Another basement insulation thread.....

[Carl Weng]

I know I know another basement insulation thread...

I have searched and could not find anything with the same situation, so please bear with me.

So we live in Bergen County NJ- right on the border of Zone 4 and 5 and I plan on finishing a roughly 1000 sqft basement.

1950s Ranch with cinderblock foundation walls. About 5’ is below grade. Foundation and exterior walls have no insulation, other than batts in the bays.

I was planning on putting up 2” XPS on the foundation walls, seal them up nicely and build the stud wall up against the insulation.

My first issue is that the foundation wall “steps” in about halfway up the wall.( see attached picture). The “step” in is roughly 2.5”.

So my issue is that if I were to build the stud wall flush against the bottom portion of the wall, this would leave a 2.5” gap between the back of the stud wall and the XPS insulation on the upper portion of the wall.

I understand from previous posts here that 2” is probably the max I want to go with XPS, so filling the 2.5” void with XPS seems to be out of the question. One other thought I had was to use spray foam instead of the XPS, but I think it gets expensive to spray 5 inches of foam on the upper wall.

Should I fill the space with unfaced fiberglass instead?

My second issue is that the waste pipe runs along half of the basement walls( see attached picture). The ~7” diameter pipe is about 2 inches off of the foundation block wall. My initial thought was to put up the XPS on the foundation wall between the pipe and block wall and then frame the stud wall on the other side of the waste pipe. This however, would leave about a 7” gap between the back of the stud wall and the XPS. I guess it’s the same question as above, how do I fill the gap between the XPS and the stud wall? Does spray foam solve my issue?


Appreciate any feedback you all have!

 

[Dana]

You really want to go with ZERO inches of XPS, since XPS has the heaviest environmental impact (by far) of common rigid foam products. It's blown with an HFC soup, the largest component of which is HFC134a (automotive AC refrigerant), which has a global warming potential about 1400x CO2. EPS and polyisocyanurate are blown with hydrocarbons (usually a pentane variant, at about 7x CO2), most of which leaves the foam at the factory where it is recaptured to meet local air pollution emissions standards, often burned for process heat. Over a few decades as the HFCs diffuse out of the XPS it's performance drops, eventually to ~R4.2/inch (instead of the labeled R5/inch) at full depletion, the same as EPS of similar density. EPS doesn't have that lifecycle performance drop, and it's labeled R remains steady for 50+ years.

Closed cell spray polyurethane has similar global warming potential issues if blown with the industry standard HFC245fa (about 1000x CO2), but there are now HFO1234ze blown products out there (< 3x CO2), if you seek them out.

There is no limit on thickness of the foam. (What's with the 2"?)

Fiberglass wicks moisture and becomes far less effective when damp. It should never be used in contact with a concrete or CMU foundation.

Harrington Park NJ is in climate zone 5A. Under IRC 2015 the code minimum R-value for basement walls is a continuous R15, or R19 if thermally bridged by framing. That would be 4" of Type-I (1lb per cubic foot density) EPS, or 3.75" of Type-II (1.5lbs nominal) EPS, or 2.5" of low density foil faced polyiso, or 3" of higher density fiber faced roofing polyiso.

It's also possible to get there with a hybrid, using R13-R15 batts in your 2x4 wall, but in zone 5A there needs to be at least R5 of foam between the foundation and fiberglass (and studs) for wintertime dew point control without interior vapor retarders, but more foam-R is better. It's important that any ground moisture getting into the cavity be allowed to dry toward the interior, and it's important that the average winter temperature at the foam/fiber interface stay above the average indoor dew point temperature. R5 is spelled out in the code as the minimum for above grade walls, and it needs to at least that for the foundation walls as well (particularly the above-grade portion.) Derating XPS to it's fully depleted level, that means it would take at least 1.25" of XPS (or EPS), or 1" of polyiso to hit that minimum.

The cheapest and greenest way to go here is to use reclaimed roofing foam (either polyiso or EPS), which can often be had for less than 1/3 the price of virgin stock rigid foam. There are several vendors trading in used and factory-seconds foam in your area, some of whom advertise here.

Measure the step back depth in the foundation, and fill it with foam board that comes out just proud of the lower level, seams taped, bottom & top edges can-foam sealed. Then install a layer of at least R5 the full height, using spacers if necessary to keep the foam from bowing in toward the foundation, leaving a tiny gap. If polyiso, the cut bottom edge has to be kept off the slab to avoid wicking up moisture from any condensation, ground water, or bulk-water/leak /flood wetting. An inch of EPS under the bottom edge with a 1/4" gap between the CMU, that extends under the bottom plate of the studwall (as a capillary & thermal break from the slab), works.

If insulating the studwall, unfaced or kraft faced batts works.

For the drain pipe section slip 2" foam behind the pipe full height, which would be sufficient dew point control for up to 5.5" of fiber. A notched 2x6 stud studwall tight to 2" foamwith cheap R19s should be good enough. At 2" any type of rigid foam has sufficient dew point control for R19 in your climate. At the coupling joints in the pipe sculpt the rigid foam coarsely then fill in behind with can-foam or FrothPak to insulate and seal.

If the pipe outer diameter is more than than 5.5", use another layer of foam above & below the pipe to get it to where notched studs can still be tight to the foam. An empty vertical cavity needs fireblocking at least for the full depth at the top, and in some jurisdictions at the mid-level.

 

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[Carl Weng]

Dana- Thank you very much for your response and your willingness to share information. I will certainly take a look at the recycled EPS outlets local to me.

Some followups-

I mentioned 2" limit of XPS as I thought that I should always strive to have some vapor permeability of greater than .5? I thought 2" of XPS was perm of .6 which means 2" was roughly the max I would want on the foundation wall?

So what I am reading is if the step back is 2.5 inches and I wanted at least 1.5" on the bottom half of the wall, then at the end of the day I will have a foam wall consisting of 4" on top( 2.5" + 1.5") and 1.5" on the bottom. Are there any issues with gluing foam to foam? Since the top portion will be 4 inches- do you know of special tap con fasteners that would work at this depth? Can I run the EPS to the floor and tape and seal against the slab floor?

I will then plan to stud the walls right up against the sealed foam wall and add further un-faced insulation and top off with mold resistant drywall( maybe cement board the bottom half).

I am not sure I follow with the pipe- The Pipe is about 7 inches in diameter. Are you saying to create a wall with a 7 inch cavity and fill with fiberglass?

Thanks for the help

 

[Dana]

The vapor permeance is all but irrelevant.

If there's a foot or more of above-grade exterior exposure on the foundation, the foundation dries to the exterior, rendering it irrelevant.

If there is a capillary break (such as an EPDM sill gasket) between the foundation sill and the foundation, it's completely irrelevant- CMU and concrete performance is not affected by being saturated-wet.

If the there is no capillary break between foundation & sill, and only very minimum above grade exposure, stop the foam board a foot from the slab to let the foundation dry toward the interior.

If you really care about letting the wall dry though the foam, use unfaced EPS. Type-II (1.5lb per cubic foot density) EPS has a vapor permeance of ~0.7 perms @ 4". Type VIII (1.25lb density- commonly use for roofing foam) runs 0.8-0.9 perms @ 4". Unfaced Type-I foam runs about 1.2 perms @ 4". (Most reclaimed roofing EPS is Type VIII.)

Gluing foam to foam is just fine if using dabs of foam board construction adhesive. Other adhesives often have solvents that will attack polystyrene and polyisocyanurate.

It's fine to have EPS in contact with the slab. While it can take on moisture in the interstitial spaces between the macroscopic beads if fully submerged, it doesn't wick moisture, and it will dry when the tide goes out. EPS is commonly use for dock floats, lobster pot buoys, etc. it's very water tolerant.

If you want the wallboard to fully cover the drain pipe, the cavity is what it is. An R25 fiberglass batt has a manufactured loft of ~8" will completely fills a 7.25" 2x8 cavity, and will perform at R24 at the compressed thickness. In zone 5 you'd need at least R2-R10 (2.25-2.5" of EPS), at least 28-30% of the total R for dew point control on R24 fiber at the above-grade section. But if you're already filling the step-back in the foundation in the above-grade section you'll have enough.

Kraft facers are fine, as long as they are above the high-tide mark of any potential flooding. When there's enough moisture in the cavity to support mold the facers will be 5 perms or more, but when bone dry they run about 0.5 perms.

 

[Carl Weng]

Is there anything special I can wrap the cast iron drain pip with? or is it ok to have it directly contact the fiberglass?

 

[Dana]

It's fine to have the pipe in contact with fiberglass insulation. You may have to do a bit of scuplting of the fiberglass with a batt knife (or an 8" bread knife) to get it to fit reasonably around the curvature of the pipe without large voids.

If you built the framed wall with out the insulation it would need a sealed top plate as a firestop, and (depending on local code) another fire-stop in each stud bay somewhere at mid-level. As long as the EPS was at least 4" thick everywhere except for the 2" behind the drain pipe it would still meet code for R-value even without the cavity fill, since you'd be over R15 everywhere except that stripe.

Whether you fill with fiberglass or not, caulk the framing to the foam with foam board construction adhesive, caulk/air-seal the wallboard to the framing, and be sure to seal all electrical penetrations of the framing (including every stud on lateral runs) with fire-stop can-foam (3M Fire Block, Great Stuff Fire Block, etc) .

 

[Carl Weng]

Spoke to a contractor that wants to use faced fiberglass because the paper has tabs that can be used to fasten the insulation to the wood. From reading these forums, it sounds like it is preferred not to have any faced fiberglass insulation below grade. How does the non-faced insulation stay in place?

 

[Dana]

Unfaced batts are designed for a friction fit. The batt is manufactured with a 15.0" width, 1/2" wider than the nominal 14.5" between 16" o.c. studs. Friction fit works just fine, and will stay in place- even crummy low-density R11s and R19s. Unfaced batts are more easily inspected for errors than kraft faced too. High density batts such as R15s or R21s (for 2x6 framing) are usually ONLY unfaced, and work quite well, as do rock wool batts, which almost never come with facers.

Kraft facers are fine if there is no history of flooding, but will wick moisture halfway up the wall and grow mold if there's even a 6" flood.

 

[Carl Weng]

Thanks Dana!

 

[Dana]

I have to wonder a bit about a "pro" who insists on using kraft faced goods. In my area most insulation contractors prefer to use unfaced batts and a full height 4-mil polyethylene vapor barrier for above grade walls, even when the vapor barrier isn't necessary due to the wall's stack-up. Stapling facers one bay at a time just adds time to the process.

Below grade you can't use an interior side vapor barrier without increasing the mold risk, and you DO still have to put some rigid foam between the foundation and fiber insulation & studs, as previously noted, both for dew point control and as a vapor retarder to slow the movement of ground moisture into the fiber insulation & conditioned space.

 

[Carl Weng]

One more question- I know that some folks suggest filling the rim joist with foam board and using spray foam to seal it up. This makes sense on the perpendicular rim joists- What about the rim joist that runs parallel to the floor joists- Should I cut very long piece and cobble them together?

Thanks!

 

[Dana]

Sure- the long run cut'n'cobble is the EASY part since it's fewer pieces to cut and fit. Leave a 1/4" gap between the sections of foam, and fill it with can-foam for a more perfect seal than just butting them together and taping the joint with housewrap tape or foil tape (though the latter can still work pretty well with foil-faced foam board.)

 

[Carl Weng]

So the contractor was not able or willing to fram around the drain pipe and instead will frame up the wall about 5-6 inches from the foam foundation wall. Obviously not ideal, but is there anything I can do to mitigate moisture and mold risk? Can I use extra thick fiberglass to fill the 1) sutd cavity, 2) cavity between foam and stud wall?

Thanks again

 

[Dana]

R8 foam is sufficient for dew point control on up to 5.5-6" of fiber insulation, so fill in the 5-6" of space between the wall foam and studwall with stacked batts, tightly fitted, but leave the stud bays themselves empty in that section.

The batts will need a close-fitting air barrier tight to the batt on the side facing the empty stud bay. Using vapor permeable sheet material (such as housewrap) side-stapled to the studs should work.

 

[Carl Weng]

So if I have R8 of foam on the foundation, it will only support dew point of 5-6 inch of fiber? Would you suggest I dont go thicker than that?


Would instead of the vapor permeable sheet material- can I use Kraft faced fiber facing the empty bay?

What is the thought behind having this vapor permeable sheet material ?

Thanks

 

[Dana]

Yes, with only R8 foam, limit the fiber insulation thickness to not more than 6".

No, you can't use kraft facers an air barrier. Any flexible sheet goods used needs to be caulked (or taped with housewrap tape) & stapled to the studs for air tightness. Kraft facers tear easily, and are impossible to make truly air tight at the edges. It's fine to use kraft faced batts with facer on the warm-in winter side of the the stackup. Even though it's not an air barrier, it is an effective "smart" vapor retarder, impeding the rate water vapor diffusion into the cavity when the surface of the foam is below the room air's dew point, and becomes more vapor open if the humidity levels in the fiber insulation become too high, allowing the insulation to dry toward the interior.

The air barrier needs to be vapor permeable in order to prevent moisture build up in the fiber insulation. At 6 mils polyethylene sheeting is about 0.05 perms, a true vapor barrier. When dry a kraft facer is about 0.5 perms, 10x more vapor open than 6 mil polyethylene. When the RH of the entrained air in the fiber insulation is 75% the kraft facer's permeance rise to about 5 perms, which is comparable to that of interior latex paint on wallboard. The amount of drying through 5 perm paint or kraft facer in one day would take 100 days through sheet polyethylene. Housewrap vapor permeance runs quite a bit higher, from about 10 perms (Typar), to 25 perms or higher (Tyvek), but all can be detailed as air barriers. The higher permeance allows more water vapor in, but it also drys very quickly.

As long as you have at least R8 of foam for every R21 or so of fiber the fiber will stay dry in your climate zone. The IRC prescriptive minimums for climate zone 5 work out to R7.5 for every R20, or ~27% of the total as foam on the exterior.

 

[Carl Weng]

Dana- Thank you for taking the time out to explain- I do hope this helps others that are in a similar situation.

But I am a bit confused as from reading some of the other threads, I came to the conclusion that it was not a good idea to add another vapor barrier (polyethylene sheeting) as that would truly trap the moisture between the foam and the polyethylene sheeting and it would not have a way out.


If you remember the top part of my wall structure is R 17.5 while the below grade section is about R7.5- I want to try and fill the entire 9.5 inch Gap with a R30 fiber so that there are no air cavities between the drywall and the foam board. Is this a bad idea?

Thanks again...

 

[Dana]

How far below grade (not from the top of the foundation) does the foam drop to R7.5?

 

[Carl Weng]

About 12-14 inches. ( but the concrete in that lower section is also 2 inches thicker)

 

[Dana]

The extra 2" of concrete doesn't add much in R-value- less than R0.2.

If it's only a foot or so below grade it really needs to still retain the 27% minimum foam-R fraction.