Moisture-resistant plasterboard: how the liner must change for MR and MRH boards
Hydrophobic sizing, Cobb targets and the bonding chemistry behind reliable green board.
Moisture-resistant (MR) and moisture-and-mould-resistant (MRH) plasterboards — the 'green boards' used in bathrooms and kitchens — depend on three engineered defenses: a hydrophobic core, hydrophobic sizing in the liner, and a tighter Cobb specification.
How standard liner fails in humid environments
Standard PBL absorbs water aggressively, swells, and detaches from the gypsum core. Mould then colonises the cellulose under the surface. The board looks fine for a year and then fails.
The MR liner specification
- Cobb 60 target: 30–40 g/m² (vs 22–28 for standard).
- ASA or AKD internal sizing for water repellency.
- Optional surface application of fungicide/biocide approved for indoor air.
- Grammage typically 150–170 g/m² face, 200–220 g/m² back.
Bonding chemistry matters more
The interface between liner and gypsum core must remain bonded even under intermittent wetting. Specialised starches and synthetic bonding agents are used. Specify them with your mill — never assume a standard PBL will become MR-grade just by adding sizing.
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