I used Home Depot's QuietBrace for my present layout as well as for the previous one because I could not find Homasote. After discovering QuietBrace, I never looked back - QB was cheaper and it worked just great.
Today I went to Home Depot and looked for QB just out of curiosity, and it was not there! Instead, they now have what they call "Squared Edge Asphalt Insulating Sheathing". It has the same dimensions; i.e., 1/2" x 4' x 8' and its price is $10.35. However, it is not as rigid as QB was, and it looks as though the coating (asphalt?) may more easily rub off.
Even though I really do not need anymore QB, I think it is a loss for model railroaders as it was a good and cheaper alternative to Homasote.
Alex