All multifamily buildings must meet code requirements for fire resistance between dwelling units, both horizontally (i.e., side by side) and vertically (i.e., above and below). They also need to meet requirements for acoustical performance such that sound transmission between dwelling units is controlled or minimized. The means for constructing these outcomes can vary depending on the types of materials and products used in floor and demising wall assemblies. One product that has been used extensively overseas is now becoming very popular in the United States as well. Boards or panels made from magnesium oxide (MgO) are showing up on projects across the country to act as a subfloor, sheathing, wall board, or underlayment material. In the process, they are bringing enhanced fire resistance and acoustical performance to many building types, most notably multifamily projects. Based on this emerging popularity, this course looks more closely at designing and constructing with MgO panels. Beginning with a description of the material and its properties, its code compliance characteristics related to fire resistance are discussed. Recognizing the latest code requirements for acoustic control in multifamily buildings, the sound attenuation capabilities are also reviewed. Finally, a discussion of its installation, particularly compared to other alternatives such as a wet-laid gypsum underlayment, is presented. Overall, the suitability and advantages of using MgO panels is provided for consideration in the specifications and design of multifamily projects all around the country.

All images courtesy of Huber Engineered Woods, LLC

Multifamily buildings commonly use wood framing as the basis for their construction. Subfloor panels made from magnesium oxide (MgO) are installed on this project, providing characteristics related to fire resistance and acoustical performance.

Magnesium Oxide (MgO) Panels Overview

What makes for a good building product for subflooring and wall sheathing? Most professionals would include traits such as predictable performance, consistent quality, ease of installation using common tools, compatibility with other materials, sustainability, and affordability all as important. Criteria such as these have typically driven the creation and proliferation of many board products such as plywood, engineered wood, cement board, and gypsum boards, all of which are commonly used for subflooring, wall sheathing, and other applications. Nonetheless, there is always room for improvement and creative product development to better meet the needed criteria.

Beginning around the year 2010, a new board product emerged as a popular choice. Made primarily from magnesium oxide (MgO) with some other added compounds, it is similar to a cement panel product. When the MgO products are formed, it is actually a controlled chemical reaction that allows the boards to become solid, much the same way that concrete is a controlled chemical process that creates a hardened material. Note that the primary ingredient is oxidized magnesium, which is a very stable and inert mineral, not the raw metal magnesium (Mg) itself, which can react dramatically with other chemicals. This is a significant difference since it makes MgO very suitable for a building product while Mg by itself is not.

Manufactured MgO board products have also been available in the United States and other countries for a number of years. The formulation of compounds or materials added to the MgO will vary by manufacturer but can include magnesium chloride (a type of salt), perlite, or wood fiber. These added ingredients help define the overall performance characteristics of the finished products or help to benefit the manufacturing process. High-quality panels incorporate an integrated mesh core as a means to increase some preferred characteristics such as dimensional stability and strength. In any of these cases, the general process of creating MgO boards is similar to creating gypsum board or cement board products. Carefully selected ingredients are assembled and run through a quality-controlled process in a factory that results in uniformly produced end products.

MgO panels are stacked and installed similarly to wood panels.

Characteristics

What has led to the development and use of MgO board in buildings? Primarily the fact that it is a demonstrated alternative to some other traditional products and offers some compelling characteristics.

Two traits in particular are especially relevant to using MgO boards in multifamily buildings, and they are discussed in more detail later in this course. These traits are:

• Fire resistance: Inorganic materials typically do well with providing fire resistance, and that is the case with MgO boards too. With a melting point over 5,000 degrees Fahrenheit, it is able to provide considerable levels of fire resistance in buildings as part of a floor or wall assembly.
• Sound absorption: The ability of materials to transmit, absorb, or reflect sound waves is directly related to their makeup. MgO boards have been tested and shown to provide very favorable performance. Additional characteristics of MgO board include the following:
• Environmentally friendly: Made primarily from inorganic materials, MgO boards rely on natural ingredients that can be responsibly obtained and processed.
• Inherent strength: The combination with other materials and compounds, including mesh cores, helps assure that the strength and rigidity of the boards is achieved and maintained over time. As a hardened board product, it handles compression loads readily.

  With the appropriate added materials, tension forces are absorbed and dissipated. It is reported that some people have a misperception about the strength of MgO boards since they assume that the product is brittle or non-structural like fiber cement. In fact, that is not the case for many products which are able to carry and withstand some notable loading. This can vary by product of course, so the exact structural capabilities of any product should be verified or confirmed with the manufacturer.

High-quality MgO panels have an integrated mesh core that enhances strength and dimensional stability.

• Dimensionally stable: The inorganic nature of the product means that it is very dimensionally stable. It is not prone to swell or expand and contract due to moisture and temperature changes.
• Resistance to mold and mildew: The inorganic makeup helps in this regard too. Products have been tested in accordance with ASTM G21 "Standard Practice for Determining Resistance of Synthetic Polymeric Materials to Fungi." This standard is designed to test the resistance of synthetic polymeric materials (molded materials, surface treated articles or similar objects) to fungal attack. While MgO board is not a synthetic polymer, this has become the accepted standard to test manufactured building products of this type, particularly those that may be exposed to moisture. The test itself is based on a 28-day period where the product is exposed to specific fungi (molds, etc.) in conditions of temperature and humidity conducive to their growth. The results of this test on MgO boards produced multiple scores of zero, meaning the tested specimens remained free of any fungal growth.
• Water vapor permeability: Manufacturers of MgO board generally do not recommend lifelong outdoor exposure, but the products have been tested for water vapor permeability. The standard test for this situation is ASTM E96: Standard Test Methods for Water Vapor Transmission of Materials. There are two basic methods provided: 1) the Desiccant Method and 2) the Water Method. While either is valid, the one that most closely matches the installed conditions is preferred with the understanding that different test methods may produce somewhat different results. MgO has been tested under both options, with the results of Method A indicating a rating 5–10 perms and Method B on the order of 15–20 perms.
• Insect damage resistance: With the absence or extremely limited presence of any wood or other organic materials in the board, insects and vermin are not typically attracted to it. Therefore, the damage from such infestations can be avoided, differentiating MgO board from common all-wood based products.