Low E glass has a special metallic oxide film that’s reflective and cuts down on energy loss. However, it needs to be cleaned in special ways.
Step 1 - Dust
Where there’s just dust on the window, as there will be during summer months, the outside of low E glass windows can quickly be cleaned by turning the hose on them and allowing them to dry normally. However, this will only work in areas with soft water. If you have hard water you shouldn’t spray the windows.
Step 2 - Hand Cleaning
Products that are safe for the metallic oxide coating include water and mild dish soap, vinegar, acetone for cleaning small spots, and hydrochloric acid when diluted with water at 99% water and 1% acid for removing tougher stains.
In soft waters areas, to clean by hand, mix together water and a little dishwashing liquid. In hard water areas, use distilled water. Rub on with a soft cloth and then gently rinse. Finish by cleaning with a soft cloth. You should never use alcohol or ammonia-based cleaners, and petroleum-based cleaners, or abrasive cleaners such as powder cleaners that will damage the metallic oxide layer on low E windows. Squeegees and abrasive cloths should never be used on the inside of Low-E windows.
You need to take particular care when drying the windows, as it can be very easy to leave ugly smears on the glass. Use a clean, non-abrasive, lint-free cloth for drying—microfiber cloths work the best—and stop before you’ve removed all the water so that the remainder can evaporate naturally, and prevent streaking. The coating will reactivate with exposure to sunlight.