Yes, it can — but only as a substrate, not as a final surface layer (façade). When used on exterior walls, MgO boards must be covered with other protective materials. In other words, the environmental conditions for exterior applications should closely resemble those of interior settings, avoiding direct exposure to sunlight, rain, and wind. Prolonged exposure to UV rays, moisture, and weathering can significantly reduce the board’s strength and service life.
1. Is There Such a Thing as Waterproof MgO Board?
Many customers ask whether we produce waterproof magnesium oxide boards. The answer is no — and in fact, there is no such thing as a truly waterproof MgO board. Due to the natural properties of its raw materials, MgO board has a porous structure, which makes it more prone to absorbing water.
However, this porosity is also an advantage: it allows the board to release moisture quickly, earning MgO board the nickname “breathable board.” We once conducted a test by submerging an MgO board completely in water for 7 days. After fully drying it under sunlight, we tested its physical performance and found no significant degradation.
2. What Do “Waterproof MgO Boards” on the Market Actually Mean?
Some manufacturers claim to produce waterproof MgO boards, but what they actually do is:
- Increase the magnesium oxide content to raise board density and reduce water absorption.
- Add water-repellent agents to further lower absorption.
So in reality, there is no waterproof MgO board, only MgO boards with lower water absorption. Moreover, magnesium sulfate is less compatible with water repellents than magnesium chloride, which means that so-called waterproof MgO boards are usually chloride-based boards with water-repellent additives.
3. What to Watch Out for in Humid Environments
Chloride-based MgO boards, if poorly manufactured — with substandard materials, incorrect formulas, or insufficient curing — and without proper soaking treatment, may develop salt efflorescence in humid environments. This can corrode steel and damage other construction materials.
At Suparna, we use an advanced water-immersion or soaking technique to eliminate excess chloride ions, ensuring our chloride-based boards remain stable and safe even in high-humidity environments.
4. Conclusion
We do not recommend using MgO boards as exposed external cladding, even if treated with water-repellent additives. Weathering will still significantly reduce their durability. If you do use MgO boards for exterior walls, they should only be used as a substrate and must be protected by other materials.
In this case, water resistance is no longer a key concern. We therefore recommend using either standard magnesium sulfate boards or chloride-based boards with soaking treatment for exterior base applications.