How to insulate and wall off basement partition for best long-term air quality outcomes?

Question

Background

Project description: U.S. heating zone 5, in a 1960s ranch-style house with basement matching original home footprint, we're partitioning the empty half of the basement (no mechanicals, laundry, etc.) to use remaining space for office, hangout, couches and kids space. Below is a couple photos of the space with framing done. In the half that's being finished, we left a 3ft utility corridor for 1 of the 2 sump pumps, kitchen plumbing, and electrical to the kitchen and garage. Framed out space includes stairs up to center of house, a new door to new utility corridor, and new door to the rest of the unfinished half (which leads to walk-out door to yard).

Water context: The half being finished is below grade and around that the surface is all concrete slabs (front porch, back porch, garage slab and driveway). Unfinished half of basement is surrounded by yard graded away from house and has a walk-out door slightly below grade, plus a few windows at or partially-below ground-level.

Basement perimeter has an interior drain, installed in August 2023 (along with 2 updated sump pits and pumps) due to some water seepage at edge of slab and walls and mainly because of flooding from outdoor stairwell drain failures. That was a high quality install, creating a drainage channel under the thick concrete floor slab, with weep holes to drain the cinderblocks, and with waffleboard to catch any condensation dripping down walls. Since that system was installed, no water intrusion has been observed, outdoor stairwell is all good.

This space has 1 dehumidifier running on and off throughout the day to maintain 45-50% RH, and if needed we'll add a 2nd dehumidifier for use while the finished space is occupied. There is no plumbing above the space to be finished (and unlikely to ever be), so water problems would likely come from condensation, bulk water on floor (flooding or basement appliance breakdown), or extreme spill/roof leak from above.

Photos

Here's a look at the half being finished, with a view into the walkway for that utility corridor:

Here's a view from in the half being finished, showing interior french drain and showing doorway to utility corridor:

And in case it's helpful context, here's a peak underground from the interior drain install (ignore the hose going out the wall):

Framing so far: I thought sill plates needed to be pressure treated but contractor insisted they did not. They had great ratings and long-time experience in the area, and I heard mixed opinions about PT sill plate, so I trusted them on it. They said the room is not an exterior space needing PT wood, since it is not in contact with exterior walls and the slab underneath is thick and drained (so it won't wick up moisture). They framed the wall 1" or more away from exterior cinderblock walls and waffleboard.

Finishing it

My question is about insulation and drywall to minimize risk of moisture / mold issues in this area. The priority is air quality, as thermal inertia is not a big issue here: it is not a bedroom, it is decently comfortable unfinished (temperature ranges 55F to 70F over last two years) and it has heating options of either wood or electric. There's an existing ceiling with old fiberglass bats, foil side down which seems incorrect but no problems yet. That old ceiling insulation extends to rim joists in a patchwork way, and this partition will block access to those rim joist cavities.

What is the appropriate way to finish this wall assembly, to minimize risks of moisture damage anywhere, mold or other air quality issues? For example, fiberglass vs Rockwool vs ??? and should it have some kind of backing or not? Vapor barrier somewhere in assembly (like between drywall and studs) or not? Drywall should be anything in particular, or installed in a particular way? Any other important considerations before we close it up?

I'm not asking for opinions based on personal preference, I'm asking to understand which construction approach is most appropriate given building sciences of air flow, vapor flow, below grade construction, material safety for indoor air quality, etc. I've ready lots about vapor barriers and convection loops but am still unsure the way to go. These various details seem like interrelated mini-questions about the overall wall assembly, so I'm including it as 1 question with all this context.

Answer 1

The contractor who framed it recommended fiberglass bats with paper-backing, attaching the paper to the edges of the new stud cavities to secure it in place. Wiring would already be installed and would sit behind the insulation, with the 1" air gap between all of that and the concrete walls. They also said no vapor barrier (plastic sheeting) should be used anywhere, as the entire assembly needs to breathe. Still, the fiberglass should have paper-facing on the interior/warmer area.

Answer 2

To me, it seems like high-quality USA-made rockwool batts with no backing, and gypsum board with no backing, would be the lowest risk for mold or other air quality problems. Fiberglass seems risky if there ever were moisture, such as from condensation. It also seems like rockwool is easier to maintain - firmer and less dusty, while fiberglass is more likely to settle overtime and be a dusty mess during any repairs.

Also, the contractor suggested fiberglass with paper-facing, but I imagine unfaced batts (of either rockwool or fiberglass) would be best for breathability/dryability, it's just that rockwool doesn't need paper to firmly fit in cavities, whereas fiberglass is easier to install with paper(?)

In either case, I heard a suggestion to leave the bottom 6" uninsulated so I can cut an inspection hatch to examine the exterior concrete, insulation, studs etc. as needed.