There are as many ways to do it as there are people who do it.
Insulating cape cod attic without soffit.
One of the best methods is rigid foam board with sealed joints over insulation in the knee wall.
Suggestions for un vented or hard to vent building roof cavities.
Since you are asking how to insulate your house here s my advice.
Two ways to insulate attic knee walls article doesn t mention what to do with the sides of the triangular area e g.
An attic s intake vents are most commonly installed directly in the soffit either as individual vents spaced every few feet or as one continuous perforated soffit running the entire length of.
The ideal solution especially if the rafters are not deep enough to allow for code minimum cavity insulation is to add rigid foam above the roof deck.
Instead of adding only 3 4 inch of rigid foam add 2 inches.
Plenty of homes have soffits without soffit vents.
In an attic without soffit vents you can add insulation in the corners and the wedges between the slope of the roof and the floor of the attic which serves as the ceiling for the room below.
See insulating a cape cod house.
Because it s hard to insulate these areas many capes leak heat at the eaves a recipe for ice dams.
Worries about the hot roof un vented cathedral ceiling designs how to.
How to avoid condensation leaks attic mold insulation mold structural damage to roof framing when roof venting is not possible cathedral roof and knee wall insulation venting air path options in older homes hot roof designs.
I suggest you use polyisocyanurate instead of blue styrofoam xps since polyiso is more environmentally benign.
Rather than having to spend days or weeks in a dirty attic carefully caulking every crack and filling all the holes with cans of foam and then carefully laying out insulation you can instead just remove the old insulation and have a foam contractor come in and in a day or less your attic will be completely air sealed and insulated.
One method mentions what to do with the sloped roof and the other method mentions what to do with the floor and knee wall.