Actually. it's not really that cheap. Beazer started using it when Gulf War II drove the cost of OSB through the roof. Later, they were in a long term contract and they were actually paying more for Thermoply than other builders were paying for OSB. It does have a water repellent surface, so, to be fair, you have to compare the cost to OSB + Tyvek. But, please -- even the structural grade is just a quarter-inch-thick sheet of cardboard.