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Are dense-packed cellulose walls a pain to deal when updating services in a wall cavity?

mjezzi | Posted in General Questions on

I’m considering a wall assembly of 2×6 studs with dense packed cellulose. My only concern is that it will make it difficult to update services in the wall if desired years later. For instance, I imagine it would be pretty impossible to fish wire through it. Or if I need to open a wall for any reason, how difficult will it be to dense pack it back up again and patch the wall without requiring large specialized equipment?

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  1. Expert Member
    Dana Dorsett | | #1

    It's somewhat easier to fish wires through 3.5lb cellulose than to punch it through high density fiberglass batts.

    If opening up a section of wall cavity and scooping out insulation it's easier to replace it with carefully trimmed rock wool batt rather than dense pack a couple square feet of wall or ceiling, even if you happen own an insulation blower(like I do), which is something I did on a small project recently.

  2. keithhoffman22 | | #2

    I'd also add that paying for a lovely insulation installation and then letting the electrician at it with a fish tape will have a predictable outcome with fiberglass batts; your A grade install will become a D grade install.

    Dana's suggestion to use mineral wool for small scale repairs is excellent. I just taught a plumber how to do this to replace wet fiberglass during a shower valve replacement.

  3. mjezzi | | #3

    Good idea Dana! Thank you.

    I’m also tempted to not insulate the wall cavity at all and instead do a Lstiburek double stud wall. That would be the least problematic and difficukt to deal with over the long haul instead of a dense packed cellulose 2x6 + 4” of exterior mineral wool.

    But maybe that’s overkill?

  4. TimTuckerCom | | #4

    If you have a good idea of what you might want, why not design for it now (putting in conduit / more boxes for outlets / etc.)?

  5. kurtgranroth | | #5

    You might also consider the option of installing a service cavity all along the exterior wall. With that in place, there should be no reason to ever intrude into the insulated part of the wall in the future.

  6. Expert Member
    BILL WICHERS | | #6

    If you’re worried about future electric changes, you could wire everything in the insulated walls with conduit, 4” square boxes, and mud rings. This will give you a lot of future flexibility to make wiring changes with zero need to mess with the insulation. I just did this in my own house where I used spray foam in a few walls (we did the roof, doing the walls at the same time only added about $150 so it was cheaper than anything else). Fishing wire in spray foam its pretty much impossible, but with the conduit I can always change stuff except for the placement of the boxes.


  7. keithhoffman22 | | #7

    First, I'm just a DIYer. But I've tried some of these future proofing ideas. Mostly, I think they aren't worth the time. Run the wire now or forget about it. Double wall and move on if that's what you want to do.

    You left out some needed information. Are you remodeling or building new? What climate zone are you in? Are you really planning 4" of exterior mineral wool (sounds awesome but I'm wondering how your search for a contractor is going)?

    Here are some ideas for you, some of which I've tried:
    - Wire is cheap. Just pull more. Your ducts and your plumbing shouldn't be in the exterior walls anyways.
    - Run the wire on the outside of the house. I don't know what you'll need this wire for but if it isn't interior use, just put it outside. Yes, conduit is expensive and making it not look poor takes some painting and nice work by the electrician.
    - Overstrap the 2x6 wall on top of the air barrier. 2x3s on the flat work well but only give you 1.5". If you air seal the drywall, the void space is probably worth a smidge of R value. I'll tell you right now that your electrician will hate boxing in 1.5" and you'll wonder why you didn't just doublewall it. Blocking for wall surface installation can require thought or it may ruin the value of the space.
    - Double wall it but create a horizontal chase between the studs using blocking that is not insulated in the inner wall. It's a lot of work just to accommodate future wire but does protect your insulation from the electrician. Definitely more work than pulling some bonus wire and landing it in or near the panel. As soon as you finish doing this, you'll realize that without holes or conduit, pulling wire will involve cutting off a 12" strip of drywall through the whole pull and it's probably not worth it.
    - Instead of building a true inner wall, build it as a strap onto the structural 2x4 wall. You can create whatever chase you want. I question whether you can get this past an inspector as there would be no vertical studs. Also, tons of work for what? Usefulness is probably limited to partial height basement foundation walls.
    - I believe Martin has discussed placing an electrical chase at floor level before. I haven't tried this idea but find it intriguing. You'd still have to cut into your insulation to land a new outlet at a standard height.

    Personally, my fantasy would be an air tight plastic or metal raceway (perhaps 12" tall) rated to support a 2x4 inner insulation wall (perhaps metal stud like supports at 16") that mounts at floor level (perhaps it could snap over 1/2" plywood that is roughed onto the inner wall). I can't think why it would be so horrible to have outlet at 8" above floor level (just above baseboard). I'm not aware of such a product.

    The thing is that the retrofit idea involves destroying the finished wall surface, might involve removing the baseboard, definitely screws up the insulation. So whether you create a chase or not, you'll be destroying stuff.

    Here are my alternative ideas that I think might be better:
    - Design a service chase to an unfinished basement or crawl space room in the interior of the house. That way you could build the chase where it could also accommodate ducting or plumbing or electrical, all without interfering with insulation and air barriers. You'll want a 2x6 void at a minimum if you think it might need drain line.
    - Order all the wire now and pull it in. I can't imagine that if you think you need a dedicated circuit somewhere it won't be cheaper to pull it and box it now than to worry about it someday. I'm guessing the problem is you aren't entirely sure where or what the future need is.
    - Design the house with an unfinished (but sealed and insulated) crawl space or unfinished basement room over area requiring future utility work. Don't put any more of the wire in the walls than you need to (jump from outlet to outlet in the crawl etc). New work can be run to directly below the location and only the wall above needs to be disturbed. Also, most electricians and plumbers can readily fish from a crawl space to a 2 gang opening 12" off the floor without disturbing more than the 2 gang outlet. While they'll muck up the insulation right there, you'll limit the damage.
    - Use mineral wool batts instead of fiberglass or cellulose. You can cut out insulation in any space easy and restore the cut out (or easily cut a new piece to shape). Of course, you'll also have to repair your air barrier after that..
    - If you have tall ceilings, you might find it better to make a decorative small drop soffit to pull any number of things though. It won't penetrate your ceiling insulation/air barrier, it won't disturb your wall insulation, and since it isn't vertical, your drop soffit doesn't (to the best of my knowledge; i'm not an inspector) require fire blocking so pulls can be dreamy. If the future proofing is home theater or data cable related, I think this is the best option. Yeah, you'll have to get from ceiling to wall mount location if your application requires that but that's not much different than coming up from 12" either. Do be aware that drop soffits can become mouse houses so pay attention to exterior details abutting the soffit.
    - Upsize your conduit so you can pull in more wire someday. Include a pull string that you leave behind if you want.
    - If the future location is known but the wire isn't, mount smurf tube (large diameter than you think you need) to the location in question from your crawl space etc.

    Anyways, I've read a lot on GBA here about ideals for adaptability of the structure and mostly think the holey grail (pun intended) hasn't been found.

    Hope that helps.

    1. Expert Member
      MALCOLM TAYLOR | | #8


      That post is chock-full of interesting ideas.

  8. jameshowison | | #9

    For electrical boxes in shallow wiring chases (which are the same as very common "fur out for drywall against masonry", I'm told these "wide but shallow" 1 gang boxes are useful:


    Possibly also relevant:

    And, while focused on products they sell, perhaps interesting, especially "Tier 3" where the structural wall is the service cavity (because all the insulation is on the outside).

  9. Expert Member
    BILL WICHERS | | #10

    Instead of “wide but shallow” specialty boxes, just use a shallow 4” square box and a mudring. You get a lot more volume for wiring this way, and you’re using standard parts that every supply house and box store will have in stock.

    Conduit can be recessed into the wall by notching and using nailing plates to hold it in place. It’s a lot more work, but worth it in some cases. It does take some skill if you want to make bends.

    Smurf tube has been banned in some locations. No idea why.


  10. mjezzi | | #11

    Lots of great replies here. Thanks everyone!

    Keith, I really appreciate you taking the time to lay out all those options. I’m definitely a consider every option type of guy, and your thoughts made it easy for me to triage all the different ideas.

    Adding a service cavity in addition to the structural wall is not an option for me because it would increase my square footage and I’m limited on my square footage.

    I don’t have a crawlspace or attic.

    I’m a tech and optimize guy, so there’s always some sort of thing I’m going to want to modify; I know this about myself. I love refining conveniences and experiences. No matter how well I plan, I’ll get some crazy idea that will require opening a wall; especially home automation related ideas.

    So my two options are:
    1. Dense pack the service cavity. Plan really well and use smerf tubes for data wire future upgrades. If I missed something, and have to open a wall and displace cellulose, I’ll patch it with mineral wool. (Thanks for the suggestion Dana)

    2. Do not insulate the structural wall and use it as an empty service cavity. Put all insulation on the exterior of the wall. This is likely much more expensive, but by how much, I’m not sure -currently exploring... It will give me the most flexibility and simplicity to upgrade and maintain my home in the future. I’m very attracted to this approach.

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