First I'm assuming that you're talking about fiberglass batt insulation. If it is just matted down because it has been stepped on, then yes you can can fluff it back up to a larger thickness. If it has been rearranged to form nests, then your best bet is probably to remove and replace it with new. Depending on the extent of the activity, the possibility exists that it is covered with urine, droppings, etc. from the rodents and (assuming that all activitiy has been arrested) you may want to clean out the attic and re-insulate anyway.
I know first-hand that this is a real crappy job, but worth it when it's done.

Check any wiring up there also to see if they gnawed on it.

Last weekend I spent 2 days getting rid of batt insulation that had been rat heaven for a while in a house that I just bought and am in the process of remmodelling. Get rid of the stuff in your attic. Mine was full of rat, skeletons, droppings, urine and other things I cant begin to identify. They had chewed thru most of the wire insulation and into many of the A/C vents. Get a paper suit, gloves, mask and goggles. Tape up all exposed skin, roll this stuff up, tape it, throw it down and get rid of it. Then do an inspection and find the damage. there will be some. Good luck

lourock:
You described my adventure to a tee. (I suckered my cousin into helping me.) The only detail I'd add to your mthod would be to purchase some large heavy plastic garbage bags and bag the stuf in the attic so you don't drag the loose stuff through the house.

Also, make sure there are no critters still there.

Feels good when it's done, doesn't it?

You should consider replacing your insulation with blown in cellulose, as it has organic additives that drive rodents away and is a far superior insulation compared to the ineffectiveness of fiberglass batts.More money, but worth it in the long run.

If you are in the west or southwest part of the country, get on the net and research the safe way of cleaning up droppings as cases of hantivirus have made people ill and in rare cases have been fatal.The clean up involves the protection mentioned above,(good respirator),not just a dust mask,and a mixture of water and bleach sprayed on the droppings before cleanup.google hantivirus,or hauntavirus,hauntivirus,not sure of the spelling.

I know of no effective way to 'refluff' attic insulation. It sounds as if you have the blown in variety, which settles normally over time. As the others have said, removing is an option---but they added excellent safty suggestions.

If the rodent problem is fixed, you could simply have more cellulose insulation blown into the attic---you and a friend could even do that yourself.

Interesting thread.

I have to upgrade my insulation in the attic. I have cellulose, but not enough of it, so I was going to go the easy diy way and install fiberglass batts on top. I realize that cellulose in many ways is an excellent material.

My cellulose is quite old, so I'm wondering if it would have the additive to repel rodents, and also if it would have the fire retardant. Were these always part of the process?

Is there such a thing as a rental for the blow-in equipment, or are we talking about a truck mounted device with compressors and the like?

Ted

Ted,

You can indeed blow new cells over the old.

A couple of lumberyards around here let you use their blower for free if you buy the bags from the. One other charges a fee. I forget, something like $40 or $70 to rent it.

Though I've never been in a Lowe's, I recall a flyer from them where they have a blower they let you use for free if you buy a certain amount of insulation from them.

You'll have the blower outside. Someone feeds the bags into the hopper, the blower moves the cells up a hose to the gunner in the attic.