Window Trim and Sill Proportions

One thing that always needs to be considered when selecting a window trim and window sill is the proportions – more specifically, the depth. The depth of the trim is measured from the backside of the moulding (where it is attached to the wall), to the furthest point it projects from the wall. Aesthetically, having thinner trims than sills standard, so that a larger “lip” is created at the base of a window. When your contractor (or you, if it’s a DIY installation) isn’t paying attention, it is possible to select a trim that is too large for a sill. Why is this a problem?


It’s not a pretty sight. The trim overhangs the sills instead of sitting nicely on the sills upper surface. Most of the time, the edge of the trim isn’t coated, in which case the styrofoam is left exposed to UV Rays (which cause it to degrade) and is susceptible to insects looking for a place to nest. While the styrofoam trim doesn’t provide any nutrients on which insects, rot and mould can feast, like any building products, it can provide shelter for them.

One solution should you ever purchase a sill with less depth than the trim, is simply to build it up with a flat band. Adding a flat band with 1″ or 2″ thickness in behind the sill effective pushes it further from the wall, creating a larger top surface for the trim to finish into.