Single Wall Boiler Vent - Combustibles Clearance

I am building a basement window cover. There is a boiler vented with 3" Stainless Steel vent pipe through the basement window opening. I would like to incorporate a 2" rigid foam Dow board into the new cover.

Does anyone know the required clearance between the vent pipe and the foam board?

Thank You,
Bill

6" is the standard for single wall vent pipe.