Garage insulatiion options
I like the reflective foam board idea.Easy to install,not too expensive,and light weight.
My heating contractor had the 2inch reflective board in his shop,and it stayed real toasty in there.
It may have to be covered to meet building code
It damages easily,should be protected anyways
So this may be an added expense.
When installing fiberglass insulation between the rafters ,you have to allow air flow over the top of the insulation.(Gable end vents and ridge vent)
The foam board is highly flammable,so give this some serious consideration.
All in all,for the cheapest out,consider just doing the roof first.If the block isn't cracked,leave it for now.Cement has a way of absorbing heat and releasing it back into the room after the heat is turned off.Most of the heat travels upward anyways.