In modern construction, there are a variety of exterior grades of sheathing. We used to have primarily the old paper-faced gypsum sheathing, commonly known as gyp-lap. It had sort of a tongue and groove edge and came in 2-feet by 4-feet sheets. The tongue side faced up and the groove side snuggled it to help control water infiltration because it was kind of a shingle style installation. The paper was completely sealed across the front with a seam around the back perimeter edges. It was difficult to not install it correctly but if I asked for a show of hands from those that have seen it installed wrong, mine wouldn’t be the only one in the air.
This sheathing was marketed as “moisture resistant.” It had a gypsum core but products with a paper face couldn’t withstand high levels of moisture over long periods of time. I may be mistaken but I believe it had an exposure rating of 45 days and had to be covered or replaced because the paper would start to turn loose. This created a hurdle for the EIFS industry early on. Even with a flawless EIFS installation, if there was bad detailing or interfacing with other components that allowed moisture in, the paper would delaminate from the gypsum core and the adhesively attached EIFS would turn loose. I have seen one or two gyp-lap installations over the last year or so but this product has been bred out by most designers because better products hit the market.