On a plywood panel stamped "exposure1" or "exterior" the glue lines themselves are waterproof. If the product is manufactured correctly the glue lines will not fail due to moisture as they are a phenolic resin that polymerizes with heat and pressure during the laminating process. If the glue line itself does fail, it is due to problems in manufacturing or occasionally allowable knots or open areas in the veneers that do not bond well. Both of which are hurried along with exposure to moisture. That being said, in my years in plywood claims, the main cause of all delamination claims are exposure to moisture causing the wood fiber itself to break down. That is why even though the glue is water proof, it is important to protect the wood from moisture, especially the panel edges and ends. Moisture can enter the wood fiber and rot it or cause it to swell and pull apart from the other plies. Heat by itself will not cause delamination, but can hurry along the process when the panels are not protected. Proper installation using paint, sealants, and flashing will avoid the vast majority of problems with plywood delamination.
