What's the difference between moisture-resistant 'greenboard', mold-resistant board, and glass-mat board, and where does each belong in a house?

Greenboard (moisture-resistant drywall):

• Has wax-impregnated green paper facings
• Designed for areas with moderate moisture (bathrooms, laundry rooms, covered porches)
• Not suitable for areas directly exposed to water
• Per IRC Section R703.1, acceptable for high-humidity areas but requires proper ventilation

Mold-resistant board:

• Treated with mold-inhibiting chemicals or has specialized facings
• Better for areas prone to moisture issues (basements, bathrooms without windows)
• Still gypsum-based but with additives or coatings
• Not rated for continuous water exposure

Glass-mat board (e.g., DensArmor, Durock):

• Inorganic fiberglass mat facings instead of paper
• Highly moisture and mold resistant
• Suitable for high-moisture areas (full bathrooms, shower surrounds, basements)
• Can handle more direct water exposure than other types
• Per manufacturer specs and IRC Section R703.1, acceptable where moisture is a concern

Where each belongs:

• Greenboard: Half-baths, laundry rooms, covered patios
• Mold-resistant: Basements, bathrooms with poor ventilation
• Glass-mat: Full bathrooms, shower areas, basements, anywhere with significant moisture exposure

Important: None of these should be used in areas with direct water exposure (like behind shower fixtures) without proper waterproofing membranes. For continuous water contact areas, use cement board or other water-resistant materials per manufacturer specifications.

Always check local building codes, as some jurisdictions have specific requirements for moisture-resistant materials in different applications.

Greenboard (Moisture-Resistant Gypsum, ASTM C1396)

• Core: Standard gypsum with water-resistant additives; green paper facing.
• Performance: Resists humidity and minor splashes. Not waterproof. Paper facing feeds mold if saturated.
• Code/Use: IRC R702.3.1 permits it on walls/ceilings in "damp" locations (basements, laundry rooms). Prohibited in wet locations (tub/shower surrounds) per IRC R702.4.2 and manufacturer specs. Do not tile directly over it in showers.

Mold-Resistant Board (Paper-Faced, ASTM C1396 / C1658)

• Core: Gypsum with biocides/fungicides; treated paper facing (often purple/yellow).
• Performance: Resists mold growth on the surface better than greenboard. Core still absorbs water and loses strength if wet.
• Use: High-humidity walls/ceilings (bathrooms, basements, kitchens) where tile/backer board isn't required. Better durability than greenboard for general damp areas.

Glass-Mat Gypsum Board (ASTM C1177 / C1658) — e.g., DensArmor Plus, GP ToughRock

• Core: Water-resistant gypsum; fiberglass mat facing (no paper).
• Performance: Highest mold resistance (no organic food source). Excellent core water resistance. Survives incidental wetting/drying cycles without structural failure.
• Use: Preferred substrate for tile in wet areas (showers, tub surrounds) per IRC R702.4.2 and TCNA Handbook. Required for steam showers. Acceptable for exterior soffits (Type X).

Summary Placement Guide | Location | Best Choice | Code/Standard | | :--- | :--- | :--- | | Shower/Tub Walls (Tile) | Glass-Mat or Cement Board | IRC R702.4.2, TCNA B415 | | Shower Ceiling | Glass-Mat (Type X if rated) | IRC R702.4.2 | | Bathroom Walls (Non-tile) | Mold-Resistant (Paper) | IRC R702.3.1 | | Basement Walls (Dry) | Mold-Resistant / Greenboard | IRC R702.3.1 | | Kitchen/Laundry | Mold-Resistant / Greenboard | IRC R702.3.1 | | Exterior Soffits | Glass-Mat Type X | IRC R702.3.1, ASTM C1177 |

Critical: In wet areas (showers), the board is not the waterproofing. You must apply a liquid/waterproof membrane (ANSI A118.10) or sheet membrane over the board