You are correct. We, however, don't have 6-12 inches to spar. The room is under 10 by 10 to begin with...

Posted By missmoo on 22 Apr 2013 02:17 PM
Thanks for all the responses. I guess this is complicated because we are trying to insulate a sound room. Thus all the materials are serving 2 purposes.
1. The outer wall is the foundation wall and we need to insulate that. We can't add another wall inside of that - thus having 3 walls along that side - because that would make a triple leaf for the sound waves. As a result, the room would actually be less sound proof with three walls than 2 walls.
2. We are using extruded polystyrene because of its density. We need this for the soundproofing.
3. We aren't doing the french drains at the interior. We worked to resolve the water at the exterior of the foundation, and then also painted the interior of the foundation wall with Ardex.
4. In order to achieve the soundproofing, we are using double stud wall construction with a 1" air gap in between. If all goes ok, it will be totally sealed for air...The wall assembly is as follows: exterior foundation wall, 2X2 stud wall within which we will put the extruded polystyrene R-10, 1" air gap and then a 2X4 wood stud wall with batt insulation, then the vapor barrier and then the 2 layers of drywall. The rim joists are insulated with a layer of drywall and then a layer of rigid polystyrene.
Given the assembly I am describing and using these materials should I not use a vapor barrier at all? Should I put it on the warm side of the inner stud wall?

Most XPS sheathing sold in box stores is 1.5lbs nominal density, just like foil faced polyiso. 

In MA there are several sources for reclaimed 2lb & 3lb roofing iso too.  (Insulationdepot.com being just one of a handful. Search the local craigslist for "rigid insulation", you'll find some of the others.)

Air tightness counts more than foam density for STC points on rigid walls.  It would seem that the rigidity & density of the foundation itself may render the acoustic properties of the foam moot anyway, but if you're looking for higher density foam, look at 3lb roofing iso (2x the density of box-store XPS.)

Do NOT put 2x2s against the foundation with foam in-between. The low-R of the wood dramatically lowers the thermal performance and the wood wicks ground moisture from the concrete.  Instead use 1x furring over the foam, through-screwed to the foundation 24" o.c. with TapCons.  Even if you have to back off on foam thickness it'll end up being higher-R than with the thermal bridging of the 2x2s, and the moisture performance would be an order of magnitude better without the wicking.

Do NOT use an interior side vapor barrier, as that will trap ground moisture inside the stud cavities, creating a mold condition.  With even an inch of foam on the exterior the wood stays warm enough to avoid excessive wintertime moisture.

The notion that you need a vapor barrier in this climate is a false presumption. The need for an interior side permeance lower than that of standard latex paint (2-5 perms) can be completely avoided with as little as an inch of exterior-side foam, and you are discussing using 1.5-2" of foam, which provides a HUGE margin.

Without any air barrier against the fiber insulation on the batts it's thermal performance is degraded by convective currents in to and out of the fiber from your 1" air gap.  (I'd think a rigid exterior-side air barrier on the studs would also lower the sound transmission too.)

Thanks so much for the response. It's so informative.
I was worried about the bridging of the 2X2 at the foundation wall. I am just really hesitant to put any kind of fastener in the wall...I don't want to create any opportunities. So I was hoping to just create some kind of frame that was attached at the bottom to the floor - and then at the top to the underside of my existing joists. Can I use the 1X furring and attach it to the inside face of the 2X2 at the sill and the head?

yes, you could build a wall of 2x2's not attached to the foundation and fasten the foam to that. be sure to use PT lumber since the moisture could rot untreated lumber and cause mold, and be sure it us completely air sealed at all edges, and tape the foam seams.

Alternatively you could trap the foam to the wall with a studwall, turning the studs sideways on the stud plates, glueing them to the foam with foam board construction adhesive.

The bottom plate of any studwall needs a half-inch or better foam between it and the slab, which acts as both a capillary break, and keeps the temp of the wood sufficiently above the subsoil temps to not take on moisture from summertime air.

We do a lot of basement remodels and focus on heating, and general HVAC including Radon mitigation. In new and old we use 2# foam for thermal and vapor retarders, then 2x4 framing, glass and rock to the interior. Double rock may be a better sound barrier and cost less than sheets of XPS considering labor costs. But I differ to Dana.

I have been following your discussion on this topic and need some clarification regarding "keeping drywall from contact with the floor" and "non-wicking polymer kick-boards". In winter had field stone basement spray foamed with 4 inches closed cell, then 5/8 drywall applied. There was significant humidity in the basement just before spray foaming and I used commercial dehumidifiers to remove humidity. I also had a mason come and repair any gaps and allowed a week to pass before spray foaming. There was never a water issue in this house prior to spray foaming, but since then there has been some "wicking" of water in a few places. The moisture appears in a corner, then dries then reappears. The drywall is probably too close to the floor in places (company did the drywall) and I wonder if I should cut out an inch or two of the drywall from the floor and see what happens. Tried to find kick-boards mentioned but no luck - perhaps there is another name for these. Thank you for any suggestions or advice

I removed the drywall that was resting on the concrete floor in the areas where there was moisture wicking and the removal of the drywall stopped the water wicking - just an FYI for others.

since then there has been some "wicking" of water in a few places. The moisture appears in a corner, then dries then reappears.

Is this caused by external leakage or by interior air getting to and condensing on a cool surface? The later may be easy to fix with some air sealing.

Slabs poured on dirt rather than clean gravel can wick substantial ground moisture too, even when surface drainage an other issues are taken care of. It depends. But for sure air sealing never hurts, only helps.