We have an all-in-one, or package, air conditioner. Much of the air handling section is insulated with insulation board of indeterminate type that is either missing or damaged. Years ago I went to the local AC parts distributor and asked for something to repair it but they wanted 130 dollars for a 4x8 sheet of whatever they were selling me so I just used some plastic wrapped wall-type insulation to try to patch it up.
I need to clean that unit up and get it back to it's best performance (it will be over 100 for the next 10 days) so I guess it is time to take it apart, clean and straighten fins, deep clean the unit, and replace the insulation, as well as some duct work that needs fixing up. Probably the smart thing would be a new AC but I'd rather try to get another year or two out of this one since we're also building a new home.
So what type of insulation is it that goes in there that would be 130 dollars a sheet? Should it be rockwool? I can't find faced rockwool in 1 inch thickness. Is it something else special? Besides what comes from the factory, what other options are reliable, safe, and healthy for insulating air conditioning components like that?