Ventilation

This is basically what you're trying to achieve, with respects to ventilation and insulation of the roof.

Soffit (eave) vents

Start at the bottom of the roof, with soffit vents.

Baffels

Baffels are used to provide a channel along the underside of the roof decking, to allow air to flow from the soffit to the roof vents. You don't necessarily need baffels, but you do want to provide at least one inch of free space under the roof decking. Baffles simply make providing this gap easier.

Roof Vents

You can use either pot vents, or a continuous ridge vent here.

^{Pot Vents}

^{Ridge Vent}

If you don't already have a ridge vent, you'll have to do a bit of work to install one.

• Remove the existing ridge cap shingles.
• Using a circular saw set to the appropriate depth, rip a straight line along both sides of the ridge.
• Install the ridge vent.
• Install ridge cap.

Insulation

Now that you have proper ventilation, it's time to think about keeping the conditioned air in the conditioned space. For this, you'll need insulation. If you used baffles in the previous step, insulating the roof is easy. If not, make sure you maintain a one inch gap below the roof decking while you install the insulation.

You can use whatever type of insulation you'd like, just make sure it will provide the R-value you want in the space available.

Vapor Barrier

Once the insulation is up, you'll want to install a vapor barrier to prevent any indoor moisture from getting up into the insulation. If moisture is allowed to get into the insulation, it could condense and cause mold problems and other water damage such as rot.

You'll want to use polyethylene sheeting, at least 4 mil thick. You're also going to want to seal all seams using house wrap tape, to ensure a good seal.

HVAC

Now that the space is sealed, it's ready to be conditioned. You'll have to install appropriate heating/cooling elements in the space, which completely depends on what type of heating/cooling you have. If you have forced air, don't forget the returns.

Carry the Load

The next item to address, is to make sure you and your stuff don't end up on the floor below. You'll want to make sure the floor of the attic is adequate to support the load. If it's not, you'll have to take steps to beef it up. For this, you might want to contact a structural engineer.

Access

Since you've been getting up and down to do all this work, I'm going to assume you have adequate access to the space. If not, you'll want to install a ladder, stairs, etc.

Can I screw the blocking instead of nailing it as I'm a little worried the nailing might cause cracking in the plaster ceiling (with wood lath) below?

Absolutely.

Is it okay to sister a 2x6 board to the current 2x6 board along the front and back perimeter of the finished area to support the ends of the new subfloor since I'll be cutting the current subfloor back to the edge of the current 2x6's? Can I use screws instead of nails?

Again, absolutely. Definitely makes things easier.

Should I replace the loose fill insulation with unfaced batts and fill in any small areas with the old loose fill?

Not necessary if over conditioned space, but would not be a problem.

Does all of this sound kosher / is there anything else I should be considering?

The joist are braced by the lath and plaster below on the bottom , and the subfloor on the top so no bridging will be needed.