Here’s are two illustrative shots of the benefits of insulating and air-sealing an attic–even in the case where there is already insulation there!
In the first picture, with R-30 fiberglass batts in place, we can see through the courtesy of an infrared camera that much heat is escaping–and the batts appear a bright yellow-orange. This is because warm air is escaping through leaks into the attic, and passing right through the batts, just like when the wind blows the heat from around you body through a loose sweater on a windy winter day. This effectively defeats much of the R-valuse of the batts, in this case robbing about 50% of their effectiveness.
In the picture to the right, of the same attic the following day, we can see from the cool blue color on the attic “floor”, that the newly blown in cellulose–blown in after extensive airsealing, is doing a much better job keeping the heat in the house.
So don’t be fooled by existing insulation. Poorly installed insulation doesn’t work well. If you’re experiencing drafts, cold rooms, hot rooms, or wasting much on your energy bills (in either the summer or the winter), a good home energy assessment can help you pinpoint the fix.