I am working on installing insulation in our home. We bought way too much insulation, and i was wondering what the effiency of doubling up and throwing an extra layer on top? We are using R-38 12" thick batts. Basically is it worth the money to add another layer if we live in a very cold environment, shady side of hill, and have a huge attic space that is at this time fully covered with 1 layer?
Short answer: Yes
Install as much insulation as you can afford. It will pay for itself in the long run. If you have space in the attic, then another layer would certainly help.
Make sure you don't restrict the ventilation of the attic by blocking eave vents.