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What are the drawbacks to a partially filled stud cavity with BIBS to reduce damage to drywall from cellulose?

Irishjake | Posted in General Questions on

I have no experience with dense pack fiberglass, and many claim it is equal to cellulose in insulating performance. I realize there are other attributes to consider, but I’m focusing my question on a specific problem or rather concern.

So my drywaller is concerned with our choice to use dense pack cellulose in the stud cavities. He says that it leads to many callbacks, due to screw pops, cracking etc. He says this happens, even in old homes that are being remodeled where the wood is dried out, and not moving like it does inherently in new construction He is a very reputable guy, and does extremely good work, so I believe him….

The insulation contractor states that the drywall contractor is correct, and there are known issues with dense pack cellulose causing problems with drywall install afterwards. He subsequently encouraged me to use dense pack fiberglass instead and guarantees that BIBS won’t bow the drywall, and shift downwards due to gravity like dense pack cellulose does. We discussed wet blown cellulose, but he isn’t a fan of “the moisture specifically”.

I then discussed this with my drywaller, who further encouraged me NOT to use ANY dense pack product, or if I was dead set on it to only partially fill the cavity since I have more than enough exterior insulation.

Side note: We weren’t going to fill the walls at all, but were encouraged to by folks who have built using just exterior insulation. They stated that the walls acted like a drum because they were empty, and ended up having to blow-in cellulose after the fact to correct the sound issue.

I’m building two structures (a house and a barn/workshop/garage/apartment). The garage/barn has 2×6 walls, and the house has 2×4 walls. The garage has 5.5″ of EPS exterior insulation and the house has 8.75″ of EPS exterior insulation. The air barrier is outside the sheathing.

So even on the garage if we fill the 2×6 walls we are at a 50/50 ratio of interior/exterior R-value which in Zone 6a I feel ok about. The house we are at an acceptable ratio too. My concern is primarily with a cavity only partially filled with a fiberous insulation. Never done it, should I be concerned? Is there something I’m missing here? Do you have experience with dense pack vs. drywall – who wins?


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  1. GBA Editor
    Martin Holladay | | #1

    The problems that your insulation contractor and drywall contractor fear are not real problems -- assuming, of course, that your insulation contractor is competent and experienced. Here is a link to an article with lots of in-depth advice on installing cellulose insulation: How to Install Cellulose Insulation.

    Due to a series of historical accidents, some areas of the country have dense concentrations of experienced installers of cellulose insulation, while other areas of the country seem to lack any insulation contractors with cellulose experience. It's not worth trying to figure out why this is so; it's just a fact.

    In areas where insulation contractors are unfamiliar with cellulose, rumors are rife, and insulation contractors tend to dissuade customers interested in using cellulose with scare stories they have heard or invented.

    Of course, some of these scare stories may be real. The reported disasters may be incidents that occur when poorly educated insulation contractors try to install cellulose without proper training.

    Here's the bottom line: If your insulation contractor doesn't want to use cellulose, you probably don't want to try to convince that contractor to use cellulose. There is a strong likelihood that the contractor you are talking to doesn't have enough experience to give you confidence.

  2. Irishjake | | #2


    Yes understood. I'm here in NH, and I'm under the impression that there are lots of dense pack installers in the area. So I will look around a bit. It was my drywall installer though that initially had the concern. It was the insulation contractor who (quite surprisingly) only validated his concerns.

    Can you provide any insight (i.e. drawbacks) to only partially filling a stud cavity with insulation? Like if only 3.5 or 4" was installed instead of 5.5" in a 2x6 cavity. I know they install foam only a few inches thick all the time, but are there any drawbacks to the fibrous insulation only being partial thickness?

  3. GBA Editor
    Martin Holladay | | #3

    If you want to use thin fibrous insulation (fiberglass or Roxul) in a deep stud cavity, you need a way to support it to keep it in place. You could use those steel spring-wire thingies that are used to install batts in joist cavities, I suppose -- like these:

  4. STEPHEN SHEEHY | | #4

    Brad. I think if a cavity is only partially filled with blown cellulose or fiberglass, you'll end up with settlement and poor insulation performance. The point of dense packing is to assure the whole cavity stays filled. Cellolose is pretty fluffy until it gets packed.

  5. Irishjake | | #5

    The insulation contractor said that the netting or Membrain just gets installed on the sides of the studs and not on the outer edge, so there are no issues with how to support the dense pack fiberglass.

  6. Irishjake | | #6


    I've had concerns about that too. The insulation contractor didn't seem concerned with settlement of the dense pack fiberglass, either in a partially filled or completely filled cavity. He did elude to the fact that in fact cellulose, even when blown-in at the correct pressure/density, can settle. So much so that he guarantees the fiberglass and not the cellulose. I wonder if it is because of the weight of the cellulose vs. the lighter weight of the dense pack fiberglass.

  7. iLikeDirt | | #7

    Just fill the whole stud cavity with mineral wool and be happy. No muss, no fuss, no drywall issues, no worries about incompetent contractors screwing stuff up, fast installation, DIY installation possible, and better R-value too.

  8. Irishjake | | #8


    Mineral wool ideally is my first product choice for lots of reasons. It isn't available in dense pack though (I don't believe).

    Here is the deal with ANY batt insulation, even the experts and folks with years of experience doing it well, have an extremely hard time getting it right. so much so, that I'd wager never. Whether it's just slightly too small or too big, compacted too much, or too little, too long or too short, there are always problems - never mind the blocking, wires, outlets, and plumbing getting in the way. Dense pack is most certainly the absolute best way to seal a cavity around all the obstacles and variations in stud cavity widths. It expands and contracts against seasonal movement, unlike spray foam. The only concern I have is the settlement - oh and if there are any issues with the fibrous insulation not being directly against the drywall or interior wall sheathing (like are there air issues with having this space)

  9. iLikeDirt | | #9

    The products really are not comparable. Fiberglass batts are hard to install right because they're so floppy and squishy, and they perform poorly unless installed perfectly. Mineral wool batts are sturdier and easier to install right in the first place. You cut the batts to size with a bread knife and friction-fit them in place. It's incredibly easy, and much faster than fiberglass. If you're worried about installer thoroughness, do it yourself and save some money. It's really easy and fast.

  10. Irishjake | | #10


    I'm not talking about batt insulation. I'm talking about dense pack insulation. I agree wholeheartedly, if your choice is batt type insulation, mineral wool is an excellent product. I've installed lots of it, so I can surely appreciate all you had to say in regards to it and how much fiberglass batt insulation basically sucks!

  11. iLikeDirt | | #11

    Definitely. But I guess I'm not sure why, if you like mineral wool batts so much, you're not considering them simply because they comes in the same basic form as a thoroughly inferior insulation (fiberglass batts). It seems like what you have chosen instead needs to be professionally installed and the professionals you can find don't express confidence in their ability to do it!

    It seems like you're considering blown-in dense-packed insulation to alleviate the problem of incompetent batt installation. But the installers you have found for the alternative product seem to be incompetent as well! Life is trying to give you a message: install the product you actually want, but do it yourself. Then you can achieve whatever level of installation quality you want. And you can use the superior product that you already like and have experience with, and is easily installed! You'll probably save a bunch of money too. Seems like a total win if you have the time needed to do the job yourself.

  12. Irishjake | | #12


    I appreciate your passion! Clearly, fiberglass batt for exterior wall or roof installation absolutely sucks, let's get that straight. I'm not considering blown-in dense pack insulation (fiberglass or cellulose) though to alleviate the problem of an incompetent batt installation. My installer is plenty confident and experienced in the install of both cellulose and fiberglass dense pack insulation. He just admits, which many might not, that there are (or can be rather) issues with dense pack cellulose having a negative effect on drywall installation or performance after install.

    Dense pack cellulose, fiberglass or any other dense pack product is far superior to batt installation though, even mineral wool. There are numerous reasons. It is simply impossible to seal a stud cavity, rafter cavity with batt insulation effectively (mineral wool or fiberglass). Expert or not, and any batt installer who tells you otherwise is lying. No offense to insulation installers who think otherwise, but there are just too many variables and other objects within the insulation cavity to ensure a good end product.

    Like I said above: (Here is the deal with ANY batt insulation, even the experts and folks with years of experience doing it well, have an extremely hard time getting it right. so much so, that I'd wager never. Whether it's just slightly too small or too big, compacted too much, or too little, too long or too short, there are always problems - never mind the blocking, wires, outlets, and plumbing getting in the way. Dense pack is most certainly the absolute best way to seal a cavity around all the obstacles and variations in stud cavity widths. It expands and contracts against seasonal movement, unlike spray foam.....)

    If only they made mineral wool in a blown-in dense pack medium, it would be the best I believe.

  13. iLikeDirt | | #13

    It's true that blown-in dense-packed cellulose (or fiberglass) is inherently better at fully filling a stud cavity than any batt insulation. That's one of its strengths. But there are drawbacks. As you've discovered, one of them is the potential difficulty finding someone confident and qualified enough to do the job properly. Another disadvantage is a lower R-value per inch: mineral wool is about R-4.2, higher than the 3.7 or so. Other disadvantages include being flammable, settling over time if the installation is less than perfect, hindering future renovations in the wall, and some others.

    You'll have to decide for yourself which option is the most attractive given the local prices, materials availability, supply of contractors willing to do the work, etc. If it were me, and I couldn't find contractors who felt confident in the work, I'd install mineral wool myself, to my own exacting standards. But I'm not you, and you'll need to make your own decision! Maybe the local tradespeople are just behind the times and everything will be fine. Or maybe prodding them to go outside of their comfort zones will result in shoddy road that will need to be re-done.

  14. Irishjake | | #14


    Thanks for the dialog. I wanted the mineral wool for the inherent flame retardancy (my full-time profession is a firefighter, so that is an obvious choice I would make!), and when I made the decision to go with interior insulation also, I had originally decided to install the mineral wool batt myself. I found out recently though that blown-in (dense pack) insulation is actually fireproof too. You've got to have oxygen, and the dense pack of the insulation prevents fire spread. (Supposedly, the IRC/IBC is going to change to reflect this eventually).

    So with the ability completely fill voids, and around obstructions, I feel that the dense pack is a good choice. I also don't need the whole r-value of the mineral wool, because that would push the ratio of interior insulation to exterior insulation R-values in the wrong direction.

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