I have a 1930's colonial in Ohio with a finished walk up attic.(Knee walls and sloped ceiling plaster lathe. My roof is slate and I have no ridge vent.)

There is no insulation above the ceiling between the roof rafters in this space. I have used fiberglss batt insulation on the knee walls in the unfinished space.

My question is can I use a foam board such as Dws Tuff-R in the rafter space above the sloped ceilings. If have a true 4 inch space here that I would like to try to fill with 3" of the Polyiso board to get an R value around R19. I do not have any ridge vents because I have a slate roof not asphalt.

Can I use this product this way, or is the heat of the attic too much for this product.

What are some other options to achieve this high of an R-value in such a small area.