Magnesium Oxide Board
cladding: MATERIALS ENCYcLOPEDIA
Applications for this system
Interior wall, ceilling and floor sheathing
Exterior wall and roof sheathing
Backer board for tile showers
Soffit and fascia
Fibreglass mat facing
Amendments for soil mixture, if necessary. Can be graded gravel, sand, road base, portland cement, hydrated or hydraulic lime, blast furnace slag or fly ash
Paper or mesh tape for joints
Ratings Chart for Magnesium oxide board sheathing
The ratings chart shows comparative performance in each criteria category. Click on the tabs below for detailed analysis of each criteria.
- HOW THE SYSTEM WORKS
- ENVIRONMENTAL IMPACTS
- EMBODIED CARBON
- ENERGY EFFICIENCY
- MATERIAL COSTS
- LABOUR INPUT
- SKILL LEVEL REQUIRED
- SOURCING & AVAILABILITY
- CODE COMPLIANCE
- INDOOR AIR QUALITY
- FUTURE DEVELOPMENT
Magnesium oxide board System
Magnesium carbonite ore is harvested and heated in a kiln at 650 ∞C (1200 ∞F) to form magnesium oxide. This material is ground, made into slurry with water (and sometimes fibers and admixtures) and formed into sheets, usually between two facing mats of fiberglass. The sheets are air-cured into a hard board in standard thicknesses (usually 3/8, 1/2 and 5/8 inch) and sheet sizes (48 or 54 wide and 96, 120 and 144 long).
The sheets are fastened to framing using screws. When used as a finish wall cladding, the joints are taped, mudded and sanded in the same manner as drywall. When used as structural sheathing the joints are typically not treated.
The boards are strong enough to be as used structural sheathing in many applications, including exterior walls and floors.
Environmental Impact Rating
Harvesting — Moderate to High
Magnesium carbonate is quarried from surface-based pits in Asia (mostly in China). It is difficult to obtain information about these quarries and their impacts, but they will presumably have a similar impact to other surface extraction operations, including habitat destruction and disturbance and/or contamination of ground and surface water.
Manufacturing — Moderate to High
Magnesium carbonate converts readily to magnesium oxide at around
650 ∞C (1200 ∞F), a much lower temperature than required to kiln limestone for portland cement, but still requires a substantial amount of energy. Carbon dioxide is emitted from the heated ore, as with portland cement, but that CO2 is largely reabsorbed by the material during its curing process, making it a much lower carbon alternative to portland cement board products.
The fiberglass matting used on both sides of magnesium board requires high energy input to melt a mixture of silica sand, limestone, kaolin clay, fluorspar, colemanite, dolomite and other minerals to liquid form. The liquid is squeezed through small orifices to make fiber strands that are chemical coated and woven or glued together to make mats. This process yields high carbon and air pollution results.
Transportation — High
Sample building uses 1,668.6 kg of magnesium board to sheath interior walls:
2.5 MJ per km by 15 ton truck
1.57 MJ per km by 35 ton truck
0.42 MJ per km by rail
0.27 MJ per km by ocean freight
Magnesium board is a heavy material and currently all products are manufactured in China and must be shipped to and distributed throughout North America.
Installation — Moderate to High
Dust from magnesium oxide board should not be inhaled, and can be distributed throughout the house, especially when making saw cuts in the material. More problematic is the dust created when sanding jointing compound. This is extremely fine dust and can become dispersed throughout the home (including heating and ventilation ductwork) and in the environment around the home. Many joint compounds contain anti-fungal agents and other toxic chemicals that installers will not be able to contain fully.
Waste: Moderate to high
Compostable — None. While the core magnesium oxide could be left in the environment or used as aggregate, it is not easy to separate from the fiberglass matting on both sides.
Recyclable — None. There are currently no programs for magnesium oxide recycling, though in theory the material could be recycled.
Landfill — Magnesium board offcuts. Quantities can be low to high, depending on the requirements of the installation.
Chart of Embodied energy & carbon
Magnesium board can be used as an effective primary air control layer on the interior and exterior of the building as long as seams are properly gasketed, caulked or taped. The product has no thermal control properties.
Material costs: High
The production of magnesium oxide board is significantly lower in embodied energy than drywall, but limited production and distribution result in significantly higher costs.
Labour Input: High
Like drywall, magnesium oxide board is a multiple-stage material to install. Sheets must be cut to size and mounted, joints taped/meshed, and joint compound applied in two or three coats with drying time and sanding required between each. Professional tools can greatly reduce labor input. Cutting will be more labor-intensive than with drywall.
Dust from mag board and, in particular, joint compounds is toxic and proper breathing protection must be worn.
Skill level required for homeowners
Preparation of substrate — Easy
Basic framing and carpentry skills.
Installation of sheathing — Easy to Difficult
Large sheets are easy to install, though level of difficulty increases with quantity and complexity of cuts, penetrations and intersections. Cuts require more effort and higher quality tools and blades than with drywall.
Finishing of sheathing — Difficult
Mudding and sanding joints, corner beads and intersections to a good degree of finish require experience.
Exterior mag board sheathing does not require jointing.
Sourcing & availability: moderate
Magnesium oxide board is not regularly stocked at all building material outlets, but can be special ordered from some.
Mag board is not affected by water and is very impact-resistant. It is less prone to wear in high-traffic locations than drywall.
Magnesium oxide board may not be recognized directly as an acceptable solution in all jurisdictions, but several manufacturers have done testing to meet ASTM E 136-09 and ASTM E 84 standards for sheathing materials. With this documentation, it should be straightforward to prove equivalence with gypsum board products.
Indoor air quality: high
Magnesium oxide board is often the product of choice for sheathing in homes for hypoallergenic homeowners. It does not support mold growth even when continuously damp, and contains no off-gassing compounds.
Pre-mixed joint compound will contain fungicides/biocides that are persistent in the environment. Most wet and dry jointing compounds will contain formaldehyde, ethylene vinyl acetate latex and other additives. These will be listed on the product’s MSDS sheet, and all will negatively affect IAQ.
Complicating the issue is the level of dust created when sanding joint compound. The resulting dust is very fine and pervasive, carrying traces of all the chemical additives as well as silica throughout the home. Heating and ventilation ducts are particularly vulnerable to being coated in this dust.
There are a few brands of joint compound that do not contain any chemical additives and are considered hypoallergenic. Note that the dust from these compounds is still high in silica and should not be inhaled.
Resources for further research
Baker-Laporte, Paula, Erica Elliott, and John Banta. Prescriptions for a Healthy House: A Practical Guide for Architects, Builders and Homeowners. Gabriola Island, BC: New Society, 2008. Print.
There are not many significant ore deposits in the USA, though there are several large ones in Canada. Currently, no magnesium oxide board is manufactured in North America, which keeps the price and shipping impacts high. The benefits of this product compared to gypsum board are numerous, in particular its excellent resistance to moisture and its structural properties. As it is so similar to gypsum drywall in sizing and installation procedure, it may start to gain a larger market share. If demand grows sufficiently, North American production could be instituted, helping to drive down costs and shipping impacts.
Currently, the lack of standards for the product can result in widely varying quality between manufacturers. This too may be addressed by increased acceptance in the market and in building codes. North American production would certainly be accompanied by high production standards.
Tips for successful Magnesium oxide board sheathing
1. In general, the same installation procedures and techniques are used as with drywall. Mag board is harder than drywall, so the scoring and cutting is more difficult and time-consuming. A carbide tipped knife is recommended.
2. Different mag board products are available, each with particular intended uses. Be sure to order the correct product for your application(s).