Dana- We use UV Plexi in our 200 year-old historic house, but we never open the windows. We drilled through the plexi and attached it right into the sashes with screws. The plexi has been in place for 5 years or more with no yellowing. If your windows are on the right type of sashes I suppose that you can mount the plexi to the sashes and still be able to open the windows. The house curator recently acquired a UV meter and checked the levels, and she was very happy with the readings. We've had essentially no damage to the furniture and textiles that are on permanent display throughout the house. If you want, I can get the information on the brand that we use. Leslie Johnston Historic New Orleans Collection [log in to unmask]