I'll have a poured concrete basement lined with rigid insulation and inner 2x4 walls.   This is in zone 6 so I'll definitely want insulation.  With 3" EPS (R15) and 2x4 stud walls filled with mineral wool bats (r10 assuming a 15% framing fraction) and no attempt at air sealing the interior will I have condensation in the mineral wool?  Will it mater?  I tried to answer my own question and must make some assumptions first basement temperature I think it's not the issue it's living space temperature and humidity that'll mater as the basement may be cooler but it'll have the same dew point as the upstairs.  So I choose 70 f and 40 % RH for a dew point of 44 f   conveniently, In the cavities of the studs is r15 & the foam is r15.  So if my concrete is below 18 deg f Ill have condensation.  If I drop the indoor RH to 30% the condensation  temp is dropped to 4 deg f.  OTH if the RH is 50% @70f the condensation onset is 30 f concrete temperature.   What is typical RH in wintertime? Which will reduce indoor humidity (in heating season) more, an HRV or an ERV?  I see this condensation as somewhat self limiting as it'll have the effect of lowering humidity and the insulation's effectiveness.  If there is enough I'll see wetness. but really how big a problem is it?

You want to keep moisture out of the basement. In addition to an exterior coating, use a capillary break on the footing before the walls are poured, then use a closed cell foam on the interior, not an open cell then your 2x4 wall. Plus 4" XPS under slab and a good underslab vapor barrier above the XPS. I use 2" of closed cell spray foam and R-20 fill in the 2x4 wall. this was the assembly I used on the house I just posted - $230 to heat mid Dec - mid June 2012.

There's no EPS that would be R15 @ 3". With Type-I insulation it would take 4" to hit R15. With Type-II EPS you can still figure on hitting R13+ @ 3" at the cold temperature extremes, R12 during more temperate weather (25F+).

The only way you can get condensation (frost, really) on the interior face of the concrete from interior moisture drives is if the insulation is air permeable, but the EPS is not. The concrete is HIGHLY permeable and passes water vapor relatively quickly from one side to the other. The condensing surface of concern is the interior surface of the foam, since when it drops below the dew point of the interior air it condenses, bringing the dew point of the air in the fiber (even the warm side of the fiber) down to the temperature at the foam/fiber interface. With any big air leaks that can become visible quantities of liquid, but if the cavities are somewhat air tight, the rate at which moisture is drawn through the paint from the vapor pressure difference is still pretty small.

A dew point of 44F is unrealisitically high, unless you're intentionally humidifying to 40% RH in winter (not a good idea, most of the time). A dew point of 40F is more reasonable and realistic, and even that won't be likely during cold snaps in zone 6 unless you back off on ventilation rates.

With roughly 50% of the R value outside the fiber layer in Zone 6 the fiber would stay above the dew point even at the outside design temp on the warm edge of the climate zone, and wouldn't dwell below the dew point for long ever. Both EPS and mineral wool are quite tolerant of moisture, so there's no risk from interior moisture drives. The condensing surface will be primarily the EPS, since that will be colder than than the bulk of the fiber, and it would take quite a LOT of condensation to appreciably affect the R-value of that outer 1/4" of fiber it would take to reduce the R-value of the rock wool by even R1.

With 3" of Type-II EPS it's a pretty good vapor retarder against ground moisture, but not a vapor barrier, and the cooler temp of the ground vs. the conditioned space also slows moisture migration. It'd not worth spending much money to lower the vapor permeance even further to quell an already quite small latent load, IMHO. EPS is closed cell- it won't take on water except in the interstitial areas between beads, which lowers it's R value less than 10% even when maxed out. The interstitial area gives it a higher permeance than XPS or spray polyurethane, but even the most vapor-open Type-II EPS has a vapor permeance between 1-1.5 perms @ 3", which is about the same as 1" of closed cell polyurethane or XPS. At 4" it would have roughly the vapor-permeance of 2" of 2lb polyurethane.

Capillary break between foundation & sill yes, closed cell foam only if you can get it for really cheap. Closed cell foam in studwall cavites is a waste of money due to the performance robbing framing (at any framing fraction). Go rock wool as long as you keep the foam-R at the above grade section roughly the same (or higher) as the center-cavity R of the rock wool.

EPS also works just fine under slabs (it's pretty standard in PassiveHouse construction), and is usually cheaper than XPS. An since it's blown with pentane instead of HFC245fa it has more than an order of magnitude lower greenhouse and other environmental footprint. Under slabs XPS has a net lifecycle greenhouse gas greater than the source fuel carbon it would offset using fossil fired heating, even at fairly low R values. (In above-grade walls in zone-6 that it's true until it's R12-R15 or slightly higher.) XPS & closed cell polyurethane both use HFC245fa as the blowing agent. EPS and polyiso are both blown with pentane, and are far lower footprint alternatives to spray polyurethane foam and XPS where ever they can be used instead. Spray polyurethane is a superior air-sealer, but when used as a high-R insulator it's un-green in the extreme.

Bob- I don't quite understand the insistence of placing the poly vapor barrier above the foam, between the foam & concrete, unless you have exceptionally low subsoil temps. In a southern New England climate with ~50F subsoil and average summertime interior dew points in a conditioned basement of ~60F, even if the vapor barrier was below the foam the low air & vapor permeability keeps the bottom of the foam dry enough (and the foam itself is tolerant of liquid moisture). In winter an interior air dew point of 40F would tend to remove the miniscule amounts of water that would have found it's way into the foam.

Dana1,
Please check this out. http://building.dow.com/na/en/products/insulation/highload100.htm  yes it is XPS, my error.

Dana1: that is a direct quote from Joe L at a recent seminar. The idea is that IF water gets under the slab, (say a high water table or flood) you do not want it trapped in the foam layer but you want the foam layer to be able to freely drain. Makes sense to me.

That seems of little consequence, given that the foam itself isn't affected by water, and post-flood the drying rate of the concrete will be pretty much the same (and toward the conditioned space no matter what side of the foam the poly lives. If you use EPS rather than XPS the drying rate of the foam to the interior would be quite reasonable too.

Liebler: Note that at the cold temperature extremes there's almost no difference in the R value of EPS and XPS at any given thickness. EPS is usually cheaper. If you use a good capillary break at the foundation sill you can use foil-faced iso, which would give you R18 nominal (R17 at the cold temperature extremes) @ 3", and it's dead-easy to air-seal with FSK tape and 1-part foam. Just be sure to put a capillary break (say 1-2" of EPS or XPS) between the bottom edge and slab to keep the iso from wicking up ground moisture (or flood moisture) on the exposed edges. Since the studwall isn't structural you can single-plate the bottom, and put an inch of XPS or EPS under both the bottom plate & wall foam that extends to the wall as break for both the wood and the iso edge.

Dana1,
From what I've found on the www the variation of r value with temperature is quite similar for EPS & XPS
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_07e6/0901b803807e65f6.pdf?filepath=styrofoam/pdfs/noreg/179-04587.pdf&fromPage=GetDoc
But degradation due to water absorption is much worse with EPS
http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_088e/0901b8038088ee26.pdf?filepath=styrofoam/pdfs/noreg/179-04637.pdf&fromPage=GetDoc
Since my bit of shopping found that 3" XPS costs essentially the same as 4" EPS. I'm leaning toward the XPS despite being less green because of less long term degradation due to water absorbtion.

I think Dana1 is Joe L, or maybe a relative.

Liebler, since you're so nuts and bolts with your project, a typical basement will run a much lower framing factor, single plates, 24" o.c., maybe 10% not 15%.

You have to be a bit careful when reading any manufacturers' comparative test- there are many ways to have a thumb on the scale that aren't always obvious. The fact that they don't state the density or blowing of the EPS in the Dow document leads me to believe it's low density Type-I which does have some issues. The short term ASTM C272 and ASTM D2842 submersion tests would exaggerate the apparent degradation rate since the interstitial spaces saturate quite quickly whereas the foam cells themselves don't. The 1983 in-situ test is completely irrelevant since the foam in question was blown with (currently banned due to atmospheric ozone layer degradation issues) clorofluorocarbons, and had higher fraction of open cells compared to mid-density goods blown with pentane. (A lot of surfboards made with low density EPS of the '70s and '80s would get waterlogged for this very reason.)

There are competing comparisons out there from EPS manufacturers that tell a different story, eg:

http://www.polyfoaminc.com/downloads/brochure/Foam-Control-EPS-Water-Absorption-Facts.pdf

Mind you the blowing agent for the XPS in the 15 year test that began in 1993 may not be completely current either, but the EPS was almost certainly blown with pentane.


greentree: I'm definitely not Joe L (I purged the L's from my name ages ago, kept the S. :-) ) But I respect his work and his opinion, and the fact that he's been willing to change his mind as evidence indicates- he actually measures stuff.

Regarding the placement of poly vapor barriers under slabs, it occurs to me that if placed under the foam and above the (typically) screenings there would be a lot more trapped space for water to accumulate between the poly & foam since the poly won't be perfectly flat, and the foam is rigid, and won't push out the air between the poly & foam when the concrete is poured. If placed between the foam and the slab it has more flat & rigid surface below it, and the the concrete would fill in any imperfections leaving no voids. Water trapped in pockets/puddles below the foam but above a bottom side vapor barrier would take quite while to dry through the foam, so placing the barrier above the foam would result in quicker drying after a flood. It may be a difference without any practical distinction, or maybe there are some real-world examples where it's meaningful, but I've neither seen nor read of a case where it mattered.

I read through this topic quickly, hence, may have missed something. I did not see any comment regarding mold and bacteria. Cement, being typically porous, will wick moisture from the soil into the concrete. That will create an ideal habitat for molds and bacteria if the inside is covered with insulation. The exterior surface of the concrete is the area requiring either a moisture proof barrier, insulation or both. Otherwise, insulation on the interior exacerbates the problem with molds and bacterial growth.

A capillary break at the footing blocks with wicking of ground moisture from the footing into the foundation wall, as does exterior waterproofing. As long as the wicking rate isn't quicker than it can dry toward the interior through somewhat vapor-permeable foam like XPS or EPS there is very little potential for mold growth in the concrete.

Mold favors organic materials like wood, and putting foam between the studwall & concrete protects the wood from both a wintertime moisture issue and ground-moisture wicking into the concrete, as long as the permeance of the fiber insulation & interior finish is high enough (2 perms or more.)