I'm working on a plan to upgrade/retrofit my HVAC and basement/foundation insulation (and possibly drainage).  Unfortunately, I plan on upgrading my HVAC before I'll have a chance to come up with a plan for the insulation.  How much clearance from the foundation wall should I reserve for possibly spraying foam and digging a retrofit perimeter drain, in case we decide to go that route, so that I don't have to move my HVAC mechanicals?

Leave 6"- that would be enough for 2" of rigid foam plus a 2x4 studwall with half-inch gypsum. With 2" of XPS (R10), and a 2x4 studwall with unfaced R11/R13 batts (~R10 after thermal bridging) you'd have a relatively low cost R20 foundation.

If you want to do it at lower profile, 4.75" would give you 3" of fiber-faced iso (R19), held in place with 1x furring through-screwed to the foundation with half-inch wallboard mounted to the furring.

Dana,
I'm presuming the furring is necessary in option #2 because the iso isn't strong enough to hold wallboard and it reduces thermal shorts... Will this airspace cause a condensation/mold on furring issue if the iso is foam sealed (closed cell) to the rim joist on top and the wall board is a paperless drywall type product (half-inch)? I'm in RI with a 10" foundation 6' of it below grade, no other insulation on it.
Mat

The fiberglass facers on the iso are somewhat vapor permeable but latex paint has 3-5x higher permeability. As long as you don't put a vapor retarder between the gypsum & furring, the furring is at the same temperature and humidity as the room, in which case if it humid enough to create mold on the furring, you'll also be experiencing mold conditions everywhere in the basement. The air-space in the furring cavity is just an air space- air-sealed or not it passes water vapor relatively freely to (and from) the room.

I retrofitted my basement in Worcester MA this way a handful of years ago. When moving a drain after a couple years service I had to remove a section. Both facers, the foundation and the furring, both facers were all quite dry, with no evidence of mold or moisture issues.

Did you have anything on top of the slab? I know they recommend R10 but to get that I'd have to dig up the slab which is out of the question. I don't have the head room for anything other than some type plastic sheeting. Is a vapor barrier a waste of time here?

I also don't have the headroom to go over the slab, and the local water table is already at the footings 8 months of the year so even digging down isn't a viable solution for me. A few days to a week after extreme rain events the groundwater level is sometimes above the top of the slab, ergo 4 sump pumps, no slab insulation, and no finish floor.

The iso stops ~6" from the slab to accommodate the power-out (no pumping) high tide mark, and the kick boards are PVC-board (yup- the evil vinyl product.)

My slab is poured on dirt, not screenings/gravel and the capillary draw is high. To limit ground moisture transfer I sealed the slab with an acrylic masonry sealer. A silane or siloxane based sealer might have been better, but it's been a measurable improvement. I may eventually get around to a top applied epoxy vapor barrier solution over the slab, but for now a 70 pint room dehumidifier set up to drain into one of the sumps works fine, and is only needed during the summer months when the outdoor dew points are high. The other 9 months of the year the ventilation air for the conditioned space is dry enough that the relative humidity in the basement rarely exceeds 50%, but it's crept up ever so slightly as I've further tightened the house with air-sealing efforts.

The sensible cooling load on this place is quite low due to shading factors, so the central air conditioning doesn't keep up with the latent load (it runs fewer than 25 hours/year) but the tiny dehumidifier in the basement keeps up with the whole-house latent load with plenty of margin, despite the high water table and lack of a true vapor barrier in the slab. Air sealing the house has reduced the summertime (and overall annual) duty cycle of the dehumidifier, while increasing the wintertime relative humidity. (It never drops below 30% RH @ 68F on the first floor now, whereas prior to embarking on air-sealing it would dwell at 25% RH or less for weeks during the coldest weather.)

Thanks, I'll put siloxane on the to do list as I plan on finishing the basement.