I ordered a roll of vellum paper and traced the inside shape of the windows. Next I took the vellum paper and placed it onto the reflectix to be cut out. Then I ordered 4″ wide gaffers black tape and covered one side of the reflectix. It just looks like dark tint from the outside.
In case there are any Honda Fit owners reading this, the hatchback window doesn’t have a rubber seal like the other windows (the entire hatch has the rubber seal), so I used these two little slits on the left and right side of the window frame to secure the Reflectix. Basically I made little “tabs” on either side of the insert that fit into those slits to hold it in place. I also cut the insert so it fit snug on the bottom, but went beyond the window on the top.
2. The advertised R-value of R-4.2 for Reflectix applied directly to a duct is an assembly R-value. The number is inflated by the air film value on the exterior of the product (about R-1.4) plus the value of any air trapped between the Reflectix and the duct.