I am about to extend a room in my house. My exterior wall is stucco but I have noticed that under the stucco is not plywood sheathing but something that looks like a drywall (not same material, buy is about as thick. I first noticed a few months ago when I had my windows replaced with new construction ones and the contractor had to breakout some of the sheating to install. Does anyone know what this stuff is? And, since it has been damaged a little with the windows, and will be when I push out the room, will that cause any problems? (I will be using plywood for the new addition. BTW, the house is in CA and was build in 1961.) Thanks