My heating bills last winter indicated I need more insulation in my house (built in the 40s).  Finally got around to poking around in the attic to make an assessment.

The current type of of insulation looks like big globs of cotton balls, so I guess it's the blown in type.  I'm thinking of using the blown in type to add additional insulation because it's cheap and I can do it myself, and I have some areas that would be difficult to get to.

So, my (probably a dumb) question is, since the old insulation looks ratty, am I better off pulling out the tongue and groove planks in the attic, pulling out all the existing insulation and installing 100% new stuff, or is it fine to just spray the new over the old?  I do realize that if I simply added to the existing it would take less material.

The current insulation doesn't look wet or infested with chupacabras - it's just old and clumpy.

Thanks to the 12% of posters that will provide helpful information.  For the other 88%, feel free to increase your post count at my expense.

You should be ok to blow right over top of it.  Is the old stuff fiberglass or cotton?  I've heard of cotton insulation making a come back due to recycling, but I have no idea on the R value of that type of insulation over glass.  Also, go around and foam any access to the outside wither where electric or plumbing comes though the walls and where the foundation meets the structure of the house.

depends, is depth a premium? IE do you want to keep the attic floor open for storage?

as long as it is dry, adding on more should be ok.

Just blow the new stuff on top of the old.

Good to go.