What is an alternating rebate?

An alternating rebate (Dutch wisselsponning) is a rebate that switches sides: for one component it is on the outside, for another on the inside. The term is used for door and window frames and for sheet materials such as insulation boards.

In frames

One jamb or sill can serve two purposes. For example, a window sits next to an exterior door in the same frame. The door needs an outside rebate, the window an inside rebate, so the jamb between them gets an alternating rebate. A rebate against which opposite-opening doors or windows close is also called this.

Tip: when ordering or making the frame, mark clearly for every member which side has the outside rebate, which has the inside rebate, and which way each leaf opens.

On insulation boards

Insulation boards, for example polystyrene, often have a shiplap edge: stepped up on one edge and down on the opposite edge. The boards overlap, so there is no straight-through joint. That reduces thermal bridging and air leakage.

Tip: always lay the boards with the rebates interlocking and push them tight. Cut edges lose the rebate, so seal those joints as the manufacturer specifies.

Related terms

• Rebate
• Rebated door
• Thermal bridge

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