One builder wants to use a 8" concrete wall. If I build a 2x4 wall on the inside what would I need to do? I am thinking of leaving 1" between the studs and the concrete and then having closed cell spray foam applied. Is 2" enough in zone 6B for moisture control? And then R13 or R15 batts on top of that? Thanks

Another option would be foam board panels of your desired thickness mounted on the basement walls and then use 1x strapping over them. You can drywall over the strapping. Using a hammer drill you can mount the foam board to the wall with plastic fasteners and then when you hang the strapping you can mount that using anchors to securely attach the strapping/foam boards to the wall. The anchors are not really available at big box stores. Your local concrete supply house will have them. Depending how muc room you need behind drywall you can run the strapping in one layer or two.

Why not go with ICF?

Honestly, if you're excavating anyhow, you're better off applying the foam both under-slab and on the exterior. This will thermally break the entire basement from the surrounding earth.
2 to 2.5" of foam on the outside of the walls will give you approximately R-7.5+. You can then make up the difference with interior batt insulation in a 2x4 wall (should be R13-R15).

Robinc's suggestion of ICF is also a good one, as you don't have to go back over the walls again to insulate them.

ICF....then just drywall it....done.

Posted By newbostonconst on 09 Feb 2019 12:53 PM
ICF....then just drywall it....done.

Well...a LITTLE more than that.

Waterproofing and all that...  6B shouldn't really see termites...  But one could always use a combo fluid-applied plus P&S to termite-proof if necessary.

While I agree that ICF is the best solution, you also might take a look at InSoFast.
1" closed cell is enough for a vapor barrier, and will also be sufficient to significantly reduce condensation potential provided the additional insulation is r-13 or less.
If you use 1" XPS, you will need to tape all seams and seal the top and bottom, and limit the additional insulation to r-10 or less.

Posted By littleboss on 08 Feb 2019 08:44 PM
One builder wants to use a 8" concrete wall. If I build a 2x4 wall on the inside what would I need to do? I am thinking of leaving 1" between the studs and the concrete and then having closed cell spray foam applied. Is 2" enough in zone 6B for moisture control? And then R13 or R15 batts on top of that? Thanks

In zone 6 the IRC prescribes R7.5 minimum for dew point control on the foam for 2x4 construction, so 2" of closed cell foam would be enough from a moisture control perspective.

But it needs to a 1.5-2 gap between concrete and studs, not 1" to meet the IRC's thermal requirements on the above-grade section. If only 1" thermal bridging of that 1" of R1/inch stud penetrating into the R6/inch foam robs significant performance from the foam.

Where do the numbers come from that say I can't have more than a certain R value in the basement walls? For example in the Mechanical room where I don't want my plumbing to freeze I would like those walls to be R25 to R30. The rest of the basement could get by with 2" of foam or R13.

Then of course there is the basement ceiling. I would think that if I was trying to keep the Mech room from freezing I would want at least R30 there. Is this good or bad thinking? My main concern is keeping the Mech room from freezing over the cold winter months.

I couldn't imagine the regulations would not let you add more R value. They only have a min. IMO....that makes no sense.  Dana....I'm I missing something?

The poster R Value said to limit additional insulation to R10. I want to know why. I don't think he is referring to building code

The numbers come from R702.7 of the IRC: https://up.codes/viewer/wyoming/irc-2015/chapter/7/wall-covering#R702.7.1

The issue is the ratio of air-impermeable insulation to the total R. For any thickne exterior foam there will be condensation accumulating on the foam if the fiber-R is too high relative to the foam.

The code prescriptives for protecting sheathing with only class-III vapor retarders points toward the ratio but doesn't state it explicitly.

So going with just an inch of foam and jamming R23 rock wool into a 2x6 studwall will end up with frost accumulation over the winter on the above grade portion of the basement wall in a zone 5 or 6 climate. But with R10 fluff/R6 foam 37% of the total R would be air-impermeable foam and the temperature at the warm side of the foam would average above the dew point of the indoor air, and any condensation or frost that formed would re-evaporate/sublimate away well before temperatures were high enough. With 2" of foam you'd be at R12 on the foam, and good for R20+ on the fiber in a zone 6 climate.

Martin Holladay's blog bit of yore covers most of the rationale here:

https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

The numbers come from R702.7 of the IRC: https://up.codes/viewer/wyoming/irc-2015/chapter/7/wall-covering#R702.7.1

The issue is the ratio of air-impermeable insulation to the total R. For any thickne exterior foam there will be condensation accumulating on the foam if the fiber-R is too high relative to the foam.

The code prescriptives for protecting sheathing with only class-III vapor retarders points toward the ratio but doesn't state it explicitly.

So going with just an inch of foam and jamming R23 rock wool into a 2x6 studwall will end up with frost accumulation over the winter on the above grade portion of the basement wall in a zone 5 or 6 climate. But with R10 fluff/R6 foam 37% of the total R would be air-impermeable foam and the temperature at the warm side of the foam would average above the dew point of the indoor air, and any condensation or frost that formed would re-evaporate/sublimate away well before temperatures were high enough. With 2" of foam you'd be at R12 on the foam, and good for R20+ on the fiber in a zone 6 climate.

Martin Holladay's blog bit of yore covers most of the rationale here:

https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

If you are a masochist, you can use the ASHRAE steady-state dew-point or Glaser methodology to ascertain where and how much condensation will form in any buildup and any climate. We built this calculator to make this somewhat easier to accomplish:

Borst Building Assembly Moisture Analysis Calculator

Thanks again Dana, I found this from the Spray Foam Institute that has good info.

http://www.sprayfoam.org/files/docs/tech-docs/SPFATechDocs2015/SPFA-147%20-%20Aug%202015.pdf

Also thanks sailawayrb for the link to the calculator

Still a little confused on this. What if you only do foam? Can you do 2" or 3" of high density and nothing else?

Posted By littleboss on 14 Feb 2019 02:06 PM
Still a little confused on this. What if you only do foam? Can you do 2" or 3" of high density and nothing else?

2-3" internal or external?

Inside

Posted By littleboss on 14 Feb 2019 02:06 PM
Still a little confused on this. What if you only do foam? Can you do 2" or 3" of high density and nothing else?

IRC code minimum for zones 5 and higher is R15 continuous insulation, which can be done with 3" of polyiso, or 4" of EPS or XPS. The labeled R-value of XPS is R15 at 3", but its only warranteed to 90% of the labeled value, which would be R13.5. You could arguably get there with 3.5", but it's the least-green foam board in common use today, and more expensive per labeled R than EPS or polyiso.

The labeled R of foil faced polyiso is R18-R19, fiber faced roofing iso would be labeled R17, but derated for temperature most foil face goods would be R16-R17, and 2lb roofing iso would perform at R15-R16.

The labeled R-value of 4" low density Type I EPS is R15.4, and needs no detrating for age or temperature. The labeled R of 4" of 1.25lbs density Type VIII roofing EPS would be R16.7, R16.8 for 1.5 lbs density Type-II

With 3" polyiso it can be strapped to the foundation with 1x4 furring through screwed to the wall with 4.5" masonry screws, with the wallboard hanging on the furring, which is about the same thickness as a 2x4 wall with the studs tight to the foundation. (This is how I retrofitted my basement a decade or so ago, using reclaimed 2lb roofing polyiso.

Thanks!