Second layer of drywall, or put plaster back up. Second layer of drywall is less costly.
If the studs are still exposed at this time, you still have time to shim them up to where they need to be for a single layer of 5/8" drywall. 5/8" IMO is so vastly better than 1/2" that you shouldn't pass up the opportunity to use it.
Casey

The suggestion to use 5/8" drywall will certainly work, but the weight of even a 4x8 sheet in this thickness is so great - almost 75 pounds per sheet - that it really stops being a DIY project unless you have lots and lots of (strong) helpers and are willing to do a lot of seam taping with the small sheets. (A 4x12' sheet is well over 100 pounds). Shimming out the wall studs is much more practical. The new lighter weight drywall is also an option worth exploring, but I'm not sure if it's available in 5/8

I guess I"m not understanding - what is the 1/2" gap? Do you mean the original plaster thickness was 1/2" thicker than the thickness of the drywall? Wow that's quite a difference.

Never heard of the lightweight drywall - will have to look into that.

For other readers who may be interested - a way of avoiding this situation is to use the drywall patch or traditional plastering method to repair only the loose areas, leaving the areas near doors/windows intact (which tends to be firmer because the trim has been holding it in place for 100yrs). Of course some plaster is so far gone that it does have to be entirely removed but that's not very common - usually you might have only a few loose spots here and there and maybe a couple of walls/ceilings that are really bad.

The 1/2" gap is under all of the casings. The old plaster was net 1" thick, so the door and window jambs are set for that thickness of finish. 1/2" rock leaves them all floating. I'm taking OP at their word that the original plaster and lath is in the landfill at this point, so plaster repair is no longer on the table.
Casey

Shim the studs so that drywall will fit correctly behind the casing--since the walls are open, insulate!

And while we're at it...is it too hard to spell-check? A LATHE is a machine for turning wood into round shapes--LATH is thin strips of wood which allows plastering for walls.

Hey Aleasha, welcome to the forum.

You may be feeling a chilly breeze because gutting out original plaster is not a popular thing with some on this forum. But, hang in there, I've been here five years, gotten loads of great advice, and had a lot of fun. I made some mistakes before I found gardenweb, and I'm happy to have a sounding board of experienced old house owners (some are even restoration professionals) to lean on for advice.

While you're pondering what to do, if you're literally feeling a chilly breeze around the windows, you could stuff foam backer rod in the gap while you're deciding on a more attractive, permanent solution.

Welcome and hope you stay a while!

Shimming the studs is the way to go. We used the old lath to do it. Simple.

Shim studs you have to with plywood strips (often areund openings) & for others attach a sister to the side of the old stud sticking out just enough to make a flat plane for the new drywall at the height you need it.

I rip the sisters from wider pieces of 2x lumber.
It is easier to handle over the table saw than sheets of plywood.

Rip about 1.5 inch wide pieces and then screw them to teh sides of the old studs.

About a 2.5 inc screw every foot is adequate.

Nail guns tend to blow out a little more often on the front edge of the old stud.

"Do you mean the original plaster thickness was 1/2" thicker than the thickness of the drywall? Wow that's quite a difference. "

Real multi-coat plaster is at least 3/4 inch to 1 inch thick, and far heavier and stiffer than drywall.

Now you know why it blocks sound so well.