Just had a home built and I need to finish the basement. Below Grade Foundation walls have 2" XPS on the exterior. Below Grade walls have 2X4 framing directly attached to interior walls. Can I add unfaced fiberglass in between those stud cavities? Reading on Building Science website was a little unclear to me. Or should I maybe cut some XPS to fit in between the stud cavities and then fiberglass over the XPS? The rim joists were sprayed with cc foam. Above Grade walls have blow in dense pack Fiberglass insulation.

What climate are you in? Do you have sub-slab insulation?

There's moderate risk that the fiberglass will keep the walls cool and if the basement is humid, you could mold and rotting 2x4s. So foam is safer. EPS is cheaper than XPS and has vastly lower environmental impact. The blowing agent used to make XPS is an extremely potent greenhouse gas.

Home depot sells EPS pre-cut to fit between 16" OC studs.

Located in North Dakota. Very cold winters here.

Thanks for the reply.

The exterior XPS acts as you vapor barrier. Go ahead and use the un-fiberglass.

Maybe lets think about this. Assuming when OP says "below grade"' there is also an above grade portion of the basement wall. That would mean that the upper about two feet of concrete would be minus 25 or worse for part of the winter. Assuming no vapour barrier on the inside, that means any moisture getting into the wall would freeze (or at least condense) against the "above ground" portion of the basement wall. It would then pool at the bottom of the wall, keeping the whole assemble in a high moisture environment.
As the concrete is highly thermal conductive, even a large portion of the "below grade" concrete wall would be below freezing, assuming that these are concrete walls. You can't switch sides with the insulation without a huge thermal bypass.
To make use of the outside XPS, it should be extended to the trim joist and covered with stucco and capped with flashing. Then the whole wall will be protected from the outside, keeping the concrete close to room temperature. If no vapour retarder is used on the inside, any damp rising could dry to the inside (and a little to the outside) without compromising the sill plate and joist work.

The Canadian Code would require a vapour barrier under the sill plate of the interior wall and up the concrete wall to the level of the outside finished grade. R20 insulation would be required in the wall and a 6 mil vapour barrier from underside of the floor sheathing to the concrete floor, place under the drywall. There is enough variables here for a good debate over the effectiveness of this wall stack but the main thought is this. You place the first vapour barrier to prevent the wall from wicking moisture through the concrete and into the internal wall assembly but leave the top of the wall unprotected to allow the wall assembly to dry to the outside through the above ground portion of the concrete wall. you then place the 6 mil v.b. on the warm side of the wall and seal it to the top and bottom to prevent moist air from entering the wall assembly.

All walls need to consider interior to interior convective moisture transport. Ie, warm air from the interior flows towards the exterior, gets cooled and condenses and then flows back to the interior. So if you use any unfaced fiberglass, make sure there is a fully sealed interior side AIR barrier. With only rigid foam (the safer option), tape the gaps. And plan on neither of these working perfectly, so the wall must be able to dry in at least one direction, preferably two. The Canadian code seems to disagree with this last point.

Keeping interior humidity low will help with all moisture issues.

"The exterior XPS acts as you vapor barrier". EXCEPT for the moisture that rises from the ground beneath the footings, up the concrete wall via capillary action, and from there into the basement (since the outside is foamed). So the wall might in fact be damp and the fiberglass would get mouldy.

Do NOT use fiberglass - or wood - next to concrete!!!! Install 2" of foam on the interior, then your studs and fiberglass.

We recommend, and I personally spray foam the rim joist of every home we work on, new or old.

As my esteemed colleagues suggest, you have to keep the dew point at bay.

Regarding the Canadian code and the use of polyE sheet to separate the concrete from the interior portions of the wall:

While I'm sure this helps protect studs/fiberglass/drywall, I believe that this option greatly increases the chances of finding water pooled on the floor. Ie, with most interior wall materials, a certain amount of moisture in the concrete can evaporate and diffuse through the wall. But use poly (or foil faced polyiso) and that same moisture is trapped - so at some point it may be enough to flow downward and leak out on the floor.

This isn't theoretical - I have a section of basement wall with some occasional water coming through. Evaporation seems to be enough to deal with it, but my conclusion is that I can't cover it up (reducing evaporation). Doing so would mean water on the floor. Of course I could drain away the water with external or internal drains - but so far "do nothing" is winning :-).

Perhaps. But my polyiso does not pool. Perhaps there is air movement common to both sides?

Every technique is only as good as it's application.

I'd say that if the wall *always* stays dry (ie, no excessive external water flow like I have), then polyiso or polyE sheet against the concrete are fine. While it might be quite humid between them, there won't be liquid water.

I have found most liquid water originates at the roof line and ends up in the basement. Naturally, there are odd exceptions.

Lucky you :-).

Posted By jonr on 17 Nov 2015 01:36 PM
And plan on neither of these working perfectly, so the wall must be able to dry in at least one direction, preferably two. The Canadian code seems to disagree with this last point.

Keeping interior humidity low will help with all moisture issues.

"but leave the top of the wall unprotected to allow the wall assembly to dry to the outside through the above ground portion of the concrete wall. "

Posted By jonr on 17 Nov 2015 02:53 PM
Regarding the Canadian code and the use of polyE sheet to separate the concrete from the interior portions of the wall:

While I'm sure this helps protect studs/fiberglass/drywall, I believe that this option greatly increases the chances of finding water pooled on the floor. Ie, with most interior wall materials, a certain amount of moisture in the concrete can evaporate and diffuse through the wall. But use poly (or foil faced polyiso) and that same moisture is trapped - so at some point it may flow downward and leak out on the floor.

This isn't theoretical - I have a section of basement wall with some occasional water coming through. Evaporation seems to be enough to deal with it, but my conclusion is that I can't cover it up (reducing evaporation). Doing so would mean water on the floor. Of course I could drain away the water with external or internal drains - but so far "do nothing" is winning :-).

We are talking about water vapours, not bulk water entering through poorly constructed walls/drainage. In Canada apart from the above, you must provide footing drainage. You must also seal the exterior of the concrete below ground. All snap ties are dabbed with tar and a moisture barrier is sprayed onto the concrete exterior below finished grade. If that has been done correctly, you don't have to deal with the results of hydrostatic water pressure. So in this case we are concerned with damp rising and with water vapours condensing on cold or frozen interior surface of the concrete. N.D. temps are similiar to Alberta's.
Of course, if you use ICF's you eliminate most of these problems.

If you have bulk water coming through concrete either as capillary action or due to hydrostatic pressure, you need to consider sulphate attack of the concrete if your soils contain them.

but leave the top of the wall unprotected to allow the wall assembly to dry..

Six feet of concrete (perhaps .05 perms) is not a drying path.

We are talking about water vapours, not bulk water entering through poorly constructed walls/drainage...If that has been done correctly...

Some of us are talking about bulk water. Which despite a similar design (footing drains, sealant), IS occurring in my case (and many others). Maybe an implementation issue or settling cracks or drain clogging - such things sometimes happen, often not showing up until years later. In my case, permeability/evaporation to the interior HAS prevented water on the floor and mold.

Even without bulk water - a google search will provide many examples of double PE basement walls creating mold problems. And recommendations that even a single layer of PE against the concrete be detailed to not dump water on the floor. I couldn't find anyone who thought that the Canadian Code design described above was a good idea. Or that EPS then studs (no PE) was a bad idea.

Two feet of exposed concrete WILL provide a drying path for DAMP RISING. That is the only moisture we are concerned about in OP's case. We only want to make sure that the moisture rising up the wall from the footings does not impact the rim joists/sill plate. All vapour moisture is prevented from entering the wall assemble by the 6 mil poly under the drywall.
Even putting foam between the studs will not completely solve the problem as the wood studs move and let warm moist air contact the cold concrete. If you are going to use foam inside, do it Bob I's way.

There is nothing that will PREVENT mould if you have bulk water ingress. Just fix the problem. Yes, it does happen over time. The answer is still the same. Dig it up. Fix the problem.

There may be problems with double poly but not if implemented the way I described and in a cold climate. The first moisture barrier only comes up to the outside finished grade, not to the top of the wall. It is sealed to the concrete with ACOUSTIC caulk. This has been documented for over thirty years as effective in Canadian climates but it has to be done right.

And no, I'm not saying this is the only way to do a basement. I am saying that in OP's case, doing fibreglass against the cold concrete without the moisture and vapour barrier will result in mould. His best solution by far is to continue the exterior insulation up to the trim joist so that all his concrete is warm and then doing an air open finish inside. Always stop the cold on the outside. If he does not continue the foam up, from a thermal perspective, the foam already in place is wasted.

Perhaps you can link some of the problem cases for us for a better understanding. I find lots of self made experts saying double is bad but not the examples. Double is bad in many, many situations but not in all situations.

In my case there is about 6 inches of concrete exposed to direct air outside. On the inside they put a layer of spray foam on top of the foundation walls. That foam is where the 2x6 wall up top meets the concrete wall. Hope that makes sense. Essentially some walls are 2/3 wood and 1/3 concrete. Other walls are 100% concrete and most walls are 100% wood. It just depends on which side of the house you are on because of the grade and walkout basement.

froboy - yes I understand. Wood walls are cheaper then concrete.
Your best scenario is still to extend the outside foam up to over the top of the bottom 2x6 wall plate (where the spray foam is.)
The next best would be to remove the interior stud framing and install 2" of eps against the concrete and up to the spray foam. Seal it to the floor and to the spray foam and seal the joints. Then put back the 2x framing and add R12 rock wool. This will give you a ledge all around your basement where it switches from concrete to wood.
Alternately, you can frame the 2x wall from floor to floor to give yourself a straight wall without the ledge. Some times the ledge works out nice, other times not so much.
Hopefully they put a sill gasket of some sort between the concrete and the wood.