Currently the ceiling of my house is insulated (2x8’s with R19 fiberglass) with OSB on top of it. My roof has no insulation in it and my upstairs HVAC system is in the attic. I am in zone 4 in the US which means I should have a min R-value of 38 in the ceiling. I am planning on adding collar ties around 8ft up in the attic and insulating the rafters with r19 with vent baffles behind it. Then on top the collar ties using r19 + r30. I also plan on leaving the insulation in the ceiling.
I am wondering if anyone with sense has had this scenario and then added insulation to the roof. And have actual energy usage data for before and after?