In a general sense, even average fiberglass windows are about 15% more energy-efficient than average vinyl windows. How energy efficient your new Marvin windows will be depends on a list of factors: type of windows, type of glass, type of glazing, type of coatings, and more.
Marvin windows and doors are made-to-order to suit your design preferences and performance requirements. Your new replacement windows will be built and designed around you — both the visible characteristics like size, color, and material; and performance features like U-factor and Solar Heat Gain Coefficient (SHGC).
You can look at a free brochure from the company about this subject that covers everything in much more detail: Open Brochure. Otherwise, your Midland Exteriors consultant — sincere product experts — will be able to talk you through each detail depending on your unique situation and goals.