The most dangerous quote a homeowner can receive is a $1,200 bid to "top off" their attic to R-38. While it sounds like a bargain compared to a $4,000 comprehensive extraction, it is an engineering trap that traps heat, hides water damage, and poisons the air.
Old insulation is filled with 20 years of microscopic rodent feces and heavy dust. Blowing fresh white fiberglass over it simply creates a biological sandwich. When your house breathes, it pulls that trapped historic dust down through your can lights.
You cannot mathematically air-seal a ceiling deck if it is covered in 6 inches of old batting. The cheap contractor skips the foam gun entirely. The new R-38 fluff will violently leak conditioned air, rendering the material useless against drafts.
Fiberglass is light, but adding 10 inches over existing 10-inch batting creates weight. The old, degraded material underneath compresses rapidly under the load, instantly destroying the R-Value you just paid for.