FYI: I have been doing some insulation of my two car garage roof. Standard attached two car garage with 2x4 trusses 24" OC. skip sheathing then osb over that with asphalt shingles. Minimal roof vent only two intake vents down low in the walls.
I have been putting two inch thick polyiso insulation in each rafter bay with 1.5 inch air gap above. Two, two inch diameter vents in each bird block for roof venting. open at top into attic space for now. I will install a ridge vent when it cools off.
The entire building is open no ceiling or anything.
It has been triple digits for the last month or so as I worked on this during mornings. Highest I have seen was 115 at head height.
south facing Roof deck is 135-125 depending on smoke haze that day. Bottom of insulation is 100-105.
South facing Wall with r15 rockwool 90-95 in full sun. 95-110 with no insulation.
I can feel the difference with the top of my head standing on the floor. The insulation has no radiant heat effect on my head. while the bare roof deck makes the top of my head warm in a few seconds.
So i can say this method is working and I will do more as time allows. It is difficult to get the insulation as most box stores have only one or two sheets if any. I do not want to order a truckload and have to do this all at once. Home depot will knock off some % for sheets with damaged corners etc.
I assume it will help hold in heat in winter as well.
Bill D