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A high-quality MgO Sulfate Board (Magnesium Sulfate(MgSO4) typically does not sweat or weep, even in high-humidity environments.

This is the primary reason the “Sulfate” version was developed.¹ The “sweating” phenomenon is almost exclusively a failure of the older Magnesium Chloride (MgCl2) boards

Why Chloride Boards “Sweat” (The Failure)

Traditional MgO boards use Magnesium Chloride as a binder. This salt is hygroscopic, meaning it naturally attracts water from the air.

• The Weeping Point: When relative humidity (RH) stays above 84% for a prolonged period, the chloride salts in the board absorb so much moisture that they turn into a liquid brine.
• The Result: Salty, yellowish droplets (the “tears”) form on the surface. This liquid is highly corrosive and will quickly rust metal studs and screws.

Why Sulfate Boards are Safe

Sulfate-based MgO boards replace the chloride with Magnesium Sulfate.

• Non-Hygroscopic: Magnesium sulfate does not have the same aggressive attraction to atmospheric moisture.
• Stable Crystal Phase: The chemical bond formed in sulfate boards (often called the 5-1-7 phase) is much more stable in damp conditions.
• Performance: In tests at 95% humidity, sulfate boards absorb significantly less moisture than chloride boards and do not reach the “deliquescence” point where liquid starts to leach out.

Key Takeaways for High-Humidity Projects

If you are building in a coastal area, a bathroom, or a basement, the differences are critical: