A mattress and foundation are engineered to work together as a set, and the foundation takes a lot of the nightly wear and tear and contributes to the bed's overall comfort and support. Putting a new mattress on an old foundation, pairing it with a foundation it wasn't designed to work with or adding a board between the mattress and foundation will impede comfort and reduce the useful life of your new mattress. It may also affect the manufacturer's warranty coverage.