Windows Without External Sills . If the windows or the sill are plastic already, then this is often the option an installer will use. If you are using 2 x 6 walls, you should have at least. Windows without external sills Windows & Glazing from forum.buildhub.org.uk Technically, a window sill only exists on the exterior of a house or apartment. The first is an interior window sill, which you will see if you look at your window from the inside of the home. Of course, there are two broad types of window sills available.