A cellular covering for an architectural opening includes a plurality of elongated, longitudinally connected and transversely collapsible cellular units composed of inner and outer cells where the outer cell is a woven, knit, or non-woven product and the inner cell is an air-impermeable film which may be treated to be a low-modulus film with acceptable surface tension so that the panel formed from the cellular units has improved insulative properties and has a relatively long life.