Installing a new A/C system and wondering about insulation

114-year-old home, no insulation, redwood, plaster and lath, mostly knob and tube. Attic gets extremely hot in summer, despite two attic fans reducing the temperature. Want to make the attic usable year-round as a gym. Need it to be cool enough, around mid-70s for a few hours every few days.

Looking to install an A/C system, but unsure about insulation. The A/C companies we're working with aren't strongly recommending it.

What are the benefits of insulating the attic room (about 600 sq. ft.)? Will it significantly cut cooling costs? Cost is a concern, especially considering future expansion plans in 3 to 5 years.

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I'm curious whey anyone would say NOT to insulate? Have you looked into the rebates from BayREN?  There are rebates for insulation, as well as for heat pumps (which also act as air conditioners) in some situations, with a bonus rebate if you combine measures. You do have to use an approved contractor, but there are loads of them. https://www.bayren.org/rebates-financing

We went with All Things Good for improving our insulation situation, which started off with an energy audit to identify issues. We weren't originally going to replace our furnace, we only wanted insulation, but we ended up going with their recommendation to do both insulation and a heat pump. We are loving it! We're finally comfortable, all the time. I recommend All Things Good and the energy audit if you can swing it.