Abstract: A doorsill apparatus for a sliding glass patio door or other exterior door is described including a heat insulating spacer fastener for securing inner and outer sill members of metal against relative movement. The heat insulating spacer fastener is preferably a synthetic plastic clip member. The plastic clip member is of a general W-shape and fastens the inner and outer doorsill members together by clipping onto first and second projections extending from such sill members. In addition, a moisture barrier member of synthetic plastic material is provided over the joint formed by the insulating spacer clip and extends along the upper surfaces of the sill members to prevent moisture from condensing and forming frost on the inside of the glass patio door.