A foamed plastic insert compresses to fit into the corner moulding or corner piece of vinyl siding or other synthetic siding. The corner piece is formed with an angled web member and undercut flange members offset from the web member and joined to it by sharp bends. The web member and the flanges define a wedge-shaped void, and each of the undercut flanges defines an outward-facing recess to receive ends of the siding members. The insert comprises a resilient L-shaped open- or closed-cell foam member, having legs that compress to fit snugly within respective spaces inside the sharp bends, and defining a V-shaped valley at its back side. When the moulding and inserts are installed, the valley biases against the corner of the sheathing of the frame building beneath the corner moulding. The insert also fills the open interior profile of the moulding. This blocks drafts, dust, moisture and insects from entering the structure through the corner moulding. The inserts can be installed without tools and without need for caulk.