I would be looking for mid-30's at the minimum. Preferably higher.

Offset glazing and laminated glass are your best options inside the normal family of glazing products. If you need more performance than that, you will need to look at dedicated STC products that have an integrated storm sash of sorts.

WoW, Thank you for the response. Is there a noticeable difference between 35 and 37? Do you have a brand preference? Is there a preference of casement or double hung?

It is not a linear scales so each point in STC is worth a decent performance bump.

Casements will be more airtight so that will help on that end of things as sound travels on air.

In terms of brands, just look for companies that can give you an STC number to start with. Andersen make some nice units and certainly has a solid company behind them.

Rule of thumb with STC is that a change of 3 points is discernible to the human ear. Look closely at the difference between unlike thickness' and laminated (not the 9/16" sound reducing laminated), the STC difference is not enough to warrant the additional cost.

Agree that casements are tighter but not all casements are created equal so look up air infiltration numbers and compare brand to brand.