Greetings, the short answer is that this is not a simple answer. Pointing you to a relevant FAQ that deals with this very question found athttp://ameritint.com/faq/ I quote from their site:
Can Window Film Be Used On Low E Windows?
Whether window film should be used on low E windows and how much you will benefit depends on three factors:
- Type of low E surface used on glass.
- Location of low E surface in the window system.
- The desired amount of heat gain reduction, heat loss reduction, or other film benefits.
There are two basic types of low E surfaces on glass. One of these is a conductive coating put on glass as it is being made. It gives some heat loss reduction, but does little to reduce heat gain into a building. The second type is a more complex system of multiple layers of metals and conductive coatings deposited on glass after it has been made. This type of low E glass gives heat reductions of 30% to 50% in addition to reducing heat loss. Obviously there will be more heat gain reduction using film on the first type. If there is any question about the type you may have, ask your glass company or the window manufacturer to send you the specific information about your glass.
The location of the low E surface in your window system is also very important in deciding whether film should be used. If the low E coating is on the room-side surface of the innermost pane of glass, the use of window film may reduce or eliminate the heat loss reduction of the glass itself. This may be more than offset by the heat gain reduction/heat loss reduction properties of the films to be used. Most low E window systems, however, consist of double pane windows where the low E surface faces the air space between the panes. In this case, film can be installed without eliminating the heat loss reduction benefit of the low E glass. The type of window film you choose for low E glass depends entirely on your desired benefit –whether you want to reduce heat gain, control glare, prevent heat loss, reduce fading or enhance the safety of your windows and glass doors. Carefully consider all these benefits before making a final decision.
As you can see there is to may variables here to give you a simple answer since where the film would be applied and the window construction (single, double pane) as well as other design details are missing. To give you and idea why this really a nasty heat transfer equation I will point you to classic technical discussion on how to solve such a problem that can be found at http://www.cibse.org/getmedia/e285dd9a-a6a0-4614-9c65-52c4c61fba71/Appendix-5-A11-Derivation-of-solar-gain-factors-updated.pdf.aspx Teh section that may be most applicable here can be found at 5.A11.5 Properties of glass and blind combinations. In this case the film is treated like a blind between the two panes of glass. However, if the film instead it was applied on the outside of the window vs the inside the window the results would be significantly different. The bot***** *****ne, I am sorry to say there there is insufficient detail to provide you an exact answer to your question. Unfortunately this is not a simple case like when you just add the R values together to get the new R value when you put one insulation in series with another.