Never put a new mattress on an old foundation. A mattress is designed to work best with its companion foundation. Mismatched sets can affect overall comfort and support. In fact, buying a mattress without its matching foundation may even affect the terms of the warranty. Plus, it's important to the safety of you and your family to ensure that both pieces of the sleep set meet or exceed federal flammability standards, and the best way to do that is to buy the set.