Magnesium oxide board

Magnesium oxide board

Applications for system

  • Interior wall, ceiling and floor sheathing
  • Exterior wall and roof sheathing
  • Backer board for tile showers
  • Soffit and fascia

Basic materials

  • Magnesium oxide
  • Fiberglass matt facing
  • Paper or mesh tape for joints
  • Joint compound
  • Fasteners

How the system works

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/s, У2 and % inch) and sheet sizes (48 or 54 inches wide and 96, 120 and 144 inches 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 used as structural sheathing in many applications, including exterior walls and floors.

Tips for successful installation

  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).

Pros and cons


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.


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.

Sheathing and cladding materials 195

Magnesium oxide board

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.


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.


The production of magnesium oxide board is significantly lower in embodied energy than drywall, but limited production and distribution result in significantly higher costs.


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.

Health Warnings — Dust from mag board and, in particular, joint compounds is toxic and proper breathing protection must be worn.


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.


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.

Transportation: Mag board transportation by 15 ton truck would equate to 2.5 MJ per kilometer of travel to the building site. Mag board transportation by ocean freight would equate to 0.27

MJ per kilometer of travel to the building site. ‘Typically from


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.


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 fungi- cides/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.


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 Bricks of a typically rectangular shape are cast from a mixture of clay, sand and small amounts of admixture and fired in a kiln at temperatures between 700 and 1100°C (1300 and 2000°F). This heating takes the clay through several stages, including the burn-off of carbon and sulfur, driving off chemically combined water from the clay, quartz inversion, sintering and vitrification. At the end of this firing process, the clay brick is hard and will not soften when it reacts with water.

Bricks are laid in successive courses, bonded by a mortar of clay/sand, lime/sand or lime/ce- ment/sand. There are many different patterns for brickwork, but most feature offset joints between

courses. Keystone arches can be formed to create self-supporting openings, or metal reinforcement is used to form straight openings.

When used as cladding, bricks are attached to the structural sheathing by means of metal ties that are nailed to the wall and embedded in the mortar joint, and a space is left between the brick and the sheathing to create a rainscreen. Weeper holes are left at intervals in the top and bottom courses to allow moisture to escape.

When used structurally, brick walls are typically one part of a double masonry wall, with an insulated core between the two wythes. Ties are placed at intervals between the two walls to increase stability. Structural brick walls are also built using a doublewide arrangement of bricks. It is rare for fired clay bricks to be used structurally in modern construction in North America.

Not Cement Brick. Many brick products on the market are actually cement-based and not clay. There are many resources available to make comparisons between cement and clay brick. Cement brick manufacturers often claim that they are “greener” than fired clay because making them doesn’t require heat. This, however, ignores the high heat input (and significantly higher carbon release) required to make the cement.

A case can be made for the use of cement brick as a green building product, largely based on its durability. It is not included in this book because of its high carbon footprint and low permeability, and because it is an intensive material to be used non-structurally as a cladding.

Fired clay brick cladding palpable. Many clay pits are in or beside waterways, and pit operations can cause silting of the water and disruptions to flow patterns. In general, clay pits are not considered high-impact mining operations, and are excellent candidates for rehabilitation at the end of their life spans.

Manufacturing — High. Clay for bricks requires some mechanical processing. It is ground up and squeezed into a homogeneous mix before being formed. This work is typically all completed by machine in factories, though traditionally it would have been done by hand on or near the building site. The largest impact comes from the firing process, during which the bricks are heated to temperatures of 900 to 1300°C (1650 to 2350°F) for several hours. No toxins are released from the clay during this process, but a lot of fossil fuels are consumed and emissions released into the atmosphere, including sulfur dioxide.

Bricks that are glazed or painted will require additional processes, and these can be more toxic. Paint coatings, in particular, can contain chemicals that are emitted into the atmosphere and/or mixed into water at the factory.

The mortar used between bricks is a significant amount of material. If a cement-based mortar is used, it carries a high carbon footprint. Lime-based mortars have a lower carbon footprint.

Transportation — Low to High. Sample building uses 26,192 kg of brick siding for the exterior: 24.6 MJ per km by 35 ton truck 6.55 MJ per km by rail

Production facilities are typically very close to clay pits, minimizing the transportation of raw materials. Regional manufacturers are close to many major markets, but if this heavy material has to travel long distances it will accumulate high transportation impacts. Installation — Negligible.


Compostable — Clay brick offcuts can be left in the environment or used as aggregate or growing medium. Quantities should be low, as partial bricks can be used throughout the wall.

Recyclable — None.

Landfill — Mortar bags.


Brick cladding will not have much impact on energy efficiency. As a rainscreen, it can reduce the amount of wind that reaches the wall behind, but will not contribute to air tightness. Bricks add little to no thermal resistance.

Bricks can be used to create thermal mass inside a building. This will not improve energy efficiency, but may help to modulate temperature swings inside the building, improving comfort. Brick walls are sometimes used as part of a hydronic heating system.


Laying brick is very labor-intensive. Each unit is relatively small and requires careful mortar joints on all sides. The material is heavy; mortar is prepared in small batches and must be kept fresh. Brick ties, weepers and headers over windows all add to the labor input. If patterns and/or arches are included in the design, labor input rises further.

Health Warnings — Mortar mixing can cause exposure to high quantities of silica dust, so proper breathing protection is required.


Preparation of substrate — Easy. Wall framing and sheathing skills are required. The process of

measuring and fastening brick ties is straightforward. Installation of sheathing — Moderate to Difficult.

There is a lot to learn in order to successfully install brick cladding. Skills required include marking and measuring for brick courses, mortar mixing, laying mortar and brick and creating appropriate headers/ arches for openings. Without prior experience or practice, brick laying may best be left to professionals. Finishing of sheathing — Easy. Jointing between the bricks is the only finish required once bricks are installed.

Lime and lime/cement plaster

several centuries. Proper window flashings and an adequate protection from roof splashback will help ensure long life. Bricks that repeatedly absorb a lot of moisture and then freeze (the freeze-thaw cycle) will deteriorate within a decade.

Interior applications of clay brick will be extremely durable.


Acceptable solution in all codes. Be sure to follow local codes for specific requirements.


No effect as an exterior cladding.

Interior brick walls should not have a negative impact on IAQ. If using recycled brick, be sure to assess the origin of the brick before using them indoors as they may have been used on a building or in a location where the porous clay may have absorbed toxic materials (e.g., old chimney brick, kiln brick or industrial brick).


It is unlikely that techniques or materials for brick manufacturing will change dramatically. Brick has a long history in building, and will likely continue to have a reasonable portion of the cladding market. Brick is widely perceived to be an “upscale” cladding, used to distinguish a quality home from a cheap home. Rising costs for brick and masons may cause the market to shrink.

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