Carolyn – the advice that PeterC’s provided is entirely logical, but as quite often applies to matters such as the one you raise, logic doesn’t always apply.
Under the now defunct Federal Government Home Insulation Program, residents of Strata Developments were required to state that they had obtained the Owners Corporation’s (O/C) approval for the placement of insulation in the roof-void above their residences.
It was also the residents of Strata Lots that were able to access the grant monies and assign that to the Installers (even though many of those were conned as they weren’t on the top floor of the building), and you will note that the current Victorian Energy Efficient Target Scheme (VEET) has the same criteria, because it’s the Lot Owners who are responsible for installing and paying or roof insulation after obtaining the O/C’s consent to do so.
So I believe that the “official” line is that it’s the Lot Owners on the top floor of your Plan who should, at their discretion, seek the O/C’s consent to pay for and install insulation in the roof-void above their individual Lots, and again at their discretion under the VEET Scheme.
Now for the logical argument. I used the word “should” (above) because the requirements of the current sustainability programs such as VEET are often written around current building designs, where as but one example common walls between Lots extend through the roof-void and up to the underside of the tiles.
Clearly that’s not the case in your 1920’s Building where your O/C also needs to replace the entire roof covering, so it may be logical (and appropriate) in your instance for the O/C to Resolve (at a General Meeting) to allocate some of its common funds to pay for the insulation of its Common Property (roof-void) concurrently with the roofing works.
Whatever your O/C decides to do, it needs to ensure that relevant safety requirements covering roof insulation are followed (e.gs. clearances around wiring, downlights, and exhaust fans) and that it Resolves and properly documents who is responsible for any on-going maintenance / inspections of / and replacements of the insulation product, how it will be made aware of any future installations of ceiling mounted items like those examples above, and who will be responsible for any damage arising from any such future installations about which it was not advised.
Decision time cometh!