According to Home Guides, caulking the seams around a window is the best and easiest way to seal a window with vinyl sidings. Due to the nature of vinyl, as a plastic, there is a tendency for a vinyl window frame to expand, contract and warp with changes in temperature over time. Therefore, it is important to choose a silicone or polyurethane-based caulking product.
Sealing around a vinyl window requires the use of an abrasive tool. This tool is used to clear out all of the old caulking around the window seal. Once all of the old caulking has been removed, the new caulking is applied using a caulking gun on the seam around the window where the old caulking was removed. Once applied, caulking should be left alone for 24 hours.
Caulking that contains polyurethane or silicone does not damage vinyl windows; and, it forms a strong bond between the vinyl and other materials, such as wood or stone.
In the case of silicone based caulking, there are two kinds to choose from: acid-cure or neutral-cure. Acid-cure silicone caulking is best when sealing non-porous surfaces. However, acid-cure silicone caulking may corrode the vinyl sidings of windows over time. Therefore, neutral-cured silicone caulking is the best bet for sealing around vinyl. Although the adhesion properties of silicone-based caulking may not be as good as polyurethane, the waterproof characteristics of silicone are unmatched.
Polyurethane caulking sticks to a variety of surfaces, making it great for adhering vinyl window sidings to wood or stone. Also, like silicon caulking, polyurethane does not corrode the vinyl window sidings. The downside to using polyurethane caulking is that the caulking itself needs to be painted over in order to protect it from the sun. UV rays can seriously degrade polyurethane caulking, leading to higher upkeep and costs.