"We have a new fireplace, in a chase on an outside wall. 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, drip down onto the unit, and runs out the front of the fireplace front air circulation vent above the firebox. What can we do for this?"