Applying a high-quality perforated house wrap behind the siding of a structure ensures an energy-efficient, healthy, and comfortable building or home. The first introduction of house wrap was in the late 1970s. Its purpose was to create airtight, energy-efficient structures. Unfortunately, in the 1990s, unhealthy and dangerous mold and mildew were found growing within the wall systems of these airtight buildings and homes, particularly those built with engineered-wood.
The discovery of mold and mildew led to concerns about moisture accumulation in the wall cavities. The house wrap industry solved this issue by adding vapor permeability to the design of the air- and water-resistant house wrap. Today there are many vapor permeable house wraps to choose from, including perforated; however, the quality is not all the same. A high-quality perforated house wrap meets or exceeds the International Building Codes (IBC 1403.2) requirements for water and air resistance, along with vapor permeability.
A high-quality perforated house wrap also considers the wrap’s tear, UV, and low-temperature-resistance, along with ease of installation. Selecting the right perforated house wrap will create an energy-efficient, healthy and comfortable building or home.
Evolution of Vapor-Permeable House Wrap
Two events triggered the need for vapor-permeable house wrap in home and building construction:
- The shift towards engineered-wood products over solid wood products during the mid 20th century
- A move towards energy-efficient building design during the 1980s
Both the use of engineered-wood products and airtight construction allowed moisture to accumulate in the wall cavities where mold and mildew could thrive. Regrettably, it was not until the 1990s that builders became aware of the moisture buildup in the wall cavities and of the health problems associated with mold and fungi. This awareness precipitated the need for proper moisture permeability within a structures exterior walls, including the use of perforated house wrap.
Early Energy-Efficient Designs Caused Moisture Accumulation in Wall Cavities
During the 1970s, growing environmental concerns and rising fuel costs created awareness and need for more energy-efficient structures. Consequently, development of the first Model Energy Code (MEC) occurred in 1983. The 1998 MEC was the successor to the International Energy Conservation Code (IECC)1.
During the late 1970s and early 1980s, builders begin making efforts to create more airtight structures that saved energy and money, primarily by adding more insulation. Also, in 1979, the first house wrap was introduced that provided an easy way to reduce air leakage and water intrusion. Unfortunately, these energy-efficient structures were so airtight, that when moisture accumulated in the walls, it became trapped, which led to mold and rot.
Engineered-Wood Products Absorb Moisture and Dry Slowly
Engineered-wood products, commonly used in modern construction, absorb and hold moisture, which can lead to mold and mildew growth. Before the 1900s, solid-wood framing and sheathing products from old-growth trees (100 to 500 years old) were used to construct most homes and buildings.
Old-growth wood is sturdy, permeable and quite resistant to mold and rot. After 1940, much of the older growth lumber had been harvested, leaving builders the option of using young wood (12-20 years), which is less sturdy and permeable compared to the old-growth wood. In addition, during the mid-1940’s there was a post-war need for faster and cheaper construction. This resulted in engineered-wood products, like gypsum and plywood, replacing solid wood as a building material.
The problem with engineered-wood products is it readily absorbs water and dries slowly, if at all, which makes buildings constructed with gypsum, OSB, or plywood susceptible to mold and fungi.
Vapor Permeable House Wrap
Creating a house wrap that was vapor permeable and air and water resistant was the obvious solution to moisture accumulation within airtight exterior walls, particularly those built with engineered-wood products. Currently, installing house wrap not only creates a high-performing building or home, but helps the structure meet the International Building Codes (IBC) minimum requirements for exterior walls.
According to the IBC 1403.2, exterior walls shall provide the building with a weather-resistant exterior wall envelope, including a manner to stop water from accumulating within the wall assembly by providing a water-resistive barrier behind the exterior veneer. A vapor permeable house warp is essential to preventing accumulation of moisture within a wall system.
Today, non-woven or woven plastic fabrics made from either polyethylene, polypropylene, or a combination of both (polyolefin) are used to make house wraps. The fabric provides the air and water resistance, and integrity of the house wrap. Permeability is incorporated to house wraps by two methods: non-perforated and perforated.
- Non-perforated house wrap is a non-woven, tough, and uniform material made of fused fine high-density polyethylene or polypropylene fibers. Non-perforated house wraps are microporous allowing water vapor to pass through the fibers of the plastic fabric as the material resists water penetration.
- Perforated house wrap is woven synthetic plastic with tiny holes that allow moisture to escape. A thin layer of polypropylene film coating on the backing of the house wrap ensures air- and water-resistance of the house wrap. However, the film lacks vapor permeability. Making the house wrap permeable involved poking (perforated) thousands of holes into synthetic plastic. These perforations allow water vapor to escape the wall system. Unfortunately, the pores also reduce the house wrap’s air and water resistance. For many perforated house wraps, there is a trade-off between moisture vapor permeability and air and water resistance; however, this is not the case with high tier building wraps.
The IECC is a model code of the family of International Codes (ICodes). The International Code Council, Inc (ICC) first published the IECC in 2000. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE 90.1) is the ICC referenced standard for the IECC.
The goal of the IECC is to establish standards and codes for the minimum design and construction requirements for energy efficiency, for renovated and new Revision of the International Energy Conservation Code occurs every three years.The most recent 2018 IECC encourages efficiency in envelope design, mechanical systems, and lighting systems as well as the use of new materials and techniques.