Basement: how to best insulate rim joists over air gap?

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dsmith060

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Hi all,
I've been researching across this and other forums for several days now, and while I've found the occasional situation that is similar, nothing seems to provide a clear answer on how to handle this.

I'm beginning work on finishing my basement (poured concrete, house built 2011), and planned to insulate the back of the rim joists using 2" XPS rigid foam board, using Great Stuff spray foam to seal the seams. After removing the fiberglass insulation that was put there by the builders, I realized that there was a gap along the bottom of the rim joist at the far back. Turns out that this gap is the air gap for the brick veneer built around the foundation that is above grade (the whole back side of the house) and the joists extend past the concrete foundation up onto the brick. The air gap is loosely covered by what appears to some white house wrap. I've attached some pictures to help illustrate.

Basically I'm just trying to determine the best course of action for insulating this area, as well as how this might affect the rest of my waterproofing / insulation plan. Should I add another piece of foam board and block off the air gap? Or does it need to be left open? If I place the foam board in front of the air gap, then I'm leaving my joists exposed behind it (which sounds like a bad idea).

The basement is generally very dry, but I don't want to leave myself vulnerable to moisture issues down the road and want to make sure I do it properly before closing everything off with drywall. Any advice would be appreciated! Thanks!
 

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Dana

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Is the band-joist tight to the brick veneer, with the vent channel going around the band joist and a gap in the subfloor or ??? The joists aren't shown in the diagram, only the sills.

Does the brick veneer stop at the same level as the top of the sill plate? What is on the exterior of the foam in the diagram?

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I need a clearer picture (or 1000 more words of explanation) to grasp what you're dealing with.

But no matter what there are almost no cases where 2" XPS would be a good choice- it's too vapor retardent for many applications (restricting drying rates), and the labeled R10 performance is only warranteed to R9, and beyond 20 years it's likely to be even lower than that, eventually falling to about R8.4, the same as EPS of similar density. The early years higher performance is an artifact of it's HFC blowing agents (a mixture of HFCs, predominantly HFC134a) which diffuse out over time.

HFC134a has a 100 year global warming potential of nearly 1400x CO2- some of the rest of the mix may be higher or lower than that, but bottom line, at a labeled R10 or higher the impact of the blowing agents dwarf any energy-carbon emissions savings over the lifecycle of house, making XPS by far the least green insulation material in common use today:

CSMP-Insulation_090919-01.png


By contrast, EPS is blown with low impact variants of pentane (~7x CO2 @ 100 years), most of which escapes the foam while still at the factory, where it is recaptured, not vented to the atmosphere. The closed cells of EPS don't retain pentane- it's quickly displaced by air, and it's performance is stable over many decades.

Like EPS, rigid polyiso is blown with low impact hydrocarbons, some of which are retained in some products, yielding a high performance at most normal temperatures, but at extreme cold (colder than SE Michigan) needs to be derated. But even the crummiest 2lb density roofing polyiso (labeled R5.5- R5.7/inch) out there would average R5/inch or better over the season on the exterior layers in a SE Michigan location.
 

dsmith060

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Thanks for the detailed reply!
I attached a couple more photos from different angles to provide a little more context. The brick just covers the portion of the foundation walls which sit above grade on the back half of the house (which sits on a slight hill), so at maximum it's roughly 5' of brick, less in many areas. The main floor exterior walls are vinyl siding. So the joists all rest across the foundation walls (on the sill plate) and cross the air gap (more like 2-3" as opposed to the more standard 1" I've seen mentioned), and the rim joist itself sits on top of the brick (about halfway across the brick, so covering maybe 1-2" of brick). I updated my drawing to reflect this.

Essentially the air gap vents into the rim joist area of the basement, and the only thing covering it is a flap of house wrap.

For what it's worth, I'm not 100% sure that the house wrap covering the air gap is intentional, but rather the house wrap goes underneath the bottom of the rim joist and sits between the wood and brick. Any extra wrap just sits loosely over the gap.

Regarding the XPS, I was actually passed along a few extra sheets from a friend and already purchased and cut a few more for this job. I've since learned of it's impact on the environment, and would definitely prefer another material for that reason alone, but I'm pretty much stuck with it now, so would like to use it if possible.
 

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Dana

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On the exterior of the band joist there is presumably some wall sheathing, housewrap & siding?

Since the brick veneer stops where the band joist and framed wall begins, it's better to BLOCK and COMPLETELY SEAL the top of the brick cavity, and install vent holes in the vertical mortar every third brick or so on the second or third course from the top, and a similar set of weeps 3-4 courses above grade. That way the cavity will convection vent moisture to the exterior rather than into the basement or wall assembly. There are purpose-made inserts to keep the weeps / vents from becoming easy paths for vermin to turn the cavity into a critter condo, some with finer mesh than others:

stainless_steel_brick_weep_vent.png
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It's important to keep the weeps at the bottom well above any potential bulk-water runoff, with some margin for any future changes in the grade level from landscaping, or erosion/deposition of runoff.

Housewrap isn't a reliable air barrier, but more importantly, it's not a capillary break- moisture can and will wick from the brick into the joists if the brick has a high moisture content. With fiberglass insulation there is drying capacity toward the interior, but it also allows wintertime moisture from the interior air to load up in the band joist. In short, it's a good idea to install a capillary break between the joist bottoms & brick before sealing & venting the cavity, especially where there is backsplash wetting of the brick. While most brick doesn't wick as strongly as poured concrete, even groundwater can make it from the footing to 5' above grade, increasing the moisture content of the joists. If it's possible to slip EPDM flashing tape between the brick & wood, do so. With a hydraulic jack, a 4x4 pillar and a chunk of 4x6 beam it's usually possible to lift the joists ~1/8" a couple joists at a time to provide the clearance for the capillary break without any damage to the finished floors & walls above.

If that seems like too much trouble, take a few moisture content readings of the band joist with a 2-pronged wood moisture meter. (Decent wood moisture meters can be had for under $50 online, sometimes at box stores.) If it's consistently in the <12% range there probably isn't a wicking problem. But those issues change seasonally, and it's just a snapshot in time. If it's over 20% anywhere it will DEFINITELY need a capillary break.

The black membrane under the foundation sill is almost certainly EPDM or a similar completely waterproof capillary break material- no need to do anything there.

A poured concrete wall with a brick veneer has pretty low thermal performance, and at your above-grade exposure level likely represents more than 25% of the heating fuel use for the house, and a large fraction of the peak heat load. A simple insulated studwall on the interior of the concrete is likely to create a mold farm, between the ground moisture wicking from the foundation, and wintertime moisture drives from the interior. There is no good place to install a vapor barrier that would manage both moisture drives. Current IRC code min calls for R15 continuous insulation or its equivalent. A fairly moisture-safe equivalent would be 3/4" or 1" foil faced polyiso glued to foundation with dabs of foam board construction adhesive, with a 2x4/R13 studwall snugged up tight to the polyiso, with NO interior side vapor barrier. An interior side vapor BARRIER (like 4 or 6 mil polyethylene) would create a moisture trap- the studs and fluff need to be able to dry toward the indoors.

With R5 or more on the exterior and only R13 fluff in the studwall there is sufficient dew point control at the foam/fiber boundary to mitigate against interior moisture drives during the winter using only standard interior latex paint as the vapor retarder (not barrier). That's a fancy way of saying that the interior facing side of the foam board stays warm enough that it won't collect enough condensation over the winter to create a mold problem inside the wall cavity, even if the interior finish wall is somewhat permeable to water vapor (but the wallboard still needs to be reasonably air tight.)

When you go to insulate the band joist, install 1-2" unfaced EPS (or 2" XPS, if you have enough) over the top of both the foundation sill (blocking the brick cavity) extending all the way to the interior over the top of the top plate of your studwall. You can then safely install trimmed up R15 rock wool batts on the interior side of the 2" XPS on the band joist to bring the wall-R fully up to code (and a bit beyond) without risk of moisture accumulation in that section, even without latex-on gypsum as an interior side vapor retarder. Cheap R13s would be OK too, but aren't nearly as air-retardent, and would be more prone to convective moisture transport, though it would still dry reasonably quickly during warmer weather.

At the bottom of the studwall & wall foam it's a good idea to install an inch of EPS as a capillary and thermal break. Polyiso should never rest on concrete, since (unlike EPS & XPS) the cut un-faced bottom edge can (slowly) wick and retain moisture from the concrete, taking forever to dry. Resting the bottom edge of the polyiso on EPS is fine. If there has EVER been any dampness or weeping at the bottom of the foundation wall, leave a half-inch gap from the edge of the slab-EPS to the wall as a drainage space. In SE MI the subsoil temps are well below the mid-summer outdoor dew point temperatures, and even if there is a vapor barrier under the slab, the bottom plate of the studwall can potentially take on moisture during the summer if it's in contact with the slab. An inch of EPS is enough to keep the bottom plate sufficiently warm in summer to avoid mold/rot levels even if it's not pressure-treated (though a code inspector would probably insist on pressure treated anyway.)
 

dsmith060

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Wow, thanks for all the great information and the explanation, Dana!

Yes, it looks like the house wrap goes up and around the outside of the rim joist.

There is already a second row of weep holes on about half the wall length (the standard one at the bottom, another about a foot from the top). But I should be able to go through and put in additional vent holes on the remainder. The intent there is just to allow more moisture to evacuate the brick to the exterior since I'll be blocking off the air gap to the interior (as you later described) correct?

I'll check into a wood moisture meter and get a few readings (I imagine spring time with lots of rain will provide a pretty good "wet weather" sample), then I can make a decision based on those readings about proceeding with installing a capillary break above the brick. I'd like to avoid jacking up the joists if possible, but would rather get the job done right if it's needed.

You pretty well described exactly what my intended plan was for the interior walls: 2" rigid foam up against the walls, 2x4 framing, with unfaced insulation between the studs. I like the idea of putting some EPS beneath the bottom plate as well.

So back to the band joist area specifically, I think I follow your description, but just wanted to confirm a couple points. Basically I should be installing my XPS across the sill plate, over the air gap, and right up to the band joist? Then sticking some rockwool in there on top? Should I place a piece of foam vertically as well (flat up to the band joist) or is that not needed? Also, if my wall foam goes right up to the top of the sill plate, won't this effectively mean I'm trapping my PT sill plate in? Or would it be better to cut it to the height of the concrete and seal the seam to the membrane / sill plate? I updated my diagram to reflect your proposed changes and the couple of additional questions I had. I just want to make sure I'm understanding correctly before I get to work on it!

Thank you again for the help with this!
 

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Dana

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Wow, thanks for all the great information and the explanation, Dana!

Yes, it looks like the house wrap goes up and around the outside of the rim joist.

There is already a second row of weep holes on about half the wall length (the standard one at the bottom, another about a foot from the top). But I should be able to go through and put in additional vent holes on the remainder. The intent there is just to allow more moisture to evacuate the brick to the exterior since I'll be blocking off the air gap to the interior (as you later described) correct?

Yes, the weeps & vents are there to allow moisture out of the cavity via convection. When the sun hits the brick and warms it up there's a pretty good "stack effect" pressure pulling air out of the top vents, and outdoor air into the weeps near the bottom, any time of the year. When the wall isn't insulated the heat loss of the house can drive the convection too, but it will be weaker once the wall is insulated. If it were only weeps, with no exit path out the top, the amount of air movement and moisture purging as air transported water vapor is severely reduced


So back to the band joist area specifically, I think I follow your description, but just wanted to confirm a couple points. Basically I should be installing my XPS across the sill plate, over the air gap, and right up to the band joist?

Yes, but extend the sill plate foam all the way to the interior side, over the top plate of the studwall. Don't forget to put 2" of foam on the band joist too.

Then sticking some rockwool in there on top? Should I place a piece of foam vertically as well (flat up to the band joist) or is that not needed? Also, if my wall foam goes right up to the top of the sill plate, won't this effectively mean I'm trapping my PT sill plate in? Or would it be better to cut it to the height of the concrete and seal the seam to the membrane / sill plate? I updated my diagram to reflect your proposed changes and the couple of additional questions I had. I just want to make sure I'm understanding correctly before I get to work on it!

Thank you again for the help with this!

*The 2" foam over the top of the foundation sill should extend from the rim joist to over the top plate of the studwall. Otherwise there will be an un-insulated/barely-insulated stripe at the top interior side edge of the sill plate.

**There also needs to be 2" of foam between the rock wool and rim joist to protect the rim joist from loading up with moisture. There can be as much as 8" of rock wool or more between the band-joist foam and the indoors without risk of moisture accumulation at the sill-plate foam/rock wool boundary.

As long as there is 2" of foam + 3.5" of rock wool at the rim joist, and 2" of foam and 3.5" of rock wool between the foundation wall and indoors (including over the top plate) it's fine. Even if the joists are 2 x 8s you have 7.25" of vertical room to work work with, so with 2" foam and a 3.5" thick R15 batt completely covering the sill-plate foam & studwall top plate it will leave a 1.75" vent gap between the subfloor above and that chunk of rock wool. With deeper joists the vent gap is deeper, which is fine.

The sill plate is indeed trapped, and won't be able to dry quickly, but it's pressure treated and can take high moisture content. The surface where the floor joists rest on the sill plate will have a 2" path to drying (the thickness of the foam) into the fiber insulation layer, which would be good for more than 1 perm- no significant risk there, since there is a capillary break between the concrete & sill plate, and an air space vented to the exterior on the exterior edge of the sill plate. The sill plate should stay pretty dry, but even if it doesn't, pressure treated lumber can take it.

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Dana

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Your assembly isn't much different from any cantilevered joisting system that needs to be insulated, but it is an extremely shallow overhang, I did a short web search to try to find a detail drawing that would be ideal, but struck out.

This get's part of the way there:

DnWKG.png


The shortfall in this drawing is that it doesn't show foam layers, and the insulation (wrongly) stops at the foundation. But it at least shows the fully insulated band joist, and the gap between the subfloor above the horizontal insuation.

This shows one (overdone) method of insulating a cantilevered overhang, but with a full cavity fill that you don't need, but shows the layers of fiber & foam insulation, but also doesn't show the interior side fiber insulated studwall:

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dsmith060

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Thanks again for the very detailed response! Really appreciate all the good information.

I think I have a pretty good understanding now, the images were very helpful.

I do have one last question on this setup: since I have that much rigid foam in the rim joist, I believe I'd still need to fire block above the foam (which I don't think is fire-rated). In this case, is the rock wool intended to act as fire blocking material? I took a quick look at the code for my township, and it appears that this would suffice:

Batts or blankets of mineral wool or glass fiber or other approved materials installed in such a manner as to be securely retained in place shall be permitted as an acceptable fire block.

Just want to make sure I'm not missing anything before proceeding. Thanks again!
 

dsmith060

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One more follow up question: I was taking another look around the rim joists to plan out how to attack this, and ran into one more obstacle on about 1/4 to a 1/3 of what will be finished basement walls (on the far side, away from the brick veneer). I have my main electrical line (going to the panel) and a ground line running along the still plate / top edge of the concrete wall, right at the seam that I'd be covering with rigid foam and sealing (image attached). Any thoughts on how to best handle that? Thanks!
 

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Dana

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You may have to stop the wall foam at the top of the concrete leaving a ~1.5" gap between the top edge of the wall foam and the foam that bridges the top of the foundation sill & top plate of the studwall.

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BTW: That blue foamy sill gasket with the shims isn't doing much for air-tightness, and it's a bigger hole than most people would assume. I would bet it leaks quite audibly at 50 pascals with a blower door test. The whole seam needs to be air sealed with either polyurethane caulk, or very low-expansion can-foam (such as ProPink or EcoSeal) before installing the wall-foam. Most box stores would have a polyurethane caulks formulated for bonding to concrete- just stay away from "self leveling" types designed for sealing floor cracks & slab edges, which are a bit soupy/runny for this application.
 

dsmith060

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Thanks for the great info Dana, and sorry to go off the grid for a few days there (baby boy arrived a couple weeks earlier than expected!)

Once I can get back to work on this, I'll be sure to cut the foam a bit shorter on that side and address the seam along the gasket as well. I definitely recognized that as a problem, but hadn't determined a solution yet so I appreciate the advice.

Thank you again for all of the great information and help with my questions!
 
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