I have a client who wants me to insulate a flexible chimney vent (7/4) for a gas fireplace because he is worried about condensation where it passes through a cold space before hooking up to the fireplace. I told him it can't be done because I need to keep my clearance to combustibles and insulation is combustible, he says he had one installed before and they used foil back ins. with the foil against the vent and then wrapped again with the foil on the outside . Is this right or am I right in sticking to keeping clearances ? Thanks.