Should I Add Insulation 

To Exisitng?  What is Payback?

It Depends:  
Retirement sewed up? Spare Income?  Care about the world's future?
If YES, then the answers is YES YOU BET!

If NO, seek government aid, and it does exist, at least in NYS.  

If SORT OF, then keep reading to see what might give you the shortest payback times.
You'd think not.  If you crunch the numbers based on what you think your insulation is, you'd think not.  However, there is a pretty good chance your current insulation is perfoming at only 30% to 50% of it's rating due to settling, moisutre damage, poor installaiont (and by poor, anything less than perfect is usually poor).  How can you tell?  Thermographic imaging, in as cold a weather as possible, with the heat on normal levels.  All the imperfections will stand out.
Other issues effecting costs:  Is your attic cluttered, and worse yet, do you want to keep (relocate / pay for storage) all of it, will you need help? 
Do you need attic access?
Is there an HVAC system up there?
Is your basement cluttered, is it easy to get around your crawlspace?
Any significant foundation plantings.
Walls are problematic, as there is no gauranteeable way to "reinsulate" walls from the outside, so you'd be trusting the abilities of your GC to pump in (probably foam) to fill gaps around existing insulation.
Where does that leave you? 
A house with typical wall and ceiling insulation, 3 1/2 inch and 6 inch, would have a 10 yr plus payback if done by a GC, 5 yr if DIY, assuming none of the other issues above will adversly impaict the project.
Synergies gained:  Your house will feel warmer, temperatures will be more uniform, it will be quieter, it will be less drafty (and if you are near a pollution source, ie highway, less outside air means cleaner), along with other synergies of a frost proof shallow foundation (See other site sections).
For an idea of what work will be required by a GC or DIY'r:
Remove trash, animal dropings, extremely stained insulation - to some extent you'll be breathing whatever is left.  Fill in low spots with layers of 3 1/2 inch unfaced fiberglass batt insulation, fill in "cracks" between batts or push together.    Install air barrier (Tyvek) taping all seams and caulking or friction fitting down to tops of walls under eaves.  Install minimum 6 inch batts lengthwise down attic.  If you aren't up to 24 inches total, add another 6, 8, 10, or 12 inch batt lengthwise offset 1/2 batt. 
Note on friction fitting air barrier to top of perimeter walls below attic:  This isn't easy.  You need to be able to reach in with hands to place sealant and barrier, sealing to not just wall below but rafters and joists to either side.  I couldn't do this as I had no attic floor, and 100 yrs of nails sticking down from the roof.  Instead I PACKED the gap between attic floor joists / top floor ceiling rafters that sat atop the perimeter wall tight with ROCK WOOL, rammed a section of foarm into that, between each pair of rafters, sealed to rafters, flush with their bottoms, then sealed the air barrier to that. (Click here for more)
Foam - Insulation
Nothing is that easy.

Click Here

For a ton of information on wrapping your building in foam.


Experiences w/

Old House Rehab


It is saving me the $40,000 or so it would cost to underpin the house, install a foundation and insulate it.  Nor would this have addressed the rim joist, a major source of heat loss.


For those concerned with vermin and bugs, termites, etc. in the skirt.  I've had one element in 3 yrs, in a vermin and bug heavy area, with only a few spiders taking up residence.  I suspect in addtion to being pretty well sealed, it's dry and foodless.


We have noticed that we no longer have ants invading the kitchen every spring, and flies infesting the area outside the kitchen in the summer.  There might even be fewer mice.


In addition, the boxout is a great ledge for planters at a real easy height to maintain, with the plants and flowers visible from inside.


I'm going to try some parsely and carrots that we can't grow in the garden, the moles keep eating them.


(Click Here For More)