Insulation slows down the conduction and convection of heat. In the wintertime your heat escapes through the insulation. In the summertime the heat pentrates your roof and attic, goes through the insulation and into your home.
Why is this? Because insulation does not stop radiant heat loss or radiant heat gain. 50% of your heat loss in the winter is throught radiation, and 93% of your heat gain during the summer is radiation.
Only a combination of Air Sealing, Insulation and a Radiant Barrier will contain the heat in your home throughout the winter, and block solar heat gain through your roof in the summer.
"Gentlemen: I would just like to say that the whole NESCOR experience has been great. The new window installation was neat and tidy and the installers moved furniture and swept everything up. I was very impressed with how well everything went. As far as home remodeling goes, this was probably the best experience one could ask for."