Abstract: Two elongated metallic profiles each formed with two pairs of flanges each of which forms a longitudinally extending groove are positioned with the two grooves of each profile parallel and opening toward the two grooves of another such profile. An elongated bar of insulating material having a pair of longitudinally extending sides is fitted between each groove of each profile and the corresponding groove of the other profile. This forms a longitudinally extending passage defined on two opposite sides by the two insulating bars and on the other two sides by the two profiles. A tool is disposed through this passage so as plastically and permanently to deform the inner flanges outwardly and into intimate tight contact with the bars.