Make absolutely certain to visit a local home improvement store for a roll of fiberglass insulation BEFORE Window World sends their installers. DO NOT let them simply install your new windows without filling the gaps around the windows with insulation.
I watched each installation carefully, the first install had a gap of about 1.5 inches around three sides. The installers added window trim on the outside and were about to add window seal on the inside and I noticed light in the gap.
I made them insulate all around the window including the bottom seal plates, if you don't, get ready for drafts. IMPORTANT, have a shop vacuum ready to clean the window openings of debris, they may not take the time to clean before install, make them clean the opening, if they don't, push them out of the way and you do it.
Review about: Window World Installation.