Reducing heat absorption through windows

During Summer it gets warm, and a lot of heat comes through our windows.

Our windows each have an area of ~1,5m², meaning up to ~800W of heating power. The German norm DIN 4108-2 defines the Fc value of a sun protection mechanism as a factor for the "incoming" power, where 1.0 is "nothing" and 0.0 is perfect deflection.

A dark curtain on the inside reaches an Fc of 0.9, a semi-transparent textile screen on the outside (e.g. lets light pass through) reaches an Fc of 0.25.

Rules of thumb:

• outside better than inside (since once heat is internally absorbed, any outgoing heat will reabsorb in the window glass by ~40%)
• if inside, reflecting better than absorbing
• anything better than nothing (as the room "feels" cooler when energy is just absorbed in the window region and not on most of the room surfaces)