By Eric Youngblood
From the November/December 2016 Issue
Building envelopes are designed to protect or cover the interior space of a building. Therefore, exterior walls need to be designed to withstand the weathering elements of its location. It is crucial for designers and architects to not only think about the aesthesis of the exterior, but what materials are best suited to keep moisture from penetrating the wall system and reduce the loss of heat thus keeping an architectural credo— form should always follow function. Moisture intrusion of the exterior wall system can lead to corrosion of structural elements, rotting or deterioration, reduction of insulation, and the growth of mold. Exterior wall systems must be structurally sound, ensure a healthful environment, and meet energy codes and regulations.
Moisture And Thermal Management
The key elements for moisture and thermal management are the climate zones and the choice of materials. Across the United States, there are several different climate zones ranging from very cold to hot and humid. As with all types of exterior wall systems, the type of materials used on an exterior metal wall system can help eliminate moisture intrusion and provide energy efficient thermal performance. Weathering elements, like wind-driven rain, can cause moisture to penetrate a wall cavity and, depending on the exterior and interior temperatures, may result in the development of condensation.
There are several preferred methods used to weatherproof or minimize water and moisture migration into the building envelope. An exterior wall system should include the assembly of metal components that work together to provide air, moisture, and temperature barriers designed for the local climate. Metal wall systems are more weather resistive, controlling water and air movement. Building envelopes were once made with wood and stone, which didn’t help to conserve energy use. Although new wall systems are thinner, new technology allows for minimization of moisture instruction and reduced energy costs during building operation.
The types of metal wall panels available include: insulated panels, exposed fastener panels, and interior wall liner panels. Insulated metal wall panels are an ideal choice for many building envelopes, providing optimal thermal performance and sustainability.
Types Of Barriers
Different types of barriers or retarders are installed to manage moisture and thermal performance. Designers can improve thermal performance by using the correct air and water barriers and the thermal barrier of insulation within the wall system.
There are several options that should be considered when selecting the right barrier for the building skin. Traditional construction methods require a combination of material and trades installing multiple components to make up a complete system. Insulated metal panels allow for one single component to deliver the desired water, vapor, and air barrier, as well as thermal performance and exterior aesthetics.
Vapor barriers or vapor retarders control or limit the flow of vapor across an exterior wall system. The material and placement of vapor retarders are important as misplacement can cause moisture to enter a dry zone of a wall system leading to concerns of corrosion, deterioration of materials, and risk of mold growth.
Meanwhile, water barriers are designed to keep water from entering a wall system. These are not to be confused with vapor barriers. Exterior wall systems may include a combination of air barriers, vapor barriers, and water barriers that are made from a single material.
Other types of barriers include vapor permeable air barriers and water-resistive barriers. Water-resistive barriers are not air barriers, and it’s important to understand the different functions when determining what materials to use for the exterior wall system.
Climate Zone Considerations
Multi-component wall systems in cold weather require a vapor barrier-retarder. Cold climates experience low temperatures outside and warmer temperatures inside. If the vapor barrier is installed incorrectly, moisture can enter the dry zone leading to condensation that can cause corrosion of the metal studs and reduce thermal performance of the insulation.
In hot, humid climates, multi-component wall systems call for a vapor barrier that is installed at the exterior of the wall system. Hot climates experience high temperatures outside with cooler temperatures inside the building envelope. The type of barrier installed should use a multipurpose material in order to limit the flow of air, water, and vapor. If the barrier breaks, water and air can enter the wall cavity causing condensation that can lead to corrosion of the metal studs and deterioration problems.
Moderate climate zones in the U.S. use a “smart” vapor barrier, installed in the multi-component wall system. The “smart” vapor barrier is installed in the interior studs and allows variable amounts of air and water to infiltrate. When installed correctly, the barrier changes with the relative humidity. Moderate climate zones experience steady changes from winter to summer, making this barrier a good choice.
Rainscreens
Rainscreens are also utilized in multi-component wall systems, and some designs are made of metal cladding. Rainscreens serve several important functions, including contributing to moisture management and energy efficiency. This exterior cladding sits away from a building’s exterior walls, allowing moisture to drain away from the building envelope. Rainscreens are designed to reduce the chances of water penetrating the wall system, but these should not be mistaken for weather-resistant barriers.
There are two types of rainscreens: back-ventilated and pressure-equalized.
The back-ventilated rainscreen allows air in at the base of the wall system and out at the top of the wall system. Large quantities of rainwater penetrate the joints and run down the back of the cladding system. The pressure-equalized rainscreen is known to be the most effective wall system. This type of rainscreen prevents water from penetrating the wall cavity, while allowing air to enter, ventilating and drying the cavity. Metal cladding used in a rainscreen helps maintain pressure equalization, which is critical for buildings located in areas with high winds.
Sustainability Of Wall Systems
Green design of exterior metal wall systems considers sustainable methods including the recyclability of existing metal materials, energy efficiency, and long-term durability. For instance, composite metal is an exterior cladding material that provides superior weatherproofing. Composite metal panels can also be used in rainscreens and barrier wall systems.
There is a lot to consider when choosing materials for an exterior wall system. With evolving building and energy codes, concerns about energy costs, and the push to use sustainable building materials, designers are continuously challenged.
Youngblood is the director of project management for IWR North America, (formerly IWR Building Systems), headquartered in St. Louis, MO. A specialty contractor focused on total building envelope solutions, IWR has been working throughout the United States since the 1940s. IWR is a subsidiary of MHS Legacy Group, a diversified national holding corporation also based in St. Louis with roots back to 1895.
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