Q: Magnesium-oxide board (ExtremeGreen board) seems to be a ideal building material, but Wikipedia and other sources say it has a problem of attracting condensation due to its salt content. Can you offer any advice? — Keith B.
A: Magnesium-oxide building products – sheathing, wallboard, siding, and other products such as SIPs – are very new in the U.S. so there is little data on real-world performance, at least in the U.S. Most of the material is manufactured in China, where magnesium oxide is readily available.
The product is cast as a cement mixture made from magnesium oxide, magnesium chloride (a type of salt), perlite, and other compounds, and formed into sheets. The main U.S. supplier, ExtremeGreen Building Products, adds wood fiber to the mix.
Suppliers of the product claim that it is an environmentally friendly, inorganic material that is strong, dimensionally stable, fireproof, waterproof, and impervious to insect damage and moisture problems, such as mold, rot, swelling, or warping. All in all, it sounds like a miracle product.
However, I would proceed with caution before choosing this product for your construction project. Few, if any, miracle building products have lived up to their initial hype.
One country that embraced magnesium-oxide board is Denmark. Starting around 2010, it was widely used as a substitute for drywall in public housing and private homes, and as an exterior sheathing in commercial buildings. About five years later, widespread failures occurred, including “weeping” of saltwater, corrosion of fasteners, and mold growth.
Apparently, the boards can absorb moisture from the air during periods of high humidity and, later, release the moisture as condensed water high in dissolved salts. The leaking salt water leads to accelerated corrosion of metal fasteners and fittings. Also the high moisture levels caused mold growth on some projects.
While some building products can resist mold growth, I’ve seen plenty of mold grow on mold-resistant caulks, plastics, and even window glass. Cement boards may not feed the mold like the paper facing in drywall, but given the right conditions, mold will grow almost anywhere.
Whether the Danish problems were unique to specific products, installation details, or climate conditions is unclear. Different magnesium-oxide building products vary in their composition, additives, density, and finishes. And the manufacturing quality of Chinese products can be highly variable.
While the ExtremeGreen products comply with many ASTM standards, I would still proceed with caution. Contractors report that the material is somewhat easier to cut, drill, and nail or screw than fiber-cement, its closest relative in the US building market. That’s a good thing. But it’s still too soon to tell how it will perform long-term.
You can read about the Danish problems at these links:
Please add your comments if you’ve had any hands-on experience with magnesium-oxide building products. — Steve Bliss, Editor, BuildingAdvisor.com