You're asking the wrong question.

It's not what insulation to use, but what other measures you need to take to ensure your insulation remains effective without becoming a mould-contaminated health hazard.

This requires the proper handling of water vapour and bulk water.

There are only two methods that work.

Source: Building Science Corp. "Don't Be Dense", Dr. Joseph Lstiburek

Unfortunately, both approaches are more expensive than simply filling the cavity. If the cost to do it right is beyond your budget, leave it alone. The building has done well for more than a century as it is.

Here is a link that might be useful: Building Science Corp. on Flat Roofs in Cold Climates

As it turns out the problem isn't the roof but the airshaft/dumbwaiter in the middle of the house, from ground floor to roof. Major chimney effect/stack effect problem we hope to fix soon.

We opened up the middle floor airshaft space (cutting through metal ducts providing passive ventilation off the ground floor bathroom) and took out the dumbwaiter space, and turns out that the duct work went from floor 1 to roof venting floors 1-2 bathrooms, with old turbines on the roof that act like an open window to the roof, above the dumbwaiter space. No venting off the third floor at all. And the airshaft is a lot of freeflowing air next to the open duct work, with a roof skylight at the top of it. So now when it's cold on floor 2 it's cold in the floor 1 bathroom - it's an open window.

Our plan now is to pull out the duct through floor 3, patch the roof, take out the skylight on teh roof and use that space to vent all 3 bathrooms with 4" or 6" vents, and put a ceiling over the 2nd floor to stop the chimney effect. Hopefully this will make a big difference.

Using a single ventilation unit may be a more efficient way to ventilate the baths. Even if you use individual units, you might want to consider some system of automatic timers and/or remote switches for all those situations where users don't activate the fans or forget to turn them off. I used to live in three-storey plus finished basement homes. A workout in themselves!