We do not recommend placing a cowl on top of a sewer ventilation pipe. However, a cowl can be used if the intention is to make the sewer pipe look the same as the other pipes on a roof. If a cowl is used, the rain collar inside the cowl should be removed in order to improve ventilation.
To avoid freezing, we recommend the insulated VILPE 110/ER/500 ventilation pipe for sewer ventilation. Uninsulated sewer pipes can be insulated later by mounting the VILPE Freeze cover on top of the pipe.