Modern day window films can be installed on either the interior or exterior face of a window, and each method has its own set of advantages and disadvantages in different situations. Understanding the differences between the two can help you choose the best option for your specific needs.

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Interior window film installation is done on the inside face of a window, and is the most common method. This type of installation is generally considered to be easier and less expensive than exterior installation. Additionally, since the film is applied to the inside of the window, it is protected from the elements, which can prolong its lifespan and better warranty period. Interior installation is also great for renters as it offers options where it does not alter the exterior of the building. The downside to an interior installation is that the film has to reject solar rays after they have penetrated the glass, and then send them back out, which heats up the glass panel and can cause some convective heat transfer. Interior-installed films are also not a great option when the window panel is very difficult to reach from the inside, obstructed by a structure, or in work environments where there are several operations that would be disturbed by the need to access windows to install film. 

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Exterior window film installation is applied on the outside of the window. This method is typically more complex and requires more skill to install. However, it also offers some significant benefits over interior installation. Exterior installation provides better protection from solar rays and can also help to reduce heat gain in the summer and heat loss in the winter. An exterior installation allows the film to be in the ideal heat rejection position, on the outside face of the glass where it can reject solar rays before they ever penetrate the glass, exterior film will always outperform its interior equivalent in heat rejection. An exterior installation is perfect for a hospital, hotel or commercial work building, where the installation of the film does not disrupt the operations of the business. The downside to exterior films is that some need to be edge sealed, which requires more material and labor, and usually there is a limited warranty period.  

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When deciding between interior and exterior window film installation, it is important to consider your specific needs and preferences. Are you a homeowner and are all your windows easily accessible from the inside? Then the interior option is likely your best bet, since you will get a lifetime warranty. Do you manage a commercial building with tenants that have ongoing operations that you don't wish to disrupt? Then exterior is the way to go. It is important to consult with the film dealer/representative and a professional installer to get the best advice for your specific case.

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In summary, Interior window film installation is an easy and cost-effective solution for providing privacy, reducing heat, and blocking UV rays. Exterior window film installation offers additional benefits such as better UV protection, heat reduction and a cleaner look. The choice between the two depends on the specific needs and preferences of the user.