Let's presume it's pink fg in the walls behind a vapour barrier and you're in a temperate climate. (Most posters are no more specific about their location than the US or Canada. That covers a lot of ground.)

I'd be inclined to change it out. It may well be supporting a lot of mould growth, even if it's not obvious.

As well, new high-density batts will give you greater R Value than you would get with the original batts.

And, of course, there's the possibility of using foamboards and spray foam.

I would also consider upgrading the insulation to provide better sound proofing - if the house is all sheet rock which is an echo chamber I would consider mineral wool which comes in batts just like the pink stuff but superior

I have no affiliation w/Roxul - I found it when we did a reno & I wanted better quality insulation

Here is a link that might be useful: Roxul mineral wool

"The GC is reusing a lot of the old insulation, is this ok? "

What does your contract say? Obviously, it is cheaper for him to just stick the old stuff back in. I'm sure he would be happy to charge you extra to buy and install new products if that is what you want. He'd be nuts to do it for free though.

As for the insulation itself, as long as it is in good shape, it will do the job it was intended to do. However, our standards have been raised substantially for energy efficiency since 1978. You would certainly get a better result from using modern, improved products.

The type of insulation material will not have any noticeable effect on the acoustic quality of the room because the walls will still be covered with drywall. At any rate, the difference in acoustic absorption between fiberglass and rock wool is only in the minds of the marketing departments of he manufacturers.

If you use rigid foam insulation boards and seal the edges with canned spray foam, you will not need a vapor retarder membrane.

the difference in acoustic absorption between fiberglass and rock wool is only in the minds of the marketing departments of he manufacturers.

A Canada Mortgage and Housing Corp study concluded that "Adding more insulation, or denser insulation improves acoustic performance, however it is not significant without resilient channels." *

Too bad that study wasn't available before I laboriously stuffed Roxul between floor joists trying to get some peace and quiet for tenants of a basement apartment. A futile effort.

An acoustic consultant told me that insulation the thickness of a stud was too thin to absorb much sound traveling horizontally through a wall but that sound that entered the wall cavity through a crack on one side had to bounce back and forth through a length of insulation in order to find another crack on the other side so that made the insulation acoustically effective. He said the difference between thermal fiberglass and "acoustic" insulation could not be measured in the field with instruments and that people were less able to hear a difference. That is for a stud width of insulation but somewhere around a foot thick the difference in insulation starts to matter acoustically.

But I had assumed the comment about sound related to the sound quality within the room and that would be unchanged in any case.