Soil pipe depth requirement

frost

It is hard to follow your "logic". But it the pipe is buried below the frost line, then the discussion is immaterial. If it is above that point, then regardless of what you wrap the pipe with it is eventually going to reach the same temperature as the surrounding ground. It has nothing to do with where the frost line is. Insulation, of any kind, does not prevent freezing, is merely slows the process down until, hopefully, events will occur to reintroduce "heat" to the pipe so it does not freeze. If the liquid in the pipe is static for a long enough period, or enough liquid is not sent through the pipe to "warm" it up, the liquid is going to freeze eventually.

The question is hard as there are to many factors for 1 answer. The answer will vary depending on, air temp, duration of air temp, snow depth if any, ground heat, and ground heat transfer rate based on surrounding soil composition.

The heat that will keep if from freezing is from the regular use of the drain. Showers and diswashers will add heat.

You want to try to keep water out of the soil because it is a good conductor of heat. Dig wider and deeper and backfill with gravel so it will drain. Do whatever you can to keep the water out. You can also add insulation around the pipe if you can find something that will not get soaked with water. There are some insulations used in refrigeration systems that don't absorb water. Then cover the whole thing with the DowBoard or whatever will protect it.

But, again, in this situation , he said the thing is now only down 10"...I don't think it will make much of any difference, depending on where you live. The insulation would also need to extend out from the pipe a good ways as well, otherwise the heat would escape around it.