Layering rigid foam insulation can increase the overall R-value, but the exact amount of increase will depend on the thickness and type of insulation being used, as well as how it is installed.

Rigid foam insulation is an effective type of insulation that provides a high R-value per inch of thickness. Adding a second layer of rigid foam insulation can help to increase the overall R-value of the wall or roof assembly. However, it's important to ensure that the foam insulation layers are installed properly and without gaps or voids to prevent thermal bridging.

In general, it's best to follow the manufacturer's instructions for the specific type of rigid foam insulation being used, as well as any applicable building codes and standards for insulation installation. In some cases, the manufacturer may specify the maximum thickness or number of layers that can be installed to ensure that the insulation functions properly. 

In general, it's best to consult with a professional or reference building codes and standards to determine the appropriate R-value for your location and climate, and to choose the most effective type of insulation for your application. It's also important to ensure that insulation is installed correctly to maximize its effectiveness, including proper coverage and sealing to prevent air leaks and thermal bridging.

In some cases, it may be more effective to use a higher R-value insulation material rather than layering up on an existing insulation. Alternatively, improving other components of the building envelope, such as air sealing and ventilation, can also help to reduce heat loss and improve energy efficiency.