches horizontally and 6 inches vertically. It also states that sheathing paper be properly integrated with the flashings used around doors and windows and other penetrations or intersections with other materials (see Illustration).
The sheathing paper specified by code (IRC) is ASTM-rated Type 1 paper (ASTM D226), an asphalt-saturated felt (tar paper) which weighs a minimum of 11.5 pounds per square. Most of the felt paper stocked by lumberyards today is either unrated #15 felt, which typically weight around 7 or 8 lbs. per square (100 sq. ft.) or unrated #30 lb. felt which weighs 15 to 20 lbs. per square. (Lumberyards often call these items 15 lb. and 30 lb. felt, a holdover from the old days when that what they actually weighed.) In most parts of the country, ASTM—rated papers must be special ordered.
As with most code provisions, this one allows other materials accepted by code authorities as “equivalent.” Most plastic housewraps on the market, such a Tyvek, Typar, Barricade, and a slew of others have all passed muster with the IRC as equivalent to Type 1 building paper.
Detailing at Windows and Doors
Leaks around doors and windows account for a large number of construction failures nationwide, often costing thousands of dollars to repair and the source of many large lawsuits in condominium and other multifamily projects. Often the problems are hidden behind exterior finishes for years, only to show up when the house or unit is being sold or remodeled. Flash doors and windows correctly the first time or you can expect big headaches down the line.
The main problem is that both windows and wall construction, and the flashing materials, themselves, have all changed dramatically over the past 30 years, but finding a reliable, industry-standard way to integrate the various components is still a work in progress.
With modern flange-style windows and doors, detailing flashings and sheathing wrap to shed water is easier said than done, so it pays to use use the best materials and techniques here. The materials used to seal around openings should last for the life of the building – or at least the life of the siding and windows. Never rely on caulks and sealants here – they will not last more than a few year. Cut corners here and you may end up tearing out your windows and the surrounding finishes long before their time, and you may find extensive damage to the windows and surrounding framing.
The code also allows alternative water-resistive barriers that are “code-approved” as equivalent to felt. This includes all the leading plastic housewraps. The housewrap manufacturers recommend 6 to 12 inches of overlap at vertical seams and 4 inches at horizontal laps, with all joints taped.
With plastic housewrap, you should wrap corners at least 6 inches each way. If the walls are sheathed and wrapped before being raised, leave a 6- to 12-inch overlap at one side of each corner, and leave a 12-inch, unstapled flap at the bottom to cover the band joist area after the sheathing is nailed off. Wide staples with a minimum 1-inch crown are recommended every 12 to 18 inches for plastic house-wraps.