I added insulation to my attic rafters and plugged the space between the outdoors and my attic with insulation. It lowered my heating bill by $150/month, but I've seen some information that I may have made a mistake by cutting off the airflow in my attic.
Did I make a mistake, and if so, how can I properly insulate my attic? There's already insulation in the floor.
You can buy air ducts that staple to the inside of your roof above the eave vents and fit between the rafters. The will hold open an air gap of approximately 1 inch to allow airflow past a layer of insulation. The ones I've used are made of styrofoam and IIRC they cost around $2. You should be able to find them near the insulation in your home improvement store.
EDIT: They are called baffles or rafter vents; this is how they're installed:
The point of allowing airflow to reach the outside is so hot air can escape. If it doesn't escape then it will heat up the roof/rafters and cause snow to melt and then turn to ice. This can damage your roof in a number of ways. Also, moisture will be trapped in the attic and can cause mold or damage to the structure.
I believe the optimum way to insulate your attic is to insulate the floor of the attic very well but allow the air above that to escape (ie. insulate the ceiling of the attic poorly)
There are a few different types of attic designs. One is insulating the floor and venting the space above the floor with soffit (eave) vents and an attic fan or two. One reason for the air flow that hasn't been mentioned is that, since the hot air will exit out the top of the roof (whether the attic fan is powered or simply turned by the force of the hot air rising), then it has to be replaced by something. If your soffit/eave vents are plugged up, it will suck air out of your house, which, of course, is not good for your electric bill.
There is another type called the CATHEDRALIZED ATTIC, which is where the insulation is put on the underside of the roof itself. CATHEDRALIZED ATTICS SHOULD NOT BE VENTED. Since it sounds like you have cathedralized your attic, you should not install soffit/eave vents or baffles.
You don't want to plug the space between the outdoors and the attic. As Joe mentioned, that will cut off airflow in the attic and cause ice dams in the winter. It will also keep your attic VERY hot in the summer, and raise your AC costs.
Ideally you want a "cold" attic - lots of insulation between the ceiling and the attic, and no insulation between the attic and the roof, along with plenty of soffit and ridge vents to keep the air circulating up there.
