Walkout basement finishing - insulating walls above and below grade

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J Blow

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Hi, All. I've picked up a lot here and realized that knowing nothing is a lot more dangerous than I thought - especially when it comes to moisture creating molds, etc. I'm still a little confused about a few things I am hoping you can clear up.

My basement is a walkout with one wall completely below grade, one completely above the other 2 half and half, of course. I've decided to XPS the concrete, then frame the walls with batt insulation between them. No problem on the all concrete wall....but, when the concrete is halfway up the wall, I'm a little stumped on what would be a problem. Currently, above the concrete is framed and insulated with unfaced batts with clear plastic covering it. There is also a thick black plastic over that as well but I'm thinking the previous owners did that for cosmetic purposes? Anyway, the concrete on the bottom half is bare - that wall sticks out a couple inches more than the wood framed/insulated wall above it. So, can I XPS the concrete and insulated wood framed wall over the plastic, insulation, and framing? Then I would frame a wall in front of the XPS and insulate with batts. What I'm unsure about is using XPS over the batt insulation and plastic and then the small gap between the XPS and the framed wall I will build. Maybe the XPS over the existing wood framed exterior wall insulation is overkill and just a framed wall in front of the current wall with more insulation is enough? I'm just real leery of locking in moisture and causing issues.

Next, I'm wondering about the rim joists above the concrete wall. Currently that also has loose unfaced batts. Can I leave those and frame in front of them or should I have some sort of a barrier in there? There will be a gap between the rim joists filled with insulation and the newly framed wall. Is that an issue? I thought it might not be as long as all the concrete is covered with XPS and then any gaps to concrete spray foamed.

Thanks in advance for any thoughts.
 

Dana

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First, use unfaced EPS, not XPS, and run it all the way up to the top of your studwall. You can leave the pre-existing vapor barriers in place (they may or may not be needed, but don't steal defeat from the jaws of victory, eh?), and fill the space between the vapor barrier and your new studwall with EPS the same thickness as the depth of concrete ledge.

Do not add a vapor barrier between the concrete and rigid foam- it's not necessary, and might even increase the moisture level of the foundation sill to damaging levels.


bscinfo_511_figure_03.jpg

EPS has a lower R/inch than XPS (about R4-R4.5/inch depending on density), but it is more vapor permeable and the R-value is stable over time. XPS is blown with HFC134a, which bleeds out over a handful of decades, eventually ending up at the same performance as EPS of the same density. But the HFC134a has about 1400x CO2 global warming potential, and it's an unnecessary environmental hit to be taking in this assembly. The vapor permeance of EPS is low enough, and not too low, it's just right. EPS is blown with pentane (about 7x CO2 GWP), and is far more benign.

Pull the batts from the rim joist and use can-foam to air-seal all of the seams where joists meet subfloor, band-joist, band joist meets sill, sill meets concrete. If you are in US climate zone 5 or colder it's worth putting an inch of EPS against the band joist and on top of the sill plate before filling up the remainder of the space with batts.


bscinfo_511_figure_02.jpg


The foam/fiber ratio on the rim joist and any above-grade concrete needs to be high enough to limit the condensation at the foam/fiber interface, which is climate dependent. Where are you?
 

J Blow

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OK - guess I wasn't too far off track but just to be sure....after I EPS the rim joists and replace the fiberglass, there will still be an air pocket between the stud wall (well, the fiberglass in between the studs) in those few inches above the concrete. The EPS of course only goes to the top of the concrete and I will cover the top of the concrete with EPS but that will still leave a pretty good sized air pocket above the concrete next to the stud wall and back to the rim joist. Is that ok or do I need to fill it in completely with insulation?

I'm zone 5 - Western South Dakota. Since the half concrete wall below grade sticks out a little more than the framed wood above grade, will I need to put in more foam above the ground so the stud wall is flat to the interior....or is an air gap there ok, too? That gap would be after fiberglass, vapor barrier, foam, and before the fiberglass in the stud wall.

Thanks for the detailed response!
 

Dana

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If you install a thicker EPS panel over the framed wall such that it is co-planar with the face of the concrete you can then go all the way up with the EPS you are putting over the concrete. Type-II EPS (1.5lbs nominal density) is still over 1 perm @ 2.5", which is plenty of drying capacity for the studwall you're foaming over.

Most of western SD is in US climate zone 6, not zone 5:

climate_zones.jpg


Using IRC 2012 chapter 7 as a very rough guide, climate zone 5 you'd want to have 25% of the total center-cavity R-value at the above grad concrete as foam. So, if you're installing a 2x4 studwall with R13 batts, that means you would want it to be ~R4.3 or so. (4.3 is ~25% of R13 + R4.3). Since 1" (R4.2 @ 75F)) Type-II EPS performs about R4.5 when the average temp through the foam is 40F, and R4.7 when the temp through the foam is 25F, an inch would be enough. But 1.5" would give you tons of margin.

In zone 6 you'd want it to be about 35% of the total R, so for R13 batts you'd need 1.5" (R6.3 @ 75F) of Type-II EPS, which would deliver R6.8 @ 40F mid-foam depth and R7.1 @ 25F mid-f0am depth.

Since there is no moisture-susceptible wood sheathing between the fiber and foam it isn't as stringent as IRC chapter 7- neither the foam nor the fiber is damaged by occasional condensation events, whereas wood sheathing would soak up moisture, and even during sustained condensing temperatures you wouldn't get sufficient moisture build up to have liquid running down the foam and puddling at the bottom (unless you have no interior side wallboard, or truly huge air leakage at the wallboard layer.

The 1" (zone 5) 1.5" (zone 6) EPS recommendations are conservative enough when you have nothing between the foam & fiberglass. The studs have a lower R value, and are effectively self-protecting from a dew-point control point of view- a 3.5" thick stud is no more than R4.4, so with even R4.2-labeled foam out there the foam is more than 50% of the total at outdoor temperatures that matter.
 

J Blow

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Somehow I looked incorrectly but definitely 5 and almost in the 'dry' category.

So, can the foam be attached the 2x4 wall I'm framing so it has something to be tied to once it clears the concrete half-wall on the bottom? Either that or the foam would need to be cut at the top of the concrete since the concrete sticks out a little. That's the space I was worrying about. It would look like this...

Above grade wall:

Exterior framed wall with fiberglass insulation | vapor barrier | air space | eps running floor to ceiling | interior framed wall with fiberglass | sheetrock


Below grade wall: concrete | EPS running floor to ceiling |interior framed wall with fiberglass | sheetrock
 

DonL

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Welcome to Terry's Forums.

Would your name happen to be Joe ?

I think I know you.
 

J Blow

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I'm seriously about ready just to hire someone....how this gets finished without being either cold or mold infested or both is a burning question. Maybe I can do the painting....errr something. Thanks for the help, Dana!
 

Dana

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I'm seriously about ready just to hire someone....how this gets finished without being either cold or mold infested or both is a burning question. Maybe I can do the painting....errr something. Thanks for the help, Dana!

Don't expect a lot of competence from your average insulation installer, especially when it comes to basements- you have to direct them. Only the very best know how to read a psychrometric chart or do a dew-point calculation on a wall stackup- few have even read the IRC chapter 7 (which isn't all that illuminating about what's going on, but works-mostly.)
 
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