You are missing my point, if the conduit is kept at above freezing temperature bypassing warm air through it from one of the rooms there will be no condensation buildup or ice. It's only 7 m of conduit we are talking about if it's lagged it will probably hold the temperature quite well so the cable will always be at room temperature as though it was indoors no ice and no brittle plastic sheath.
ASHRAE standard 55-2013 - "Thermal Environmental Conditions for Human Occupancy" specifies that to decrease the possibility of discomfort in indoor environments due to low humidity, the dew point temperature should not be less than 2.8°C (37°F). This equals to 30% relative humidity in 21°C. The upper dew point limit is specified to 16.7°C (47°F), which equals to 76% relative humidity in 21°C.
With -10°F exterior temperatures, to maintain the duct walls above the dew point, its going to need extremely thick insulation, have a large interior diameter and a lot of air-flow through it. *IF* the cable has to be protected against extremely low temperatures, circulating warm humid air from inside the building is *NOT* the way to do it - trace heating tape can easily keep the conduit above freezing with lower energy consumption for the same insulation thickness, and doesn't introduce extra humidity.
Anyway, if the cable is lying slack in the conduit, cooling it to -23°C (-10°F) is unlikely to do any damage or significantly compromise its electrical properties. Most interior Cat5 cable is rated for operation down to -20°C (-4°F) and going 3°C colder is unlikely to be a major issue. That's neglecting heat loss through the building wall, which will help keep it within its rated temperature range.