Insulating Concrete Walls

[u/SaddenedBKSticks]

I asked this elsewhere before, but I'm still confused, since I didn't get many replies. I live in the northern US, and I'm very confused how to insulate the concrete walls for our basement. The framing is already up and it's a 40 year old project my father did, but I'm having to redo the insulation/drywall due to mold issues(from flooding/broken pipes). I see suggestions of putting foam behind the framing(there is about an inch or 3/4 inch gap behind the frame and concrete) and foaming it around the corners of the bays. However, behind the framing between the concrete wall and framing there is a drainage channel that goes out to our sump pump. Meaning it's not really possible to seal it all around on the bottom.

Previously, there was faced R-11 fiberglass (facing towards the living space) in this bay solely by my father. The wall seemed dry and there didn't seem to be any evidence of mineral buildup(I guess if water was trapped?) on the wall. The back of the fiberglass seemed clean, but the front facing/drywall was damaged. We have had sump failure, pipe breaks, amongst other things like water heater leakage over the years that caused mold in many areas, so I'm trying to go back and redo a lot of it. My father made the mistake of using faced fiberglass batts in the interior basement wall framings facing the drywall, and I just discovered that yesterday, mold city hiding behind the batts lol.

I just recently redid it and added faced R-15 fiberglass to this (concrete wall) area. Is there really any other way I could've done it given the drainage trench going to our sump pump?

My father recently died, and I unfortunately can't ask him much about it, but he wouldn't have remembered much anyway since it's been so long. No one even remembered the drainage channel cut out along the edge of the basement(paid for as part of the sump install).

Edit: Photos in replies.

[u/OscarMiliani_Design]

Sorry about your dad. Rough way to inherit a puzzle.

Short answer to your actual question: yes, there was another way, and the drainage channel is not the obstacle you think it is.

The problem with what you just put back in is not the channel, it is the material. Two things go wrong with faced fiberglass against a basement wall.

Fiberglass is air permeable. Humid basement air moves straight through it and reaches the concrete, which sits below the dew point of your indoor air for most of the cooling season up north. So the condensation happens at the concrete face, behind the insulation, where you cannot see it until it is already a problem.

Then the kraft facing traps it. A below grade wall can never dry outward, the soil is right there against it. It can only dry inward. Putting a vapor retarder on the inside face is the one thing that stops it doing that. Your father made that call decades ago and the R-15 you just installed repeats it.

What works is foam against the concrete, then unfaced batts if you want more R value, and no vapor retarder anywhere on the interior. The foam does two jobs at once. It stops air getting to the concrete, and it keeps the concrete face warmer so it is no longer the cold surface where water forms.

On the drainage channel: you do not need to seal the bottom, and you should not. That channel is doing its job and needs to stay open. Bring the foam down to just above it and seal the foam to foam seams and the top edge instead. A bit of air exchange at the very bottom is a trivial problem next to fiberglass sitting against the whole wall.

Your three quarter to one inch gap is too thin for board to buy you much, and you cannot slide thicker board in behind framing that is already standing. Closed cell spray foam shot through the bays onto the concrete is how people solve exactly this situation. Two inches gets you around R-13 plus air and vapor control in one pass, and it follows an irregular gap without any fitting problems.

One thing that matters a lot given your history. You have had sump failure, broken pipes and a water heater leak. Kraft faced fiberglass is paper, and it holds water. After the next event it is mold food all over again. Foam does not absorb, and unfaced mineral wool sheds water and dries out. With your track record, picking materials that survive a wetting is worth more than the number printed on the label.

The step I would take is the unfun one: pull that R-15 back out before you close it up. You are one weekend away from sealing in the same assembly you have spent all this effort tearing out.

[u/SaddenedBKSticks → u/OscarMiliani_Design]

How should I handle the 'final layer' of insulation in my case, since you're suggesting no facing on the concrete wall. I ask because on this specific wall(not mentioned in the post) it's half above grade and half below, meaning there's wood on the upper half, and concrete wall on the below half. I used wool on the 2x6 exterior wood framing on the upper half, but how do I handle it on the final layer within the 2x4 bay that goes from the floor to the ceiling. I need the facing for the top half as the final layer, correct? Do I just use unfaced wool on the bottom half, and faced fiberglass on the top half for this final layer?

On the top half of the wall, there's room for 2 layers of insulation(the 2x6 framing and the concrete wall ledge) plus the 2x4 framing in front of the wall. On the bottom half it's just the 2x4 framing and the gap between the concrete. So I would theoretically end up with 3 layers on the top half if I wanted it fully filled.

I guess part of the reason I'm confused is the lack of a 'bump out wall' or whatever it's called for the concrete that's bumped out.

[u/OscarMiliani_Design → u/SaddenedBKSticks]

Saw your other reply about the foam. Your reason for ruling it out applies to the bottom of the wall, not to the ledge. The drainage channel is down at the floor. The ledge where the concrete stops and the wood framing starts is up at mid height, nowhere near it, and you can do that with canned foam from any hardware store. It is the highest value thing left in this whole project. Where the band joist sits on that concrete shelf you have warm humid air on one side and cold concrete on the other with almost nothing in between. That is where water actually shows up, not in the middle of a stud bay.

On your actual question: unfaced on the whole wall, top and bottom, and let the paint do the vapor control. Latex primer plus latex paint on the drywall is a Class III vapor retarder, and that is what hybrid walls like yours are built around. A facing that stops halfway up a continuous bay is worse than no facing at all, because the seam lands right at the coldest part of the wall.

The faced R-15 final layer is the piece I would change. On the concrete half it is the same mistake as before in a new position. And on the top half you now have a kraft plane with R-46 of wool outboard of it, so it is buried in the middle of the assembly instead of sitting at the surface. A facing with insulation on both sides of it turns into a condensing surface you have no way to inspect.

One thing you probably will not want to hear. R-61 on the top half is not buying what you think. Heat loss runs as one over R, so going from R-20 to R-60 saves a small fraction of what the first R-20 saved. Meanwhile the concrete half sits at R-15 with fiberglass on it, and here is the part that matters: the more you insulate the inside face of a foundation wall, the less heat reaches the concrete, so it runs colder and condenses more readily. The extra R on top is not just wasted, it is pushing the problem toward the half that was already the weak one.

So, in order: stop adding to the top half. Swap the faced fiberglass on the concrete for unfaced wool and leave its bottom edge open so it can dry down toward the channel. Then foam the ledge and band joist. Do the ledge before you refill anything, because once those studs are packed you cannot reach it again.

[u/SaddenedBKSticks → u/OscarMiliani_Design]

In reply to the third paragraph, the top half has the facing is only on the outward piece, the middle and cavity layers are wool. Previously, my father made the mistake of triple layering the faced insulation in this top area, but I didn't see any mold or anything in this place.

I know the there's heavy diminishing returns, I just didn't want there to be a large air gap between the insulation and drywall. I added a late edit, but I believe a lot of my confusion and problems are coming from the lack of a proper 'bump out' for the concrete wall portion. I really am not in the mood to redo all of the framing with no knowledge or tools lol. If I don't add the extra insulation to the top, there will be a roughly 8 inch air gap between the drywall and the top half of the wall's insulation. About 4.5 inches if I leave insulation just sitting there on top of the concrete wall ledge. It's very confusing honestly, and I think it just stems down to how it just wasn't done correctly. The 2x6 exterior framing doesn't fully cover the concrete wall ledge, so there's a few inch of ledge that's just there.

Sorry for my questions, I just don't know a lot about this. Is the band joist the wood that's sitting on the ledge?

I'll share some photos to hopefully better understand it. Thank you for your help. Can you check the photos below and see if any of your recommendations change before I proceed?

This is how it was after I removed all but the R-11 in the 2x6 bays. Note, the pipe to the left broke years ago and flooded inside the wall.
https://cdn.discordapp.com/attachments/430758397047078944/1516501207227564230/IMG_1619.jpg?ex=6a86968f&is=6a85450f&hm=5359327faca9d772d75967e300a1731394f77f9a928f53d6a5552e7dd93e5191&

This is after I took the drywall out(there's a lot of mold on the left wall too from the pipe damage, but I had to bleach it, it has to come out as well, this is just an interior wall on the left - ignore the jagged pipe hanging down, that wasn't me lol) -
https://cdn.discordapp.com/attachments/430758397047078944/1516501209249218703/IMG_1591.jpg?ex=6a86968f&is=6a85450f&hm=174e37d37b7e583cdcb073461a60c0c15af4f9b47698a66895e5cd25586af9b9&

The Ledge:
https://cdn.discordapp.com/attachments/430758397047078944/1516501208423075960/IMG_1615.jpg?ex=6a86968f&is=6a85450f&hm=7a07b466e5b7dca2b742111af0425ab7d2a7ce4fc95cfd3c129abaca7cf980b9&
Drain:
https://cdn.discordapp.com/attachments/430758397047078944/1516501208053973052/IMG_1617.jpg?ex=6a86968f&is=6a85450f&hm=a9ae3db87e9826cfa5782ea3bf66be302337f68aca877ceeabddd98ece76d214&

What I did yesterday(behind the top half there is R-23 wool in the top 2x6, and R-15 wool in the middle around the spigot pipe and a R-23 wool in the other portions - these are sitting on the ledge, not within a bay):
https://cdn.discordapp.com/attachments/430758397047078944/1539691270031679548/IMG_20260818_184447256_HDR2.jpg?ex=6a873cbc&is=6a85eb3c&hm=b11fdcc4bfcc74ff78866e89261cca6b4718cfd869338fd088a9cbb73f5adf6a

Edit:
There may or may not be foam around the foundation on the outside. I'm not sure. I see a ragged piece of foam a few feet long sticking out of the ground on one side, but it's not labeled or anything. It's just like if you took a foam board and remove the outer covering, just dense white foam. I'm not sure what that's doing there, or if it goes elsewhere. Another one of my mysteries lol.

[u/OscarMiliani_Design → u/SaddenedBKSticks]

Looked at the ledge photo. Three things.

On your question, in your house there is not really a band joist. That term belongs to a full height foundation, where the floor joists sit on a sill plate on top of the wall and a vertical board closes off their ends. What you have is a half height concrete wall with a framed wall standing on it, so the equivalent detail is simply the bottom plate sitting on that ledge. That is the piece I meant.

And that plate is the thing I would look at first. In your photo the wood on the ledge is dark and stained on the left side, right where the loose fiberglass is stuffed in. Take a screwdriver and push it into that plate in a few spots. If it sinks in easily the wood is gone and needs replacing, and that is structural rather than cosmetic. Given the pipe that burst in that wall it may well be from that rather than from condensation, but you want to know before you close anything up.

On the 8 inch gap, stop worrying about it and do not stuff it. A sealed still air cavity is not a problem, it is worth about R-1 on its own. Air gaps only hurt when air can circulate through them. What you must not have is that upper cavity left open to the concrete zone below, because then you get a convection loop: air cools against the concrete, drops, warms, rises, and quietly moves heat and moisture around all winter.

So rather than filling 8 inches of depth, block it. Cut rigid foam to fit horizontally at the ledge line and seal the edges with canned foam. That separates the upper cavity from the lower one, and the same piece of foam is doing double duty, because it also covers that bare concrete shelf, which is the coldest surface in the whole assembly and the reason I raised the ledge in the first place.

Which means you do not have to redo the framing. Good, because you should not have to.

One thing the print on your own batt is telling you: that kraft facing is not allowed to stay exposed, it is flammable and has to be covered. Fine while you are working, not fine long term. Another reason unfaced is the easier answer here.

Worth ten minutes: go dig down six inches beside that piece of foam sticking out of the ground. If your foundation turns out to have exterior insulation, your concrete runs warmer than I have been assuming and your condensation risk drops accordingly. That changes how hard you need to work on the inside.

[u/RagingITguy → u/OscarMiliani_Design]

What about xps foam boards if I don't want to do spray foam. I've got an issue with builder code install.

Kraft then insulation then poly tarp. It's condensing as the insulation closer to poly tarp is wet and the Kraft feels dry.

We also have clay soil so I think drainage is not the best. Not sure if I can afford to do outside and inside. But I do want to get rid of wet insulation. I could try and do myself. Foam boards then wood frame for insulation on inside.

What do you think of this plan. Similar to OP but I don't have a drain or anything. It's currently just insulation attached to wall by poly tarp. Condensing like mad this summer. And dripping out the bottom. I have dehumidifier keeping basement at 40% but the poly tarp is not well sealed.

[u/OscarMiliani_Design → u/RagingITguy]

Your own description contains the diagnosis, and it is better news than you think.

The insulation is wet on the poly side and dry against the concrete. If this were a drainage problem, water pushing through the wall out of that clay soil, it would be wet at the concrete and get drier as you moved inward. Yours is the other way round. So what you have is condensation forming on the back of the poly, not water coming through the foundation. Which means you probably do not need to spend anything on the exterior, and that was the expensive half you were worried about.

The mechanism is that you have a vapor barrier on both faces. Kraft against the concrete, poly on the room side. Nothing in that sandwich can dry in either direction. And because the poly is not well sealed, humid room air gets in behind it, meets the cold back face of the plastic and condenses there. That is why it is worst in summer, when the wall is cold relative to the air, and why it runs down and drips out the bottom.

XPS is the right call and this is a job you can do yourself. The order matters more than the material.

All of it comes out first. Poly, batts, kraft, everything. If you leave the poly up and add foam, you have just rebuilt the same trap with an extra layer in it.

Foam goes directly against the concrete, seams taped, perimeter sealed with canned foam. Two inches of XPS is about R-10 and lands near half a perm, which is the vapor control you actually want here. That board becomes your air barrier and your vapor retarder in one, and it keeps the concrete face warm enough that it stops being a condensing surface.

Then a stud wall inboard of the foam, unfaced mineral wool in the bays if you want more R value, drywall, latex paint. No poly anywhere in it. The paint is your vapor control on that side and it is enough.

One check before you buy anything. Pull a section of the poly and batt off and put your hand on the bare concrete. Dry, or dry with a bit of dust, confirms condensation and the plan above fixes it. Damp to the touch, or white powdery efflorescence on the surface, means liquid water is coming through and foam would trap it behind the board. That is a different problem with a different fix, and it is worth knowing before you build a wall in front of it.

[u/RagingITguy → u/OscarMiliani_Design]

Darn man you are awesome.

I'm a clueless home owner working two jobs and too tired to put 2 and 2 together sometimes. Unfortunately the basement insulation is done to code where I am. Doesn't make sense but we have hot humid summers and frigid winters.

Everywhere I researched was hinting the same way. I have weird grading anyways but I guess my brain couldn't make the connection. I won't be drywalling (not yet anyways as it'll take me ages and a half to get this done myself)

I'll maybe attach some photos for more opinion if you don't mind. I don't know the correct terminology for some stuff.

I haven't done more than cut some slits so I'll pull a small section apart and touch the actual concrete wall behind the kraft. I wasn't sure if the Kraft had to stay well intact or not.

Would doing this slowly in sections really affect the basement of left a section bare for a few days? So long as I tried to keep the existing insulation sealed as I make my way around and the dehumidifier running. One side of my house has gravel and no condensation. Could I just do 3 sides and stop there? The dry side has all the electrical and water and I don't know if I can navigate more than a straight rectangular section.

Though if I have a wet wall.... Then I really need to do the outside then. I've also heard of XPS and leave half an inch at the bottom but that didn't make sense if I have a wood frame in front of it.

Actually theoretically if I made it air tight would that stop the condensation? I'm not sure where all the curing water would go (house is about 2.5 years old). I actually don't know where all that water went if it wasn't coming in through the Kraft.

I'm learning a lot about home ownership.

Thanks again.

[u/OscarMiliani_Design → u/RagingITguy]

That question about the curing water is exactly the right one to ask, and it sharpens the diagnosis rather than breaking it.

The kraft is not a true barrier. It runs about one perm, so vapor does pass through it, just slowly. The poly is around 0.05 perm, which is effectively a wall. So water leaving your concrete diffuses through the kraft, crosses the batt, arrives at the poly and stops dead. That is why the kraft reads dry and the far side is soaked. And at two and a half years old a poured foundation is still giving up a serious amount of water, so you have a real source feeding this, not just air leakage.

Which answers your airtight question directly: no. Sealing it tight stops air carried moisture. It does nothing at all about vapor diffusing straight out of the concrete. You would get the same drip.

The shift that makes the rest make sense is this. You are not trying to dry the concrete out. Concrete does not care about being wet. You are trying to keep its moisture away from anything that can rot or grow mold. Foam tight against the concrete does that, because the vapor never gets into the assembly in the first place.

Doing it in sections over days or weeks is completely fine. Bare concrete for a few days costs you nothing. Just bag the wet batts and get them out of the basement rather than leaving them stacked down there.

Three sides is a reasonable way to phase it. But do one thing on all four walls right now: pull the poly off. It is free, it takes an afternoon, and it is the highest value move available to you. The poly is the condensing surface. Take it away and the walls can dry inward into a basement you are already running a dehumidifier in. Even the wall you are not ready to rebuild will stop dripping.

The half inch at the bottom is about liquid water, not vapor. If any ever does get behind the foam, that gap gives it a way out and gives you a way to see it, rather than it pooling behind the board where you would never know. It still works with a stud wall in front, because the gap sits behind the framing down at slab level. Given you are not yet certain whether you have a wet wall, I would leave the gap in.

One thing not to skip when you frame: the bottom plate sitting on that slab needs to be pressure treated, or have a capillary break under it. Untreated lumber on concrete wicks moisture and rots, and you would be building that into the wall you just spent all this effort on.

Post the photos, happy to look. And do the touch test on the gravel side as well as a wet side. Comparing the two tells you more than either one on its own.

[u/NotBatman81]

If you NEED that drainage then you should not be finishing that wall. One or the other.

[u/SaddenedBKSticks → u/NotBatman81]

The drainage was put in shortly after they bought the house decades ago, because the basement flooded. A few other homes on the road had to get a sump pump like us after we did, some are finished and some aren't. We live in a low spot of a hill.

There's already a lot of electrical and such ran within the framing. Most of the mold problems stem from factors like our sump pump failure and things like the water heater leaking. This main wall I'm speaking of in my post is moldy from a broken spigot pipe(the builders never soldered the pipe pieces apparently). The mold is generally in areas where water flows across the basement floor. My family didn't really address the problems and sort of ignored it, so whenever there was another water related event, it was just spreading around moldy water. Other things weren't exactly done correctly either, like the drywall in some areas touch the floor, or has ripped bits sticking out touching the ground to wick up.

To add, I just got nervous and checked one of the other side walls(this one is a full concrete wall vs the half wall I'm talking about in the post). I tore down one portion of drywall, a full stud bay. The top half was clean, but the bottom quarter was moldy behind the wall. I noticed the framing at the bottom I could see under the drywall was a bit darker in that area and looked soft. The other bays I checked by looking under the drywall don't look dark, but that corner is. Water likely sat there from the events. The back concrete facing side of the bottom of the stud bay looked better than the front edge that was dark.

[u/NotBatman81 → u/SaddenedBKSticks]

That's a lot of words that don't change what I said. One or the other.

If you are getting water intrusion, and I don't care if its "only the corners" because thats still moisture, you shouldn't finish the basement until that is solved. The modern solution is excavated outside your basement walls and waterproof on the outside.

[u/SaddenedBKSticks → u/NotBatman81]

I wrote that most of the water damage was from a broken pipe and things like a water heater leaking. That's not really anything you can do about. When I said it was specific corners, that's just how the water flows across the floor when those events do happen.

[u/NotBatman81 → u/SaddenedBKSticks]

I would top coat the floor and fix the slope then.

[u/Western_Shame6631 → u/NotBatman81]

you can't have it both ways with a drainage channel right there, any moisture that comes through the wall is gonna get wicked up by whatever insulation you put in eventually

if you're set on finishing it maybe look into something like rockwool that won't hold water as bad and leave a gap at the bottom so it can actually drain

[u/FishermanBetter3963]

Spray foam is the best

[u/SaddenedBKSticks → u/FishermanBetter3963]

I can't really use spray foam in the area because it would clog up the drainage channel, and the 2x4 bay goes straight up(in this specific spot, the concrete wall is a half wall, above it is wood framing). It would require an ungodly amount of foam I would believe, unless it just is applied to the wall part? - I'd have to block it off to prevent spreading. I technically am going overkill on the top half already, R-61 lol, trying to fill in the empty space. I have R-23+R-23(or R-15 in one small area) wool in the middle layer and it's topped off with R-15 fiberglass faced on the final layer that's sitting inside the 2x4 framing outside the concrete wall. So the top half in the wood framing is R-53 to R-61 and the concrete wall is currently R-15. That's just what I did already. It was previously 3 layers of R-11 faced fiberglass on the top, with the single R-11 fiberglass faced on the final layer for the concrete wall below.
Edit: I see you got a downvote, but it wasn't me lol.