Basement – How to find what code is in the area for insulation in an unfinished basement

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.

You are in the same climate zone as me. Your walls do not have to be insulated from top to bottom according to building science reports (please read their definition of basement which is totally below grade). They only suggest floor to ceiling insulation in our region given spray foam or rigid foam - in "typical" home basements.

What I have been doing the past 6-7 years with great results - including my own house - is only insulating about 1 foot below grade, using BS advice. I use rock wool. I stuff any joist areas until they meet wall level. A decent size house costs about $250. The results can be felt right away. My crews ask that this not be done until right before the drywall is put up because the basement instantly retains a ton of heat.

This is the exact reason why it is not necessary in our region to go floor to ceiling. By over insulating you are not allowing for any cool air to come in during the warmer 6-8 months out of the year. I for sure have noticed an increase in AC usage while a decrease in furnace usage in my own home.

Also by leaving your bottom half exposed you are creating a high volume area for air flow. Meaning any moisture that gets in - and there is always some - will evaporate quickly.

So we simply stagger our scrap 2x4s about 1 foot below grade (sometimes this is just the top 1-2 feet of framing), stuff rock wool against wall and then above our framing into the joists all the way to the outer wall. This is super cheap and super effective and all inspectors in our area were more than pleased with this. Again this is not for cold climates but will work in the Midwest.