Many people think that UV exposure is the reason that fabrics fade. Actually, UV light is only about 40% of what cause fading. See UV Fading Facts.
Nonetheless, UV light is still the major reason for fading of fabrics and damage to other valuable materials.
Certain glazing materials when used in our storm windows can provide up to 99% protection from some or all of the UV light. Typical alternatives are shown in UV Glazing Options. In our experience 3/16” laminated glass, UV filtering acrylic, and polycarbonate are the most popular glazing materials for this purpose. Each material has its advantages and disadvantages.