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Interior Insulation of Embedded Joists in Cold Climate Basement

navigator16171 | Posted in General Questions on

I was about to pull the trigger on installing some insulation in my basement until I read about the “problem” of insulating embedded rim joists and have seen a lot of conflicting information.

First, I live in SE Wisconsin (Climate Zone 6) and have a 1930s house with cinder block basement walls, concrete floors, embedded rim joists, no HVAC sources in the basement, and likely no exterior insulation or waterproofing.  Basement is almost entirely below grade (even the joists are mostly at or below grade). 

My goal is to add drywall walls around the perimeter of the basement and some of the ceilings to brighten things up but not looking to make it a fully finished area or necessarily seek to meet the code requirements thereof.

My plan is/was to simply install 2″ of XPS rigid insulation on the walls around the perimeter of the basement, and then 2×4 stud walls with 1/2″ drywall and no insulation in the cavity (again, not looking to make this a finished space).  I understand that level of insulation would be enough to provide protection to the studs and drywall from moisture intrusion and thick enough to avoid condensation issues that can come from using thinner XPS insulation with a lower R value.

Then I started reading about embedded joists, which I have (here is a photo of mine: https://bit.ly/38bOHyD).  The joists appear to rest on the top of the cinder block walls and then concrete or some type of masonry is pretty tightly infilled between the joists all the way to the top, and this vertical surface is flush with the cinder block wall surface below.   The joists do not show any visual signs of prior water damage or rot.

I had thought I was going to run the 2″ XPS wall insulation up into this joist space and seal the XPS around the joists and floor, but then started reading about concerns with this causing the joists to rot.  I’ve read some different things about what to do here, such as max 1″ XPS insulation (per Canada’s energy website).  Some say don’t do it at all.  Some say add insulation, but leave a gap between it and the joists.

I’d like to know what the realistic options are here for interior insulation that are least likely to moisture or rotting issues.  Not looking to insulate the exterior, cut off joist ends, etc. Again, I’m looking more to protect the studs and drywall walls I’m building from moisture than to provide energy efficiency.  Above all, I want to avoid doing anything that could create moisture/mold/rotting issues.

1) Are there any risks if I install the 2″ XPS from the floor to the bottom of the joists and put no insulation in the joist space?

2) Are there any risks if I install the 2″ XPS from the floor to the bottom of the joists and install 1″ XPS in the joist space?  If so, should I airseal the XPS around the joists and subfloor? Or leave air gaps between the joists and XPS?  I’ve read both ways….

3) Are there any risks if I do same as (2) but with 2″ XPS in the joist space?

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Just to confirm, you have some joists ends that are below grade, and all the joists are embedded? I’ll give your post to see what the experts advise.

    1. navigator16171 | | #2

      That’s right, the grade is essentially near the top of the joists. It’s a city lot and the grade is pretty much level from the house to the adjacent sidewalks; there’s not much that can be done with it. While the whole setup is probably not ideal on paper, it has seemingly lasted 85 years without issue.

      All joists are embedded that way on the east and west sides where they terminate into the walls, minus a few that have openings for radiant heat pipes and water pipes for exterior hose spigots. The north and south walls run parallel to the joists but there is also masonry built above the cinder blocks all the way up to the subfloors on those walls.

  2. Steve Knapp CZ 3A Georgia | | #3

    I'm a non-expert, so YMMV with my input. It seems risky to do anything with the basement if you are unwilling or unable to build new bearing walls and cut off the ends of the embedded joists. I suspect the joists have not rotted over the last 85 years because your leaky old house has enough air infiltration to dry out most of the moisture that is trying to move into the ends of the joists. While wanting to tighten up the house is commendable, you may reduce that drying potential and increase the chance that the entire home will end up in the basement.

    Let's see if any of the experts feel differently.

    1. navigator16171 | | #4

      Most homes in the area have this embedded joist construction, many have newer finished basements, and I'm fairly certain no one is cutting off beam ends and changing the structure of their entire house to add a basement rec room or work shop. I haven't heard of any homes collapsing either, which seems a bit alarmist. I still understand the concern and am trying to figure out a practical, cost-effective solution for it. If that means the joist spaces aren't fully insulated, then so be it. As I mentioned, my goal with the rigid foam is more a moisture barrier than to provide warmth.

      Given that, I'm mostly curious about whether it would be better to leave the joist spaces free of insulation or go the 1" insulation route suggested by Canada's energy agency (although I still have questions about the finer details of doing so and whether to air seal).

  3. navigator16171 | | #5

    After reading just about everything out there about this, do any of the experts have an opinion as to whether it is better to leave the joist spaces completely free of insulation? Or would I be better with 1" of rigid insulation in the joist space as suggested by Canada's energy agency with airsealing around the joists? What about the on two sides of the basement where the joists run parallel to the walls?

  4. Steve Knapp CZ 3A Georgia | | #6

    Maybe you read this discussion: https://www.greenbuildingadvisor.com/question/what-to-do-when-the-ends-of-the-floor-joists-are-embedded-in-concrete. It outlines the pros and cons of different approaches in some detail.

    I'm not sure any of the experts will venture an opinion that supports the plan you want to use. It may be that there is little risk of catastrophic failure, but that doesn't mean there is zero risk. My concern would be what happens 20 or 30 years down the road. In your case, the piled up soils on the outside of the home are probably keeping the ends of the joists warm and air leakage and stack effect are keeping moisture from accumulating in those structural elements. But what happens when you add insulation, conditioned air, and people into that spaces?

    If you decide to move forward, I would install a few humidity sensors near the exterior and monitor things (using current conditions as your baseline).

  5. Charlie Sullivan | | #7

    I think you are safer leaving insulation out of that area, maybe even ending it 6" below the joists.

    When you go to buy insulation, make sure you get the new "NGX" version of Owens Corning XPS foam that has low global warming potential, 1/10th that of their old stuff. Or use a different type of foam--EPS, GPS, or polyiso--that doesnt' have that problem.

    You could insulate that area from the outside, perhaps with mineral wool board, to keep that part even warmer and lower risk more. If you aren't fully insulating on the exterior it's a lot easier than if you are going down 6'.

    1. navigator16171 | | #11

      Thank you, that's what I might be leaning to. I guess I’m a bit confused how having the very top of the insulation touch the bottom of the joists would materially change how they dry out. But if it’s safer to keep them not touching, I can add some space. Would it still be a good idea to caulk around the joists to seal any gaps? Or are those gaps beneficial for drying?

    2. Hammer 🔨 | | #13

      How do you protect the mineral wool from weather and insects? I’m not using exterior insulation for fear of termites and ants

      1. Charlie Sullivan | | #17

        It seems that ants at least, and probably termites, either don't like or can't chew through mineral wool boards. We protected ours with stuccoed concrete backer board. That worked well where we back filed with good soil; where we made the mistake of back filling with clay, frost heaving cracked the backer board. There could be concerns about the board wicking moisture up from wet soil, so it's best where the soil is well drained.

  6. Ikibbe | | #8

    Hey there,

    I have a similar situation. (Link to my post below).

    What I’ve decided to do is spray foam 1’ in the rim joists, and then inject any suspect joists with borate rods. The rods will largely be for peace of mind. My rim joists are embedded in brick and a bit higher above the grade then yours but maybe this approach would work for you.

    In response to my question, Khoa Ueno links to some good resources and seems to endorse the idea of borate as a reasonable, not too extreme solution. Maybe you’ve read the links, but I found them useful enough to read a few times.

    Good luck!

    https://www.greenbuildingadvisor.com/question/embedded-joists-in-cold-climate-brick-basement-insulate-heat-borate

    1. Deleted | | #9

      Deleted

    2. navigator16171 | | #10

      Thank you! Is there any reason to do spray foam in the joist space versus something like 1" of rigid foam? And I assume you are air sealing tightly against the joists then? Do you have rigid foam underneath the joists down to the floor?

      1. Ikibbe | | #14

        I’m using spray foam in the joist space only because the brick and mortar there is so uneven it makes installing rigid foam impractical if not impossible. I have a few other areas in the basement that are like that so it just made sense to buy a spray foam kit and not drive myself nuts trying to cut and patch a bunch of rough areas with rigid foam.

        The rest of the walls below the joists are a lot smoother, so yeah, those areas are rigid foam. I used 2” of EPS, foamed and taped at the seams.

  7. Hammer 🔨 | | #12

    I have a similar situation in 1 wall in my home. I also wonder why you can’t put your rigid foam up to the joists and leave it. The joists will be free to dry and it’s worth the sacrificial heat loss

    1. Ikibbe | | #15

      Hey,

      If I understand the literature correctly, the issue is that you risk condensation in the cavities. The 1” of foam is working as an air barrier, which is almost more important than the insulating value, though you lose a lot of r-value as well.

      “The elimination of insulation at the rim joist area resulted in temperatures close to original conditions. However, static thermal simulations indicate that there might be significant risk of wintertime condensation within the cavities at typical interior humidity conditions, based on the beam pocket surface temperatures relative to indoor dewpoints. In addition, this measure loses a significant fraction of the energy savings of the fully insulated case (roughly double the heat loss, from R-14 to R- 7).”

      https://www.buildingscience.com/sites/default/files/migrate/pdf/CP-1201_Masonry_Wall_Interior_Insulation_Retrofit_Embedded_Beam_Simulations.pdf

      Of course, I’m just an enthusiast not an expert, and a newbie to boot, so hopefully someone with more expertise can weigh in here, ha.

      1. navigator16171 | | #16

        I thought that you didn't want an air barrier though to avoid the potential rotting and moisture issues? Isn't the theory that the joists and masonry or concrete infill between the joists should be exposed to warm air to allow for drying to occur?

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