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I have the following situation. I have a raised ranch and we're having drain tiles installed. I wanted to use this time to install some additional insulation while we've got wood paneling down.

The basement has 4 feet of wall below grade and then above grade cinder blocks are moved two inches toward the exterior and start going up. Studs go against the bottom section and then above the 4 foot mark, these studs are separated 2 inches from the wall. I made some ASCII art below where the X's are the wall and floor and the S represents the stud. Note the gap between the stud and the wall near the top. 

X  SS
X  SS        - Cinder block above grade wall
X  SS
X  SS
XXXSS
  XSS        - Poured basement wall
  XSS
  XSS
  XXXXXXXX   - Floor

We have a vapor barrier (plastic sheeting) stapled directly onto the 2x4s (the exterior side). Snug in between each pair of 2x4s we have R-13 fiberglass batts.

As for my questions...

1) Would it be okay for me to put some rigid foam insulation into that gap? I figure I could take down a 2x4 and start sliding insulation against the wall.

2) If I do that, should the vapor barrier be on the exterior side of the rigid panel or the interior?

3) Upstairs we have drywall attached to 1x2s which are then attached to cinder block. This situation sucks because barely any insulation fits in that tiny gap between the cinder block and drywall. This house is in Chicago too. I'm looking at my options here.

A) Rip off all the drywall and little 1x2s and put in some real 2x4s in there w/ some batt insulation. Fix all the power outlets, window sills, etc.

B) Attach foam board insulation directly onto the drywall. Then put new drywall on that. Of course fix all the power outlets, window sills, etc.

C) ???

What would you guys/gals do in this situation?

Thanks,

mj

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2 Answers 2

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Well... THAT's an expensive house to heat, what with Lake Effect and all!

I think it'd be fine to slip some styrene (pink) foam board into the gap. You could also use polyisocyanurate, but you wouldn't get the benefit of the foil facing because you wouldn't have any air gap.

If you add foamboard, or even if you DON'T, the vapor barrier should ALWAYS be on the HEATED side of the insulated wall in cold climates. That means on the LIVING SPACE side of those studs. Right now, your vapor barrier is gathering moisture (frost) between it and the fiberglass, and when it melts your fiberglass gets wet. On the CORRECT side, it stops any moisture from getting into the fiberglass in the first place, trapping it within the living space where it belongs.

Upstairs... I'd leave the existing fiberglass in place, but strip off the drywall. I'd span the "studs" with heavy (2" thick) horizontal furring strips, and I'd fill the gaps with 1-3/8" polyisocynanurate foam board (foil faced) snug against the existing "studs". I'd seal all joints with aluminum tape. Then new drywall can be added to the horizontal strapping, leaving a 5/8" air gap between the foil-faced foam and the drywall.

In fact, that's exactly what I DID in this house - it's a transitional-timberframe 5/4 cape in Vermont, and we now burn just over two tons of wood pellets (about $500US total) per year, just about 1/3 as much as we burned the first winter, before we did that foamboard job. It was interesting to get the switches & outlets out to the new surface, but I accomplished it by cutting pieces of plywood with box-shaped openings & stitching them into place across the studs. I screwed the boxes into the plywood carriers (mounting ears reversed), so the boxes are just less than flush with the new drywall.

Funny to run a studfinder on these walls, though - studfinder only finds HORIZONTAL studs. 8)

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  • That's a nice example w/ your own house. What did you have to w/ the windows? Did you add strips of wood around all four sides to cover the drywall edge?
    – mj_
    Aug 17, 2014 at 3:24
  • I haven't actually built the jambs yet for this house - health issues slowed me down. I'll get there. I built all the jambs for the last house, again customized for custom walls. For anyone else... most window manufacturers sell "jamb extensions" in various sizes for situations like this. Lacking premade jamb extensions, one can always just buffer the jambs out with appearance-grade stock molding.
    – TDHofstetter
    Aug 17, 2014 at 5:07
  • TD - I'm a compete amateur, but doesn't the logic about the vapor barrier on the heated side of the insulation only apply above grade where the main water source is condensation vs a below grade basement where water could come through the walls?
    – elPastor
    Aug 8, 2019 at 13:36
  • @elPastor... a vapor barrier isn't what you need below grade. For that you need a much more robust moisture barrier at the very least, preferably even sufficient drainage to keep the ground next to the basement walls as dry as reasonably possible.
    – TDHofstetter
    Aug 9, 2019 at 14:42
  • Thanks TD. If I understand you correctly, the 2" foam board would be the moisture barrier directly on the concrete and a vapor barrier would be between sheet rock and fiberglass insulation if I wanted to double-up the insulation, with a gap between the studs and foam board for air circulation. Might be worthy of a separate question...
    – elPastor
    Aug 9, 2019 at 14:56
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Rigid insulation is a good idea here. Polystyrene would be fine, but poly-iso would give you a higher R-value. I personally would look to get at least one layer on the poured basement wall to get a bit of insulation there.

How to deal with a vapor barrier is complex; it depends on the construction and the kind of insulation that you are using. The best source I know is Building Science Corporation; here's their information on basements.

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