I'm also curious on people's opinions on this man's methods: https://www.youtube.com/watch?v=8oXacPjdmZU

What is the head height and how is it currently insulated?

Currently there's no insulation at all. The height? Hmmm, well I'd have to measure it in meters. I'm guessing it's about 3m tall. There's no light in there so I have to wait till tomorrow to measure.

Is the attic vented right now? Sounds like it is not.

Crash Course

Well, I'm not even sure what it means to "vent" it, because from what I saw on youtube, it just means there's an air gap between the roof and the insulation? of about an inch? I looked outside, and didn't see any gaps in the seam under the roof, connecting it to the side walls. All I know is it's very breezy in there.

Venting = ridge vent, gable end vent, etc.

If you don't see it, it means that the attic is currently being treated as conditioned. At that point, you need to insulated between the rafters with a non-air/moisture permeable insulation to avoid any condensation issues against the sheathing.

You need to be cognizant of the fact that the rafter ends are going to be much colder where they interface the roof decking once you start insulating. It is critical that you airseal that insulation so that no air/moisture can migrate to the edges and be a possible condensation point.

You also need to make sure that any foams are covered with a code approved barrier to prevent any combustion concerns.

A lot will depend on your ultimate goals for the space. If you want legal habitable space, you need to run all of the required infrastructure first. Like wiring. Or plumbing. Insulation is no where near the first step.

By the time you are done with running the necessaries, you should know what a vented vs invented attic assembly is. And the pros and cons of each.

You are way jumping the gun.

Or just hire a contractor to do all of the necessary steps to convert the storage space to habitable space. Like shoring up the joists. And framing it out properly for the type of assembly that you choose. You don’t just throw some fiberglass at it and hope for the best. You can rot out your house if done improperly.

Here are some photos of the roof. I should add it's a new home, only about 5-6 years old: https://www.flickr.com/photos/62310091@N07/albums/72157661977033578/with/24136620478/

Thank you all for the advice. So far as wiring goes, that's a good question. I don't need plumbing up there, but yeah, this is quickly going out of my area of expertise. I do want nicer lighting up there, and I'll need more outlets...

So far as "non-air/moisture permeable" is that like that rigid insulation with the alluminum siding? like in this video? https://www.youtube.com/watch?v=8oXacPjdmZU

Those pictures appear to be a mixture of applications. The insulation in the floor indicates that the attic is being treated as vented, but the lack of any apparent venting (save for what appears to be one small gable end vent) says otherwise.

Step 1 - decide what you want the space to be like (i.e. livable, additional living space, temperature controlled storage, etc.).

Step 2 - Insulate and seal according to step 1 decisions.

I definitely want to make it livable. I want it to be a library and play room for kids - with a big Lego town set up. I think this first step is going to be to get the old contractor who built the interior in the first place, and then see what legal hurdles we need to go through - that's usually fairly simple around here. I'm in Slovakia and the town hall usually just wants a plan and a bit of money. I'll need an estimate on all this, and then talk with my wife about everything. :) Thank you everyone for your advice. I see now that this is really beyond my scope to do it alone...