Basement finishing: existing insulation half covering foundation walls?

[IndyGopher]

Hi all! I've read way more on this topic in the past couple days than I'd wish on any human being! Basically, I've got a brand new 2000+ sf basement in the suburbs of Indianapolis and plan to finish a lot of it myself. The builder bolted insulation to the top half of the walls, probably due to some code requirement. I know the preferred method of insulating a basement is polystyrene foam board. I just can't imagine removing all this existing insulation then cutting off bolts/fasteners and grinding them down flush with the wall to facilitate the foam board insulation. How would you handle this? I see a lot of options, including:

1) Leave the existing insulation in place and cover the bottom half of the walls with foam board.
2) Leave in place, use full sheets of foam board, sliding as much as possible up under the existing insulation.
3) Leave in place, no further insulation on bottom (to facilitate air flow).
4) Leave in place, frame over existing insulation (either just outside of it or right up to it, slicing where studs would be place), then insulate bottom afterwards with typical batts of R-13.
5) Remove existing insulation and do the work of prepping for foam board.

Appreciate any advice!

 

[Dana]

Fiber insulation will wick ground moisture from the foundation to the interior side finish materials. Yard it out and start over.

Rigid polystyrene isn't necessarily the best solution here, and XPS (extruded polystyrene, pink/blue/green board) is the least-green solution. You'll get more R/inch out of polyisocyanurate (foil faced or fiber faced.) The IRC codes call out continuous R15 for zone 5A (that would include Indianapolis), which would take 4" of polystyrene, or 2.5" of foil faced polyiso, or 3" of fiber faced roofing polyiso. To hit the same performance level in with low moisture risk you could also install 3/4"-1" foil-faced polyiso (R5 minimum, R6 better) trapped to the foundation with a 2x4/R13 studwall, which would still meet IRC code on a U-factor basis, and has sufficient foam-R on the above grade section to prevent wintertime moisture accumulation where the fiberglass meets the foam. If taking the batt insulated studwall approach it's best to use unfaced batts, kraft facers are still OK, but avoid foil facers, which would create a moisture trap. Do NOT install a polyethylene vapor barrier under the wallboard.

The only caveat with polyiso insulation is that it is mildly hygroscopic, and can wick moisture if resting on a damp slab, or in the event of a minor flood. Installing 1.5" of polyiso under both the cut bottom edge of the polyiso and the bottom plate of the studwall as a capillary & break prevents moisture from wicking into the wood (no need for pressure treated) or the polyiso, and keeps the bottom plate closer to room temp rather than slab temp in summer, where it might otherwise be taking on moisture from the room air during humid summer weeks.

Reclaimed roofing polyiso with fiber facers (fiberglass, asphated paper etc) is usually dirt-cheap- cheaper than batts(!), and 3" is a common thickness used in commercial building roofing/re-roofing. Gluing the boards the wall with foam board construction adhesive, taping the seams with housewrap tape, then strapping it to the wall with 1x4 through-screwed to the foundation with masonry screws works, mounting the wallboard on the furring. (That takes up less room area than 3/4 polyiso + insulated 2x4 wall.) Building an uninsulated studwall tight to the foam works too, and easier to route plumbing & electrical lines through.

Be sure to seal both the top and bottom seams where the foam board meets the foundation, and insulate/seal the top of the foundation, foundation sill, and band joist to at least R15 as well. It's sometimes easier to deal the foundation ledge with 1.5" foam board, which is the same thickness as the foundation sill. With the cut'n'cobble between joists it's best to cut them with a loose fit to allow air-sealing with can-foam in the gaps.

Some variations on the theme: