I've got a detached garage workshop. I want insulate the roof a bit to help with occasionally heating it up in the winter to make it more comfortable to work inside. I'm in Denver, CO where it's usually nice enough during the day in the winter to work out there (inside of the garage in the high 50s), but it gets colder in the nights evenings. Right now, uninsulated, I have 2, 110v heaters in there. One radiator, one regular. With that, if it's in the 30s outside, it's in the high 50s inside after running the heaters for a bit. I'm upgrading that to a since 220v 6000w heater, which gets installed tomorrow. That alone should make a difference.
Here is a picture of the roof: https://bit.ly/3icL4My The roofing is directly on those planks and there are some janky, off square 2x6's screwed in to the stick built joists. I want to reinforce those a bit, and and make them even, but that's a little beside the point.
How do I better insulate this thing? I want it to be better than nothing, but in no way will this be a conditioned space. The only goal is to make it retain the 6000w generated heat a bit more, to make running that heater more efficient.
I got some advice about just installing some R13 batts up there, but since it's a unvented space, I was told there could be condensation issues. Is that something to even worry about? I'm in a very dry climate, and I won't be heating the inside more than a 30 degree differential, and at most, it'll be 10% of the time in the winter only. My current plan is to install the fully encapsulated R13 batts and leave a bit of space above them, since those are 2x6 studs. In case condensation forms, it has room to dry out, and the fully encapsulated batts won't absorb it.
Are there any other/better options here? For around similar price points as R13 batt ($350 for this size roof)? Is this something to even worry about because of the very occasional heating of the space? Spray foam is not an option because of coast. It would be 4x the price of the R13 batts.
Thanks!