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Community and Q&A

Attic insulation reality check

iLikeDirt | Posted in Energy Efficiency and Durability on

It’s time to get my attic air sealed and re-insulated and I’ve committed to hiring it out since the 6/12 pitch truss attic is a horrible place that I hate to work in. I’m having trouble finding people willing to blow cellulose. Everyone around here seems to want to blow fiberglass. Is this a major issue?

The first bid I’ve gotten is $3.20/sf for attic floor air sealing and R-30 worth of blown fiberglass. Sane/insane?

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  1. exeric | | #1

    I think it is. When I first got interested in this stuff a few years ago one of the first things I saw was a youtube video showing a demonstration of bulk water being poured on cellulose vs fiberglass. The cellulose shrunk drastically while the fiberglass just shed it with the water going to the bottom of the container. They then stated that fiberglass is material of choice. If you didn't know better and had an incurious mind one would actually believe the urban legend shown in that video.

    Apparently in large parts of the country the insulation blowers also have incurious minds that will latch onto the easiest, laziest explanation for anything. When I tried to find cellulose blowers locally it was hard to find. They all wanted to blow fiberglass and when pressed for an explanation they said it would handle water better. Yeah, right.

    What those youtube demonstrations were really showing is that if you have any bulk water intrusions, either through air paths or simple leaks, the first thing that will happen with fiberglass is water will reach sheathing and rot it. In contrast the cellulose will absorb it. The experiment in the video wasn't realistic at all though because the biggest problem isn't rapid bulk water intrusion that collapses the cellulose, but slow intrusion. In slow intrusion of water in cellulose the cellulose has the ability to spread the moisture to its neighbor and decrease the local density.

    The thing that gets me is that so many people who blow this stuff for a living can be so ignorant. It's very disappointing. As its said, people see what they want to see and disregard the rest.

  2. Expert Member
    Dana Dorsett | | #2

    At 10 cents/ per R-foot installed price that would be a bit expensive for fiberglass in my neighborhood, but if that also includes extensive blower-door directed air sealing it might not be out of the question.

    The wintertime performance issues of low-density fiberglass from 25 years ago have largely been resolved- it's way better than it used to be. It's not clear whether they have solved the infra-red translucency issue that cuts into cooling season performance. But even if that's still an issue, at R30+ it's not going to be anything like the comfort & efficiency disaster it was back in the pre-1990 era when R19 was code min attic insulation many climates.

    Eric- there are no rocket scientists (or building scientists) wasting their careers installing insulation, true. But that's OK- I'd hate to be paying rocket-science rates for that service. Whether it's fiberglass, cellulose or something else, it's the quality of the installation that makes it or breaks it, not the depth of knowledge of the installer. Give me somebody who air seals first, installs the insulation at a correct smooth, and even depth, with no gaps, depressions or voids, doesn't block attic vents, and cleans up after themselves and I'm a happy guy, even if the installer flunked high school math and believes that the moon landings were nothing an elaborate hoax. There are a lot of details to get right, and an installer who takes some pride in the craft of it is more important than what they know (or don't) about the relative properties of different insulating materials.

  3. exeric | | #3

    Dana, I agree that pride in workmanship is important, maybe most important. But you may be ignoring an important point. If a person just handles the basics by rote and never thinking much about it, then stuff happens. If something happens out of the ordinary to that individual, or the circumstances are slightly different from what he/she is used then that person will likely bungle it. That's because they are used to assessing things without analysis. Happens all the time. People like that are thrown for a loop easily. That's because people like that generally don't know what they don't know. It's the Dunning-Kruger effect.

    To quote Wiki: "The Dunning–Kruger effect is a cognitive bias wherein unskilled individuals suffer from illusory superiority, mistakenly assessing their ability to be much higher."

  4. iLikeDirt | | #4

    Oh that reminds me... How important is it to also do the attic floor that's over an attached garage? The garage isn't used for anything other than typical garage stuff, but it does get scorchingly hot in summer and very cold in winter. And there's a water heater in there.

  5. LucyF | | #5

    Dana has a term for that he calls it "creative idiocy". On another question about insulation and air sealing, Dana wrote, "And yet... the more idiot proof you make something, the more creative the idiots become." - See more at:

  6. GBA Editor
    Martin Holladay | | #6

    The answer to your question depends on your climate. Most garages aren't insulated. But if your garage gets "very cold" in winter, I'm surprised that your water heater is located there. Obviously, you don't want your garage to freeze if there is plumbing there.

    I'm guessing that your garage doesn't freeze, and that's why the plumbers put your water heater there. I'm also guessing that you live somewhere where "very cold" is pretty mild.

  7. DIYJester | | #7

    Nathan, I just got a quote to blow cellulose in my addition's attic about 20" deep (R60's) for about $1 per sq. ft. This doesn't include air sealing.

    In response to the moisture thoughts, why would you have bulk water in your attic anyways? And if you did have bulk water, you will have problems whether you have fiberglass or cellulose. The problem isn't the type of insulation at this point is ensuring your roof isn't leaking and ventilation is correct.

  8. iLikeDirt | | #8

    Decided to go with a $2/sf bid for R-30 fiberglass and two blower door tests (one before, one after) with blower door-directed air sealing. I'd prefer cellulose but the price was right and I got tired of trying to chase down anyone who could do it. Even if I could I highly doubt I'd be able to find anyone who could get the stuff that doesn't have ammonium sulfate in it.

    Relatedly, did you guys know that Owens Corning is the maker of Owens Corning insulation and that Owens Corning can help you save money in your utility bills using Owens Corning eco-friendly fiberglass insulation that is installed by Owens Corning trained certified expert installers who are green certified to install Owens Corning insulation? What an affiliate program these guys are running...

  9. Peter_Rogers | | #9

    I hate blown fibreglass. It seems to collapse like a souffle if you so much as look at it, and unlike cellulose it won't fluff back up to the depth it was previously. Also, lower R value per inch means that eaves will have even less insulation above the top plates (depending on construction). And wind washing is much more of an issue. I am loathe to disagree with D Dorsett on anything (extreme folly!) but in this case I think the product does matter, and it's not simply about quality of installation.
    Oh, and I forgot to mention recycled content 85% for cellulose, compared with 30% or so for fibreglass (?) that's from memory; I may have that wrong or it may be brand specific or outdated. Picked it up on one of these articles somewhere...

  10. Expert Member
    Dana Dorsett | | #10

    Newer fiberglass blowing wools aren't as easy to "fluff" to a low density/easy collapsing level as the stuff of 15+ years ago, but I suppose anything is possible if you have a dishonest contractor trying to skimp on material. Unlike the older stuff, some of the new open-blow wools (such as Owens Corning AttiCat) is bagged as granulated higher density kernels/beads of fiberglass that maintain much of that structure as it's blown, with virtually zero settling over time.

    While I personally dislike the stuff enough (on a number of basis') that I would DIY cellulose with a rental blower rather than have a contractor blow fiberglass, it's not quite as lousy a product as it used to be. As long as the blower door testing verifies the air-tightness prior to blowing the fiberglass you'll avoid the issue with infiltration dragging airborne glass fibers into the conditioned space, a common disaster when blowing fiberglass into air-leaky assemblies.

    It's not hard to find sulfate-free cellulose though- even some box stores would special order "stabilized formula" goods (designed for damp spraying, and almost always sulfate-free, and performs just as well when dry-blown), from their standard suppliers without a significant up-charge. But if you're not up for a DIY and there are no contractors handling cellulose in your area you're kind of stuck. In my area you almost have to seek out sulfated cellulose when buying through materials distributors catering to contractors, but the box-store retailers catering to DIYers seem to only stock sulfated goods.

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