What is a Magnesium Oxide Board?

A magnesium oxide board (MgO) is a type of board that is typically used in construction. It’s a white board which has been treated with magnesium oxide to make it fire-resistant.

Magnesium oxide boards are made of cement, sand, and water. They have a high tensile strength and are resistant to chemicals, so they can be used in different types of environments such as hospitals and laboratories.

The benefits of using magnesium oxide boards include fire-resistance, chemical resistance, and durability.

What Causes Magnesium Oxide Boards to Sweat?

Magnesium oxide boards are a great alternative to plywood and OSB. They are a lot more environmentally friendly and they have been known to be much more durable. However, there is one major drawback that you need to be aware of before you purchase this type of board – it will sweat.

It is important to understand the causes of magnesium oxide boards sweating before you install them in your home or office. It may not seem like an issue now but once your project is complete and the boards start sweating, it will start to cause problems.

Used as a sheathing, Magnesium oxide boards can be prone to moisture damage and this can often cause serious mould. They were used quite often as facades between 2010-2015 but were found to be causing problems in the long-term.

The binder that’s found in MgO boards is created as a chemical reaction between magnesium oxide and magnesium chloride. Typically, there are 2.5 to 3.5 parts of magnesium oxide per one part of the latter.

The MgO boards are strong and hard, but decomposes when exposed to water or high relative humidity. MgO boards also have organic fillers such as sawdust, and inorganic fillers such as sand, lime or volcanic ash.

High relative humidity environments can cause MgO boards to absorb the the moisture in the air due to the magnesium salt (MgCl2) composition. MgO boards absorb humidity and then subsequently release/sweat salty water. This “salty water” is highly corrosive, and leads to moisture and mould problems, as well as corroding steel frames. It can also be highly corrosive on metal fasteners and profiles.

At 95% relative humidity, MgO boards showed severe sweating after one week, while boards at 80% relative humidity apparently did not sweat after 80 days of exposure. Sweating of the MgO boards did not occur until relative humidity increased above 84%, while at 82% relative humidity, it would not sweat for 30 days. Risk of mould growth increased at relative humidity higher than 92% RH.

How to Prevent Magnesium Oxide Boards from Sweating?

Sweating is a common problem for MgO boards. The magnesium chloride in the mag board reacts with moisture in the air to create salt and water. This is why you need to take some steps to prevent your mag board from sweating.

MgO board production methods have changed in recent times to combines the durability of magnesium oxide cement with the strength of heavy duty alkaline resistant fiberglass mesh. However, other alternatives have replaced Magnesium Chloride with Magnesium Sulphate. Because it has no chloride in it, MgO boards are far more stable in humid environments.

What are the benefits of MgO Boards?

MgO boards are fire resistant Class A1 materials. These boards are lighter, more flexible, and stable. They also reduce construction costs by a significant amount and is a more sustainable production process with a reduced carbon dioxide footprint.


Select Magnesium Oxide (MgO) boards without Magnesium Chloride in it to avoid sweating. There are a range of colour options and surface finishes that may provide a more suitable environmentally sustainable product for your construction. MgO boards should not be used outside of the home in any situation where they’re in contact with moisture.