Foam would probably be your best bet. Even an inch all over the ceiling
would do what needs to be done. Your problem probably isn't conductive
heat and cold transfer, as in through the ceiling drywall or plaster,
given your stable climate.

That means you will probably get the most benefit from a good air seal.
Foam is dustless. No worries there. I'd say you could get the whole
attic floor covered with 3 of the foam kits at http://www.tigerfoam.com
and they do have a way to get it in there through the roof if you
remove a couple of slats here and there. You'd have to talk with them.
Ask for Sam. He knows about the extension if you need it.

This will cost about $1,800-$2,000, but it'll do the job dustless (my
favorite) and seal like nothing else can, but may be outside your
budget.

Just an option. Personally, I like PU Foam for insulation and sealing.
It doesn't settle, as any blown in insulation will. Cellulose is better
than fiberglass, but I don't like the dust, eventhough it is just
nuisance dust. I have enough problems trying to breathe in this world
as it is! (asthma)

It's also E-84 fire rated, but you should definitely get covers on the
lights. Safety first!

abi