HO & O - High Quality Scale Modeling Architecture and Railroading

Because the storefront windows and doors cover most of this wall, I chose to cover the entire wall with the window material. You can see the window material has been glued over the entire front of the
building. This picture also illustrates the corner joint of the foam core building and the clapboard veneer that will be the side wall. The window material I'm using is absolutely perfect for lighted windows where interior detail is not to be seen. What is it?