Insulation has two uses. The primary use being to keep your house warm. The second is for noise reduction.
R25 can be many different thicknesses. It depends on the insulation product. Styrofoam SM would need to be 5" thick. Pink fiberglass would need to be about 7" thick. White (beadboard) styrofoam would need to be about 6.75" thick. Blown in attic insulaton would need to be about 10" thick.
It has a thermal insulation structure placed around a central core of longitudinal axis and it is characterized in that the thermal insulaton structure comprises at least one layer consisting of separate solid strips, each strip being wound with a very long pitch and a maximum wind angle to the horizontal axis of less than 30°
Yes, it is possible to get a grant to replace a leaking roof and replace insulation with spray-on foam insulation. You can explore options such as government grants, nonprofit organizations, or energy efficiency programs offered by your local utility companies. It's recommended to research and apply for grants specifically designed for home improvement and energy efficiency projects.