a substantially smooth concrete slab that is larger than the panel 10 such as the pad of a foundation of a house or a parking lot the insulating material 12 is placed flat on the casting surface 30 grooved side up within a concrete form 31 which will determine the shape of the panel 10 if the insulating material 12 is divided into segments 13 the segments 13 are fit together within the concrete form 31 the segments 13 are then attached together and the seams 18 between them sealed by a sealant 17 see fig7 the sealant 17 may be any type of adhesive compound or other substance that will adhere to the insulating material 12 and seal the seam 18 such as adhesive backed paper or plastic duct tape joint tape caulk joint compound or expanding foam glue and preferably has insulating properties similar to the insulating material 12 in the preferred embodiment the sealant 17 is a heat and water resistant expanding foam glue examples of such a glue are made by hilti u00ae and touch u2018 n u2019 seal u00ae in one embodiment shown in fig6 the shape of the insulating material 12 is the same as the shape of the concrete form 31 this embodiment is useful when casting a complete wall as a single panel 10 the joints between walls can then be capped as described below in another embodiment shown in fig8 the shape of the insulating material 12 is smaller than the shape of the concrete form leaving space between the insulating material 12 and the concrete form 31 on at least one side this space may be filled by a spacing material 32 which is composed of or coated with a material that will not bond to wet concrete preferably.