An insulating microwave material comprises a dimensionally stable substrate having a first side surface and a second side surface; a receptor film, which is connected to the first side surface of the dimensionally stable substrate via a substantially continuous adhesive layer and comprises a microwave energy interactive material supported on a first polymer film; and a second polymer film, which is at least partially covered by an amorphous polyethylene terephthalate layer, so that the amorphous polyethylene terephthalate layer is directly contacted with the second polymer film. The amorphous polyethylene terephthalate layer is in pattern configuration and is directly bonded to the second side surface of the dimensionally stable substrate, so as to define multiple sealed chambers between the first side surface of the dimensionally stable substrate and the amorphous polyethylene terephthalate layer. In this way, the boundary of the multiple sealed chambers is defined by the second side surface of the dimensionally stable substrate and the amorphous polyethylene terephthalate layer, wherein the multiple sealed chambers can expand in response to the microwave energy.