Insulation
Why upgrade your insulation? By doing so you will improve the efficiency of your a/c unit, and that will make it last that much longer. This is also one of the few upgrades you can do on your home that will immediately lower the cost of your energy bill.
When do you want to upgrade your insulation? When you have an odors, see damage, your electric bills are larger than you feel they should be, or notice a lot of dust in the rooms underneath your attic.
