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MgO Sulfate Board is highly recommended for high-humidity areas. In fact, its superior performance in moisture-prone environments is the main reason architects and builders choose it over the cheaper Chloride-based MgO boards.

Here is why it excels in humid conditions:

1. Zero “Sweating” (Non-Hygroscopic)

The most critical advantage is that Magnesium Sulfate (MgSO4) does not attract moisture from the air.

• Chloride boards are “hygroscopic,” meaning they pull water vapor out of the air. In high humidity, this causes “weeping”—salty water droplets forming on the board surface that destroy paint and promote mold.
• Sulfate boards stay dry and stable even at 95% relative humidity.

2. High Dimensional Stability

MgO Sulfate boards absorb significantly less water than most other building panels.

• Minimal Swelling: Tests show that even after 22 weeks in high-humidity conditions, quality sulfate boards show almost zero change in size or thickness.
• Resistance to Warping: Unlike gypsum (drywall) or plywood, it does not soften, sag, or delaminate when exposed to steam or dampness.

3. Strength Retention

All boards lose some strength when they get wet, but MgO Sulfate boards are much more resilient:

• Sulfate Board: Typically retains about 70-80% of its strength in high-humidity environments.
• Chloride Board: Can lose nearly half of its structural strength ($~45\%$) as the chloride bonds weaken in wet conditions

4. Mold and Mildew Immunity

Because the board is entirely inorganic, it provides no food source for mold spores. In a humid bathroom or basement where organic materials like paper-faced drywall would rot, MgO Sulfate board remains unaffected.

Best Practices for High-Humidity Installation

While the board itself is moisture-resistant, a successful installation depends on the system:

• Seal the Edges: The cut edges of the board are its most vulnerable point. Always use a waterproof sealant or primer on all edges before installation.
• Tile Backer Use: It is an excellent substrate for tiles in showers and steam rooms, but you should still use a waterproof membrane (like a liquid-applied guard) over the joints and screw heads for total protection.
• Fasteners: Even though the sulfate board is non-corrosive, always use Grade 304 or 316 Stainless Steel screws in high-humidity or coastal areas to ensure the entire system lasts as long as the board.

Where can you use it?

• Bathrooms & Wet Rooms: As a tile backer or painted wall.
• Indoor Pools & Saunas: Where humidity is constant.
• Basements: Where dampness often ruins traditional drywall.
• Kitchens: Especially behind sinks and industrial cooking areas.