A method for securely mounting an elongated, insulating strip in an elongated metal profile section, the strip having opposed longitudinal edges and the profile section having an elongated groove with a slightly open mouth. The method comprises placing one edge of the strip into the elongated groove in the profile section, the strip extending out of the groove and the groove loosely holding the strip and then partly closing the mouth of the groove, simultaneously along the entire length of the groove, onto the strip to securely retain the strip to the profile section. An apparatus for use in forming a thermal break between a pair of profile sections, each profile section having an elongate groove along its length, and an insulating strip loosely held in the grooves and extending between the sections. The apparatus is a press having a bed to support a first die; the first die holding the first and a second profile sections in place, slightly spaced apart, parallel to each other, and with the grooves facing. The first die is at least as long as the profile sections. The press has a member carrying a second die spaced from the first die, the second die at least as long as the grooves. Moving means on the press relatively move the second die toward the first die for partly closing the grooves in the profile sections, simultaneously along their entire length, onto the strip to securely hold the strip to the profile sections.