I am curious about other's experience as I get asked the following question all the time:
Is it necessary to pull out a poorly installed batt of insulation or will a subsequent blow of cellulose or fiberglass fill in most of the missing gaps? What are people's experiences?
When topping off an existing ceiling with batts, there's a cost to pulling out the old. True, the blown-in may not fill in all the gaps, but you'd likely get more benefit by spending that money on additional blown-in on top. A more worthy reason to pull out the old would be to gain access to the ceiling to do proper air sealing. Even then, you don't necessarily need to remove it unless it's full of mice droppings, etc.
Best practice would be to remove the old poorly installed stuff to gain access for comprehensive air sealing, eave vent baffle installation, chimney air seal and insulation dams, etc. and then re-insulate with blown in. If best practice is not your goal or is too expensive then you still shouldn't just top it off - too many voids will exist under the poorly installed stuff and air sealing is still the primary savings opportunity. An alternative - After strategic cost effective air sealing is completed (and proper eave vent baffle and chimney dams, etc.) you could consider blowing under the poorly installed batt while leaving it in place to fill the joist cavity, then top off as well. I've also heard of people moving it aside and reinstalling while working in sections at a time but labor to do this leads one to might as well throw it away. Obviously if its mouse ridden or otherwise of no use then dumpster it.
The "sad" part is the attic insulation is new and clean. The house was remodeled a few years ago and they want AC b/c it is so hot. Poor batt insulation quality with big skylight shafts have to be part of the problem.
After lifting specific batts for blower-door/IR/experience guided air sealing, (rarely with 2 part foam) my crews will use a narrow rake to push batts down against the ceiling and around wiring and framing anomalies. Then we will blow on top. Don't forget to drop any ducts to the attic floor and seal and blow over. For walls, we will at times dense-pack into a batted cavity that is obviously poorly done. Unless the attic batt is full of bat and mouse yuch, insulation is doing more good in the attic than in the dumpster.
I have seen this done well too and may be the best cost effective approach considering the insulation is in good condition. It does have value if installed like it was meant to be. Air tightness is the indicator of success. If you are good with blower door zonal testing and IR and finding the leaks in a fully covered attic then success is achievable. Make sure to get at those skylight shaft walls, maybe dense pack those or enclose in rigid board if you can get board up there.
I agree, it is wasteful to remove and dump old insulation. It still does its job. Air sealing is important so after that's done blow cellulose and a good job will be had for a much lower price.
YES, YES, & YES!!!
Bat droppings, mouse droppings, dead mice, nuts, seeds, acorns, lack of access to properly air seal, these are a few of my least favorite things . . .
"A 1994 report by the International Agency for Research on Cancer (IARC) listed fiberglass as a human carcinogen, according to the American Lung Association. To warn users about the danger of this product, OSHA and the IARC required that all fiberglass insulation packages contain health warning labels. As of 2009, these labels were still required, despite a 2001 IARC report that fiberglass is not a likely carcinogen."