So I've had a plumber friend tell me the best way to see if 3 pipes in my basement are drains is to put a Garden hose in them and run the water. What if the pipes under the slab are cracked? Won't water just sit there then under the slab? Sounds bad
Plumbing – How to determine whether pipes in the basement are actually drains
plumbing
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If you dont want to run a new pipe you will need specific insulation
Notice that this insulation has a shiny outside layer. This acts as a reflection for heat, and usually has a black/dark side on the inside.
The foam/glass wool used to wrap the pipe needs not be anything special, but I am sure there are special ones that locks out heat better.. instead of the conventional one that keeps heat in.. is there a difference? I am not sure.
Look at this insulation- They wrap silver around the pipe and then insulate it. You DONT want to do this.. because this keeps the heat INSIDE the pipe, and the insualtion slows down absorption of coldness form the earth
As commented- it has no difference--inside or outside- it will deflect the heat in both cases... This is open to debate though.
So you are looking for Glass Wool Pipe Insulation with reflective outer shield
Lots of questions (related to this Meta discussion). I will just tackle a few points I feel confident about answering.
Subfloor
The Schluter DITRA does not need to be on CBU so I would skip the 1/2" layer of cement board you are planning on using and make your plywood subfloor thicker (or just skip the CBU and keep your 23/32" OSB). CBU is not "stiff" enough to be a subfloor, it can be used instead of DITRA but it must be secured to a subfloor. It rarely makes sense to use 1/2" on the floor unless you are trying to make the finished floor come out to some specific height (for example to help the transition to another room with a different floor). If you are not goin gto use DITRA, you will want some CBU on the floor but the 1/4" stuff is fine.
Apply the modified thinset and DITRA directly to the plywood subfloor. Checkout Schluter's DITRA Installation Instructions for details on the subfloor prep and how to secure to DITRA to the plywood.
Toilet
Without a picture or diagram it is not really clear to me exactly what the issue is with the toilet drain, but that won't stop me from taking a guess :)
You may be right about getting away with cutting a larger hole in the subfloor to handle the bend of the toilet drain pipe. Are you sure the PVC drain will stick above the finished floor? Realize that the thinset between both the subfloor/DITRA (not very much) and DITRA/tile (more so) will add a little to your height, maybe between 1/8" to 1/4".
Have you measured the height of the toilet drain with the flange installed (just dry fit it to take a measurement)? The flange can be installed on the top of the finished floor (tile), on top of just the DITRA, or on top of the plywood subfloor. In your case you would probably want it as high as you can, so install the tile first and then glue the flange to the PVC closet bend. I think once you dry-fit the toilet flange you will see you are OK here.
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Best Answer
The ground under your home is loaded with water. It may be down a ways, but it's there. Any water you add will percolate down to the existing water table. The only real concern is temporary flooding. Use common sense. Monitor the entire area while you run your test. Use an assistant if available. Look and listen.