Finishing fumes and flames don't mix. Electric heaters, preferably without an exposed heating element to ignite stray fumes. Just finished insulating my roll down garage door, but the garage ceiling needs heavy insulation and dry wall. Will hire someone to do that, too darn heavy for me anymore. I'm thinking I need a fan to vent air out of the attic space that will result. Without that insulated ceiling, a heater will never be able to keep up.
May also have a fresh 3-4 inches of blown in insulation put in the attic of the house itself. Had new ductwork installed last year and I think the existing blown in got trampled. Winter can get frosty here, but insulation is even more important during a Mojave Desert summer.
The more I do, the less I accomplish.