We have a new fireplace, in a chase on an outside wall. Cold climate. Outside of chase is insulated. Nearly flat roof, no attic. Cold outside air vent is in the outer chase wall for air to the firebox. When nights are freezing, we get condensation/freezing water between the two flue pipe layers, because it melts in the day if it warms up with sun, water drips down onto the unit, and trickles out the front of the fireplace front air circulation vent above the firebox. Roof leaks have been eliminated as a possibility. It is a Desa unit, with air space between pipe layers. What can we do for this?