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Insulating basement that’s already framed with electrical

IndyGopher | Posted in General Questions on
First, let me own my novice DIY status and say I screwed up. I bought a new house last year in the Indianapolis area and immediately went to work finishing the basement. The builder attached batts of insulation to the top half of the poured concrete walls to just below grade, but that’s all. I framed the walls just off those batts — so several inches from the foundation. Didn’t consider any other insulation as I assumed like a novice that I’d just insulate the stud cavities after everything was done. It’s only recently I’ve become aware that the preferred option is rigid foam attached to the concrete walls for moisture control. I’ve now got a 2000 sf basement fully framed with electrical run and I’m wondering what my best course of action is for insulation. Thanks for any advice!

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  1. alexqc | | #1

    Do you have any insulation on the outside of your walls?

  2. GBA Editor
    Brian Pontolilo | | #2

    Hi Indy.

    Alex asks a good question. If there is only insulation on the above-grade area, perhaps below grade is insulated outside?

    If not, and if the framing is held away from the concrete wall, you can still insulate with rigid foam, I would think. It might even be worth taking out a stud here and there to slide full sheets behind the wall. Get the rigid foam nicely air sealed and then, if you'd like, you could use the existing fiberglass in the stud bays. Make sure to detail the rim joist properly too. Here are a few articles that you've probably already seen. But just in case:

    Insulating Rim Joists
    How to Insulate a Basement Wall
    Stay Dry, No Mold Basement

  3. IndyGopher | | #3

    I really appreciate both replies -- and so quickly! I'm not sure whether there's exterior insulation, but you raise a good point about why the builder would've only insulated the top half of my walls. I imagine the only way to find out would be to contact my builder, correct? If I find out those walls are insulated below grade, how would you proceed? What would you do with the top half of the walls already covered in fiberglass?

    In the event the outside is not insulated, are you suggestion removing the existing fiberglass and doing full sheets of foam all the way up the wall...then using the fiberglass in the stud bays?

    Thanks for sharing the articles! You're right I'd seen 2 of the 3 and learned a ton -- including the importance of insulating the rim joist. I obviously wish I'd given more thought to this before eagerly jumping into such a large project, but hindsight is 20/20. Thanks for any further advice!

    1. cussnu2 | | #4

      A shovel would be another way to confirm...........

      1. IndyGopher | | #5

        Haha, yeah you're right about that. Supposed to be 80 this weekend, so perhaps I'll do a small dig. I did confirm with the builder that they use Tuff-N-Dri Hydra-Guard spray on membrane on all the basements, but the guy I emailed wasn't sure if they insulate on top of that. I'd guess they don't given the expense, but I put an email in to someone else.

  4. Expert Member
    KOHTA UENO | | #6

    I assume the existing fiberglass insulation was the white vinyl facer "bag and sag" product (picture below), except only on the top half of the wall.

    In addition to the rigid foam board insulation option, another option that works just as well is closed-cell spray foam against the concrete basement wall surface (after removing the fiberglass). See link below, under Figure 12.

    BSD-103: Understanding Basements

    It might be easier to shoot "through" the framing, rather than "threading" rigid foam boards. Of course, there are environmental issues with spray foam, but the same applies to plastic rigid foam insulation. Also, it is not that great of a do-it-yourself option--it would require a *lot* of Froth Paks.

    Lastly, the concept of insulating only the top half of the basement wall is not all that irrational. The greatest heat loss occurs through the top of the wall, where it is exposed, and only "shallowly" buried below grade. See infrared image below. Note the way that the "line of cold" follows where the window well occurs.

    1. IndyGopher | | #7

      Thanks for the options and photos! The top-half insulation is actually unfaced fiberglass batts with no vapor barrier at all. Here's a link to a couple photos of what my basement looks like at the moment:

      I've read a lot today and seen spray foam mentioned multiple times. I'm definitely considering it despite the cost and, sadly, environmental impact. Seems EPS is a more cost-effective and green option, so I'm also looking into that if I can stomach removing the fiberglass and "threading" the boards in as you mentioned. I'm just sick over not taking care of this prior to framing, but live and learn....

      1. Expert Member
        MALCOLM TAYLOR | | #9

        I bet you could quickly knock out quite a few of the studs and replace them afterwards. Although the wiring goes through them, you can still stack them all up at one end of the wall.

        1. IndyGopher | | #11

          Good point. I'm going to experiment with threading a piece of EPS in there today and then make a decision on what I might need to remove. Hoping it's easier than I think, but I guess we'll see.

          1. Expert Member
            MALCOLM TAYLOR | | #13


            You may only need to remove one stud per wall. Cut the nails at the sill plate with a reciprocating saw and it will come out easily.

  5. Andrew_C | | #8

    Idea - buy a single sheet of the rigid foam insulation that you plan to use, at the appropriate thickness. Try "threading" it into position behind the stud walls that you have. It might be easier than you think. Also, some sheets come pre-scored so that you can snap them into smaller pieces, say 2' x 8'. Personally, I'd avoid the spray foam if at all possible.

    When you're done, you'll love having a comfortable basement that doesn't make your house smell like it's connected to a moldy/mildewy crawlspace.

    Now is also the time to think about whether or not you're going to insulate the may have to height adjust a number of things...

    1. IndyGopher | | #10

      That's a great idea, Andrew. I'm headed out today for some plumbing supplies anyway, so I'll grab a sheet of EPS. So I'm assuming I should remove the fiberglass already installed so that I can use full sheets of foam board on the walls from top to bottom? The fiberglass is attached with masonry screws or nails through big plastic washers. Would I need to remove all those as well? Seems like a huge job...

      As for the floor, I think I'm going to pass on that. I already had to drop the ceiling a foot in multiple places, so I'm hesitant to lost anymore headroom. My plan is to use LVP directly on the concrete as I've heard many people have good success with it being a totally waterproof material. Thoughts/advice on that are welcome as well!

  6. MattJF | | #12

    It looks like your wall is set out a reasonable amount. I don't think you will have much issue getting foam board back there after ripping out the fiberglass.

    Check Craigslist for reclaimed foam option in your area.

    1. IndyGopher | | #14

      Thanks, Matt. I'm starting to feel more confident as well. I think I can do this! I found a place about 90 minutes from me that sells reclaimed foam, so I emailed them for info. I've seen that tip on this site a few times and it's definitely a good one! Hope it works out, otherwise I'll go with 2" EPS from Menards it looks like.

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