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Musings of an Energy Nerd

How to Install Cellulose Insulation

Insulation expert Bill Hulstrunk provides detailed instructions for blowing loose-fill cellulose on attic floors and dense-packing walls

A 3-inch hose works fine for loose-fill cellulose. The fastest way to move cellulose to an attic floor is through a big 3-inch hose.
Image Credit: National Fiber

In some parts of the U.S. — notably northern New England — cellulose insulation has been widely used for more than 30 years. In other parts of the U.S., however, cellulose insulation is just beginning to gain traction.

Of course, cellulose insulation is installed with different techniques than those used to install fiberglass batts or spray foam. To help explain these techniques to builders who are unfamiliar with cellulose, we decided to interview Bill Hulstrunk, the technical manager at National Fiber, a manufacturer of cellulose insulation in Belchertown, Massachusetts.

Hulstrunk has worked as an insulation installer, an energy auditor, a weatherization program director, and a trainer. He has presented workshops at national conferences on a variety of topics, including the design of superinsulated buildings, air-sealing techniques, insulation performance, pressure diagnostics, and thermal imaging.

Q. What type of equipment is used to blow cellulose?

Hulstrunk: If you are going to be an installer, you need to own your own blowing equipment, which typically costs from $5,000 to $10,000. We don’t recommend that our installers use rental machines.

These machines will be reasonably sized. Typically an installer will show up in a box truck or pulling a trailer. The equipment draws from 15 to 30 amps, depending on the machine. The 15-amp machines can be plugged in, but the 30-amp machines need their own generators.

Q. What is the most important thing to remember when installing loose-fill cellulose on an attic floor?

Hulstrunk: Since you are installing the insulation at a lower density, be sure you do all of the necessary air sealing work beforehand. Air sealing is critical. When homeowners say, “I don’t have enough money to do both air sealing and insulation,” I tell them, “Then it’s better to wait until you have enough money to do the air sealing — otherwise the insulation…

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  1. 5C8rvfuWev | | #1

    thanks again
    This blog entry has answered more questions than I knew I had. Very much appreciated, especially as I go forward and anticipate or set expectations for contractors.

    Joe W

  2. wjrobinson | | #2

    Another very very good blog
    Another very very good blog Martin. You consistently deliver what we need to know to do our best work.

  3. kevin_in_denver | | #3

    "The argument we make is that
    "The argument we make is that when you consider the combination of the dense-packed cellulose and the drywall, the assembly is air-impermeable."

    "I tell them, “Then it’s better to wait until you have enough money to do the air sealing — otherwise the insulation doesn’t make any sense.” "

    Martin, do you think this is a contradiction? Could you have Bill explain it ?

  4. user-917907 | | #4

    Thanks, Martin and Bill, for
    Thanks, Martin and Bill, for a very helpful blog.

    Kevin, the air-sealing comment pertained to a loose-fill attic, while the air-impermeable comment had to do with dense-pack walls or ceilings.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Kevin Dickson
    I agree with Jack's comment. Obviously, attention to airtightness improves the performance of any assembly.

    The ceiling below an unconditioned ventilated attic tends to be leaky, unless attention is paid to sealing air leaks. This air-sealing work is particularly important with older buildings, where holes can be large enough to swallow up a small child. So of course Hulstrunk is right that it makes no sense to blow insulation on the attic floor of an existing house unless air sealing work has been completed.

    The second case -- negotiating with a building official about whether it is permissible to insulate an unvented cathedral ceiling with cellulose -- is obviously a new-construction situation, not a retrofit situation. In such a situation, the builder has control over the airtightness of the drywall. I agree with Hullstrunk that, with care, it's possible to make a drywall ceiling airtight. It certainly behooves the builder to do so, because if the ceiling assembly leaks air, the level of risk rises -- and the builder will be at risk of facing an expensive callback.

  6. tXWMcUDnxx | | #6

    Best product I have used to date.
    I can’t speak highly enough about the national fiber product. We started installing their product over this past summer and fell in love with the product right away. The project in question was in Hoboken and we had to dense pack a 16” cavity with cellulose. The thing that caught my attention right away was the lack of dust and how cleanly we were able to fill the cavities. At the end of the project we found that we have 30 bags left that we didn’t expect to have. Using less product is always a nice plus!

    This past fall we visited their facility for a training class and to see how the product was made. I was surprised to see that they exclusively use over print materials like news papers and phone books. It’s good to know that the New Your times, Boston Globe, and some of the less reputable New York papers we being put to good use. I was hoping for an overdraft playboy or two, but no luck. FYI that was a joke.

    If you have a distributor in the area I highly recommend trying their product. We are building a passive house and we just placed an order for three truck loads. If you are interested in reading about our passive house project visit

    Feel free to call me if you have any questions about their product or how to install cellulose insulation.

    411 Energy Services LLC

  7. kfield | | #7

    Reasons to use that brand
    I tried a cellulose product that is manufactured about 20 miles from us and thinking that they were all the same was a mistake. My installers complained about plastic bags and inconsistent texture while blowing. In addition to having Mr. Holstrunk available for tech support, the product is great. Dust is almost nil and we havent clogged a hose in weeks. I will happily pay more for the shipping from CT. to PA because it allows me to give my customer a better job and charge for it accordingly. Garbage in=garbage out.

  8. WEG | | #8

    Great Article
    Really enjoy the tricks of the trade section. Still trying to work out setting on machine when dense packing walls. Gate closed 50% with air reduced to 70% of max. Kendle 425. Any suggestions?

  9. MZmLTh8Y9m | | #9

    Walter- contact me, Chris White at 413-531-4308, to set up a machine optimization visit. As long as you are using National Fiber, there should be no problem figuring out what to do to get the results you want (I work for the same company Bill Hulstrunk does).

  10. GBA Editor
    Martin Holladay | | #10

    Bill Hulstrunk responds to Walter Gayeski
    [Bill Hulstrunk sent the following response to Walter Gayeski by e-mail:]

    With the Krendl 425, the air should be set at 100% and the feed gate closed to approximately 25% depending upon the size of the installation hose being used. The bag coverage chart can be used to confirm proper installation density of 3.5 lbs/cu. ft. If not enough bags were used, than close the feed gate more, reinsert the installation hose and reinsulate the cavity to increase the installed density to achieve a dense pack.

    Bill Hulstrunk

  11. jklingel | | #11

    very good
    I, too, learned from this. Thanks very much for that. I will have to check on whether or not that particular cellulose is available here. john

  12. dankolbert | | #12

    Another vote for Bill
    I've known Bill for a while and he's my go-to guy for cellulose questions. Thanks for the piece, Martin.

  13. uG7QG8xSq2 | | #13

    Comprehensive Cellulose Article
    This articles matches up well with Community Environmental Center’s experience installing cellulose (mostly damp-spray) in New York City residential buildings. It definitely took trial and error before we came to the right mix of installation strategies, but we’ve been enjoying great success with both home performance and weatherization jobs. We recently wrote an article on cellulose insulation from an advocacy perspective (much less comprehensive on the installation details!) that you can check out here:

  14. user-884554 | | #14

    To Raymond Evangelista: 30 "left over bags"?
    Really, a few may be explainable, but 30? If your customer reads this blog he might be asking for an explanation!

  15. albion10 | | #15

    Dense pack R-value is higher?
    Great article. Thank you for the information. I am confused -- how does dense-packed cellulose have a higher R-value then loose fill? Is it not the air spaces that influence the R-value?

    Oh, this sounds way dumb, but reading the labeling on cellulose, how can I tell if it has borates in it? Lowes sells "Greenfiber" but nothing on the bag about additives for insects or fire resistance. Their web page has a different package stating it's got borate on it. (I thought all of it had borate.)

  16. GBA Editor
    Martin Holladay | | #16

    Response to Tim Miller
    The R-value per inch of installed cellulose generally increases as installed density increases -- up to a point. Exactly where that point is depends on the characteristics of the cellulose manufacturing process. Modern "fiberized" cellulose is manufactured using a process that results in smaller pieces of material than older hammer-mill manufacturing processes; this "fiberizing" results in better thermal performance.

    You'll get slightly different R-value information depending on which manufacturer you talk to. According to one source, loose-fill cellulose has an R-value of R-3.8 per inch, while dense-packed cellulose (at 3.5 pounds per cubic foot) has a slightly lower R-value of R-3.65 per inch.

    However, according to Bohdan Boyko, a technical rep from U.S. GreenFiber, the fiberized cellulose introduced in 1990 has about the same R-value per inch whether it is installed as loose-fill or dense-pack. The main advantage of dense packing is NOT an improvement in R-value; rather, it is a reduction in air infiltration rates.

    Dan Lea, the executive director of the Cellulose Insulation Manufacturers Association, has a slightly different take from Boyko. He told me, "The guys at ORNL and R&D Services ... [performed] research that we paid big bucks to do. Their conclusions: at nominal settled density, cellulose R-value is R-3.6 to R-3.7 per inch; at wall density it is R-3.8 to R-3.9 per inch. They base this on studies of multiple materials."

    Dan Lea also sent me an e-mail in which he noted, "It is clear that the R-value of current fiberized cellulose products increases with density up to a point somewhere between 3.3 and 4.0 pounds per cubic foot." So if you are installing the product at a very high density, you will probably see the R-value per inch start to drop off.

    Concerning your final question: I'm fairly sure that the Greenfiber cellulose sold at Lowe's includes borates.

  17. tXWMcUDnxx | | #17

    To Raymond Evangelista: 30 "left over bags"?
    I based my number of bags on how much green finer I would have needed. When I use green finer I find that I normal y need 14% - 20% more then they recommend. With the national fiber product I needed a little less than they said I would need.

    Trust me when I tell you the job was done right. I don’t take a job if I don’t think I can do it correctly. I also scanned the project with the infrared camera when we were done. The infrared camera doesn’t lie!


  18. Starbright Steve | | #18

    coverage charts
    Most of my work is retrofit of existing buildings but in the last year, I have blown some new attics. On all of these jobs, I have used 18 to 20% more insulation than the coverage chart called for. My supplier has recently had their material recertified but the coverage chart changed very little. These blows ranged from R-30 in a commercial building to R-50 and R-60 in residential. I never point the hose up at 45 degrees and I constantly measure carefully. What do you think is going on here? Am I to blame?

  19. GBA Editor
    Martin Holladay | | #19

    Response to Steven McCarthy
    Re-read Bill Hulstrunk's answer to the question, "How much settling should an installer expect?"

    In his answer, Bill said, "The settling chart on the bag assumes that you hold the hose at a 45-degree angle and shoot it upward." But you just told us that you don't do that. So the coverage chart is likely to be inaccurate for installers like you.

    You seem to be installing the insulation at a greater density than the chart assumes. There's nothing wrong with that -- in fact, the cellulose won't settle as much in your attics as in attics insulated by your competitors -- but your method uses up more bags.

    If you are willing to risk the possibility of unexpected settling, you could try to install less cellulose next time, but I think it's safer to give the customer a little more than expected rather than risk installing too little.

  20. user-788447 | | #20

    dens pac install from exterior
    Much appreciated article.

    My office has designed a retrofit scenario where we would nail I-Joists vertically over air tight re-sheathing of a 1930s stud wall assembly. Currently we are considering the 9 1/4" I-Joists bays be filled with dens pac cellulose.

    In this article Bill Hulstrunck mentions two things that informs how we should do this:
    (1) "When it comes to thick walls, it’s all about density. One thing I have learned over the years is that when we get to thicker walls, we have to increase the density a little more — to just over 4 pounds per cubic foot for a 12-inch thick wall, and to as much as 5.1 pounds per cubic foot for a 27-inch-thick R-100 wall."
    (2) [Concerning dens packing behind drywall] "Another problem: you are injecting the cellulose pneumatically, so the wall needs air relief. If the wall is too tight, the installer can mistakenly think he has reached the proper insulation density when there is really a problem with air relief."

    We were thinking of spacing the I-Joists 32" apart and sheathing with a fiberboard or exterior grade gypsum sheathing and then installing the cellulose through the sheathing. However in this scenario maybe the density of the dens pac needs to be increased above 3.5lbs/cf and it would be difficult for the installer to achieve the proper density installing through a hole in the rigid sheathing.

    I'm wondering would Bill's recommendation then be for the 9 1/4" I-Joists to be spaced at 16" O.C. and the dens pac installed through the InsulWeb prior to sheathing? (Note we are trying to cut costs by installing the I-Joist @ 32" O.C. and create a 16"O.C. nail base for lap siding with 2 layers of furring strips the first horizontal across the I-Joists and the next vertical for a drainspace).


  21. GBA Editor
    Martin Holladay | | #21

    Response to J Chesnut
    First of all, your plan to install the I-joists at 32 inches on center, sheathe with fiberboard, and insulate with dense-packed cellulose won't work. Even at 24 inches on center, your fiberboard can belly.

    You should install your framing at 16 or 24 inches on center -- no more -- and use Insulweb so you can see what's going on. Then when the insulation work is done, and the cellulose bellies have been rolled, you can install your fiberboard.

    I just visited a job site in Washington state where they tried to dense pack cellulose through fiberboard sheathing. Some of the fiberboard bellies were 3/4 inch proud -- enough to touch the back of the siding (the job had a 3/4-inch rainscreen gap). Moreover, the installers had a lot of density problems, and they ended up drilling a zillion holes to get enough cellulose in there. Conclusion: "We're never going to do that again."

  22. Starbright Steve | | #22

    coverage charts
    Thanks for your response. I did read that coverage charts are based on cellulose being blown up at 45 degrees. I would add, that no one installs cellulose that way. No one! Including the person in the image at the beginning of this article. I believe that manufacturer supplied coverage charts for open blown cellulose are grossly inaccurate and not helpful for any contractor that wants to be fair to customers, legal and competitive. I suggest to contractors that they develop their own coverage charts for common insulation blows in new construction by going back to check on settled depth. In my case, Blowing a quarter ton of cellulose EXTRA and the time to do it is a tad too much of a giveaway.

  23. jarpicGuHj | | #23

    To Insulweb or not to Insulweb
    I'm curious about the pros and cons of dense-packing using Insulweb versus just using the drywall as the cellulose container and leaving a 12 inch gap in the middle of the installed drywall in which to blow the cellulose -- i.e. the method described by Riversong in a previous article (and also in an old Fine Homebuilding).

    I can see the benefit of using Insulweb in terms of visibility into the completeness of the job and also a sense of the denseness, versus the opaque nature of blowing behind drywall.

    But it would seem like a much increased cost to install Insulweb on all of ones walls, both the added material cost (mostly the Insulweb) as well as the extra time to properly install it.

    And I feel like I will have less to worry about with the cellulose bulging if I install it behind thick (5/8") drywall than if I install behind Insulweb and then have to roll out the bulges before installing drywall. I could well be wrong about this, and would love to hear if that is the case. Which method best controls the "bulges"?

    Thanks for a great article! Really useful.

  24. GBA Editor
    Martin Holladay | | #24

    Response to Robert Dickinson
    I think you have adequately summarized the advantages of using InsulWeb. If you want more perspective on your question, you are likely to get more answers if you post it where more readers can see it -- on our Q&A page:

  25. jnarchitects | | #25

    2 Questions
    1. Is Insulweb needed with a damp spray application?

    2. Is there a way to confirm the appropriate density of a dampspray installation?
    I.e. should you be able to compress it with your hand?

  26. GBA Editor
    Martin Holladay | | #26

    Response to Chris Harris
    Q. "Is Insulweb needed with a damp spray application?"

    A. No.

    Q: "Is there a way to confirm the appropriate density of a damp-spray installation -- I.e., should you be able to compress it with your hand?"

    A. Good question. I assume the answer is yes; you can always feel the density by holding a scrap of housewrap or poly against the exposed face of the cellulose and pressing with your hand to see if the cellulose feels like a firm mattress. But I will e-mail your question to Bill Hulstrunk.

  27. Bill_Hul | | #27

    Damp Spray Application Question

    In vertical damp spray applications, the starch in the wood fiber allows the material to be self supporting if applied at the 3.5 lbs/cuft density. For overhead applications, slopes and flat ceilings, than the Insulweb will need to be stapled up in order to support the dense pack cellulose overhead.
    In both damp spray and dry dense pack applications, the cellulose should have the feel of a firm mattress when pressed with the palm of your hand. In damp spray applications, low density, too much moisture, or poor spray technique can cause small gaps "sags" to develop, usually overnight, between the top plate and the top of the cellulose. If this is observed, the installer will need to recompress the cellulose below at least six inches and than respray these areas at a higher density. Spray applied cellulose is more sophisticated from an equipment and applicator perspective and is best performed by those installers who spray frequently with large equipment specifically designed for damp spray cellulose, i.e. NuWool 1600, Krendl KS-260 or equivalent.

  28. saucymonkey | | #28

    Hello, I'm getting closer to
    Hello, I'm getting closer to insulating the addition I put on my house this summer and I have a few more questions, I have an unvented cathedral ceiling made of 2x12's 24 o.c supporting a SIP made of two layers of 1.5" extruded foam between a sandwich of 5/8 advantech.

    1) Given that the ceiling cavities are free of wires and electrical fixtures, could I achieve a comparable level of insulation using fiberglass batts rather than cellulose, since that would be much easier for a DIYer like myself to install? Would fiberglass batts be better for the ceiling in some ways?

    2) Does the aluminum roller trick work on the ceiling or is that a good application for lip stitching?

    3) Are there any products like insulweb that are available in smaller quantities? When I inquired with my local supplier the smallest quantity they had was about 4x what I needed. Granted it was only $120 a roll but still I hate to buy more than i need if I can help it.

    4) I see now in the latest addition of FHB that I messed up when choosing my 3" foam thickness for my climate zone 6 house. All the info I had at the time (J. Lstiburek fhb 221, and C. Wing The Visual Handbook) showed an R10 layer of foam over the sheathing on a hybrid roof insulation plan. They seemed to be only concerned mostly with the final R value. Why does the roof require more insulation than the walls to prevent the dew point form being reached? Is my roof doomed to premature failure?

    Thanks so much for all your help so far, -Chris

  29. GBA Editor
    Martin Holladay | | #29

    Reply to Chris Steiner
    In your climate zone (zone 6), the code requires at least R-25 of foam above your roof sheathing for your approach to work. Have you talked to your local building code official about your plan?

    As far as I know, the reason that the code is more conservative about roof dew points than about wall dew points is that roof failures are more common than wall failures. Because of the buoyancy of warm air, there is more air movement through roofs than walls.

    I can't really recommend your approach, since it doesn't meet code. However, if your installer does a very good job dense-packing the rafter bays, your roof will probably be fine.

    Concerning whether you will get the same performance with fiberglass batts as dense-packed cellulose: no, you won't get the same performance. Although perfectly installed batts might have the same R-value, they will always be more air-permeable than dense-packed cellulose. The fact that fiberglass batts don't slow down air movement will make them more risky in your case.

    If I were you, I would spend the $120 and just buy the Insulweb. Are you doing the dense-packing yourself? If so, I hope you have a powerful blowing machine.

  30. saucymonkey | | #30

    My local code officer is
    My local code officer is about 70 years old and really only cares about the footprint of the addition. If I tried to describe what I was doing he wouldn't understand/care. I called him today to set up an appointment to have him review my work and he said he had totally forgotten that I was over here building something. I had him come look at my framing in July and he seemed confused as why I was having him look at my construction project. I live on the mid coast of Maine. If I went inland a few towns I'm pretty sure I could build WHATEVER I wanted without ever having to worry about the building code. For me building codes are like this inaccessible text that really doesn't apply to me anyway, except of course when it comes to the laws of physics.

    I tried to do my homework on this one but it never occurred to me that the dew point would be different for the roof. All the references I had at the time explicitly showed R10 foam on the roof deck with no mention of the foam thicknesses varying for different climate zones. I thought I was playing safe by making it R15.

    So what should I do now, should I add 2'' of foam to the inside of the rafter bays or would that just create a new moisture problem? Whats going to happen if I do nothing? Is it just that on super cold days I will get some condensation on the underside of the sheathing? If I have a dense packed rafter cavity 11 1/4" thick is there really going to be any interior heat getting to sheathing?

    I had this house insulated last year (there was absolutely none, not even the attic!) and they dense packed the rafter bays in the sloped ceiling part of the interior knee walls with cellulose. The rafters are only rough cut 2x4's (It's an old house). I was a little freaked out when I came home and saw what they had done, but my energy auditor assured me that this was common practice and proven to work. It seems like if that is OK, then what I did should be 10x better.

    As far as doing it myself, yes I was going to, I had gotten an estimate to have it done and it was way more than I had expected. They had broken it down into its parts and seemed a little outrageous to me, for example they were going to charge me $840 just to blow the 24'x14' open attic space. That's almost $600 in labor for a couple of hours of work. I can't afford to waste that kind of money on something I can do myself. They confessed that they had never done a job with insulweb before, they only did retro fits. I figured why pay them to learn when this is something that i want to know how to do anyway. But I think now I'll shop around a little for some other estimates.

    Thanks again for your prompt replies to my questions, I only wish I had read your articles before putting my roof up, Oh well, live and learn. I did it once, I guess if I have to I can do it again.

  31. GBA Editor
    Martin Holladay | | #31

    Another response to Chris Steiner
    My advice is the same: "If your installer does a very good job dense-packing the rafter bays, your roof will probably be fine." If you are the installer, and it's your very first dense-packing job, you'll need (a) a powerful blower -- not a rental machine -- and (b) a lot of self-confidence.

  32. Wnk3xdaxwU | | #32

    Cellulose weight against sheetrock in cathedral ceiling
    How do you keep the dense pack cellulose in a cathedral ceiling from excessively pillowing out against the ceiling sheetrock?

    We are building a house in western Wisconsin with an 8:12 cathedral ceiling formed from parallel chord trusses. There is room for 20" of insulation (distance perpendicular to the face of the ceiling - truss heal is over 2'), and it would seem that no matter how tight the fabric forming the cavity is stretched, the weight of the cellulose will cause it to bulge out and interfere with installation of the drywall.

    One suggestion is to staple the fabric on the inside of the truss, about a half inch from the edge, to create a space for the bulge above the sheetrock. Is this practical for the weight of the cellulose in our ceiling? Is 1/2" the right distance?

    I'm also having difficulty finding a contractor to do the work, so any references would be appreciated. The house is located in Wisconsin about 50 miles east of Minneapolis.

  33. GBA Editor
    Martin Holladay | | #33

    Response to Dennis Cornhill
    There are two possible ways to do this:

    1. Install the fabric, and then strap the ceiling with 1x4s or 2x4s. The bulges between the strapping won't interfere with the drywall.

    2. Install a continuous layer of rigid polyisocyanurate insulation under the trusses, followed by 1x4 strapping installed with screws. Cut holes in the polyiso and blow the cellulose through those holes. Patch the holes with tape or canned spray foam. (See more information on this technique in the article on this page.)

  34. Wnk3xdaxwU | | #34

    Cellulose weight against sheetrock in cathedral ceiling
    I've had three cellulose insulation contractors tell me how they propose to insulate the cathedral ceiling:

    1. Install a layer of fiberglass at the bottom of the truss cavity, staple fabric to the bottom of the trusses and dense pack the cellulose above the fiberglass. The purpose of the fiberglass is to keep the cellulose in place. I've asked via email why this is necessary in a dense pack installation; awaiting his response. He is confident that the bulge below the trusses formed by the pressure of the dense pack cellulose will not impede the installation of the sheetrock.

    2. Install sheetrock up to within a few feet of the top of the ceiling. Loose fill the truss cavities up to the opening, fill the last few feet with fiberglass and then cover the fiberglass with sheetrock. He is confident that the loose fill cellulose, as it settles, will not slip down the truss cavities.

    3. Install sheetrock up to within a few feet of the top of the ceiling, using one extra screw on each joist (6 screws per joist per sheet) to help mitigate the pressure of the cellulose on the sheetrock. Install fabric over the last couple of feet by stapling inside the joists to create a small cavity for the cellulose to fill before coming into contact with the sheetrock. Pack the truss cavities to 110% (1.8#/cu. ft.) and finally complete the ceiling by installing sheetrock over the top couple of feet.

    Alternatively, the third contractor could put fabric over the entire ceiling (stapled to the inside surfaces of the trusses), but at an additional cost of a couple days of labor.

    Given that we have 21" of space for insulation, even a loose fill would settle to 18" which should still give us more than R-60. How important is dense pack for our application? Do we need the dense pack to keep the cellulose, as it settles, from slipping down the 8:12 truss cavities?

  35. Bill_Hul | | #35

    Cellulose weight against sheetrock in cathedral ceiling
    Hello Dennis,

    The installation process will vary depending upon if this roof is vented or not. Before proceeding with a non-vented roof, I would first check in with your local code official to see if he/she is comfortable with using cellulose in this application. Also, all non-vented roof assemblies must be able to dry in at least one direction, and in both directions if possible. This means if an Ice and Water Shield type product is being used on the roof sheathing, than a vapor barrier or any material acting like a vapor barrier (i.e foil faced foam board) should not be used on the inside of the roof assembly to allow drying.

    In a non-vented cathedral ceiling of this rafter depth, you should first staple up Insulweb vertically along each rafter trusses, creating individual rafter bays to make it easier to fill and provide some extra lateral support for the cellulose to be self supporting. Next, staple up Insulweb to the underside of the rafters and than strap under this with 1 x 3 or 1 x 4 strapping ever 16" of 24" o/c. In this non-vented roof case, the entire rafter cavity should be dense packed with cellulose at 3.8 lbs/cuft density (the deeper the cavity, the denser the cellulose needs to be self-supporting). I prefer using the Insulweb in this application over foam board since it allows us full quality control of the density above. At the 3.8 lbs/cuft installed density, the cellulose should be very firm when pressed against from below. The installer may need to roll any bulging below the strapping, an aluminum roller works good for this. For a 21 inch rafter bay, each 25 lb bag of cellulose will cover 3.3 sqft.

    For a vented roof cavity, you should first install the wind blocks, vent baffles up to the ridge vent. Next, staple up Insulweb vertically along each rafter trusses, creating individual rafter bays to prevent the cellulose from one bay from spilling over to the next and preventing the hose from sliding downward. A three foot wide piece of Insulweb can than be installed on the underside of the rafters down from the peak on each side. Next, dry wall can be installed on the under side of the rafters and covering the first lower foot of the Insulweb. This will leave a two foot strip of Insulweb on each side of the peak so that the insulation hose can be inserted through and dropped down, loose filling the rafter below. Once filled, the cellulose will not slide down the rafter bay.

  36. Wnk3xdaxwU | | #36

    Cellulose weight against sheetrock in cathedral ceiling
    Thanks for your help. Bill. The roof is vented.

    The process proposed by my third contractor is similar to the one you describe. There are one or two differences that I need to check out.

    First, he did not mention covering the trusses with InsulWeb to prevent the cellulose from migrating between truss bays. Second, I don't know that the fabric he uses is air permeable, like InsulWeb. He described it as a reinforced poly.

    I'm expecting that the InsulWeb covering the trusses should be installed a bit loosely to allow the pressure of the cellulose to fill the space around the truss members.

  37. Wnk3xdaxwU | | #37

    Cellulose weight against sheetrock in cathedral ceiling
    We insulated the ceilings using the process for a vented roof described in Bill Hulstrunk's message above. The insulation contractor asked us to install the ceiling sheetrock with an extra screw in each row. The contract called for 110% fill so the bays should stay close to full even after settling. The same contractor also blew dense pack cellulose into the walls.

    We are having a mild winter but even so the heating system seems to be loafing, so at least the early signs are good. I guess I'll know better when we start paying the utility bills.

    Sound insulation is also good. The noise from a snowmobile passing by within 150 feet of the house was severely muted.inside the house.

  38. ricky_005 | | #38

    Ceiling Sag - Maximum depth of cellulose insulation
    I find it hard to believe that 20 in. of cellulose in a vented attic on a 1/2" drywall ceiling with trusses 16" o.c. wont cause some ceiling sag over time....

    I have plans on doing flat smooth ceilings and very concerned that here in the hot humid Lagrange, Ga area given enough time the sheet rock will begin to sag. The home will be located 400 feet from West point lake which is a 26,000 acre lake.

    Assuming this is a problem and I believe it to be so, I plan on attaching Insulweb to the bottom the ceiling joist across the entire ceiling through out the home. Next running 2x4 furring strips 16" o.c.

    Reason for 2x is it will give the Insulweb a maximum deflection of 1 1/2 before it could touching the top side of the ceiling drywall. I have not tested the deflection of Insulweb, but will perform some test to get a rough idea of what the maximum deflection is at 16" o.c.

    Not quite sure how an 1.5" gap between the drywall and cellulose will affect the r value etc...... but I would assume it will add r-value.

    I have thought about maybe using 1/2" ridged insulation board attached to the underside instead of the Insulweb that way I could use standard 1x4 for the furring strip..... not quite sure how that would work out as the ridged insulation board is rather soft. Might make it easier leveling the ceiling boards, or could make it more difficult. Not sure if building codes would allow it also.

  39. GBA Editor
    Martin Holladay | | #39

    Response to Richard Maxwell
    Your approach -- strapping the ceiling -- is common in New England. While New Englanders normally use 1x3 or 1x4 strapping, there is no reason you can't use 2x4s.

    Make sure that the perimeter of your ceiling assembly is carefully air sealed, however, since you are creating an air gap between your air barrier and your insulation. If any exterior air circulates in that air gap, the thermal performance of your ceiling will be compromised.

  40. user-483661 | | #40

    Storage attic with cellulose
    Hi, we use our attic for storage. We just had a contractor insulate our attic. They did dense pack cellulous under the floor boards in the main parts, and then loose fill around the edges, separated by a barrier. my wife doesn't like the barrier and losing the space around the edges, and she's concerned about any health/dust issues related to going up to the attic to put or retrieve storage stuff. Is it typical to do loose packed on the sides and dense pack under floorboards this way? Any comments about this approach would be helpful.

  41. GBA Editor
    Martin Holladay | | #41

    Response to James Christy
    You wrote that the two sections of your attic are "separated by a barrier." What do you mean by a "barrier"?

    Q. "Is it typical to do loose packed on the sides and dense pack under floorboards this way?"

    A. Yes. There are no particular health concerns arising from visiting the attic occasionally. It's true that attics can be dusty. If your wife is allergic to dust, she probably shouldn't spend too much time in the attic.

  42. user-483661 | | #42

    Thanks for the quick reply!
    Thanks for the quick reply! Trying to enclose a pic, the barrier is just made of cuts of wood. Does this look typical? (they aren't done with cleanup yet).

  43. user-483661 | | #43

    Here's that pic.
    Here's that pic.

  44. GBA Editor
    Martin Holladay | | #44

    Second response to James Christy
    That looks like a good job, as long as there are insulation baffles to maintain an open ventilation channel connecting your soffit to the attic.

    The main issue raised by this photo is whether the insulation installed between the floor joists provides the R-value you need. My guess is that the R-value of the central part of your attic is too low.

  45. pvahid01 | | #45

    I live in a condo built in '85. I have neighbors to my sides and above me. I am the ground unit built on a concrete slab. It was around 10 deg difference in the winter between the floor temperature and ceiling in the coldest corners. One of the exterior walls is a kitchen wall with cabinetry. The contractor gave me a quote for dense packing the exterior walls, but is 10 degrees worth it? Will my neighbors need to insulate as well for my unit to realize the benefit?

    Lastly they said they wouldn't fill the wall with the cabinetry due to the way it could be framed, the dense packing might not fill up the entire wall, and although there is vinyl siding on the outside they wouldn't be able to insulate from that side. Is there really nothing I can do from the exterior side to alleviate the cold? I felt significant drafts from around my wall-mounted microwave and surrounding cabinetry in the winter.

  46. GBA Editor
    Martin Holladay | | #46

    There are many issues here. The fact that the floor temperature is so much lower than the ceiling temperature implies that your main problem is air leakage, not lack of insulation. While dense-packing the walls is one way to address air leakage, it may not be the best way. You should consult a contractor experienced with blower-door-directed air sealing.

    Another factor may be that the concrete slab is uninsulated. (This is a guess; I don't know whether your slab has insulation or not.) If the slab is uninsulated, the addition of vertical rigid foam at the slab perimeter can make a big difference. This type of work is probably best performed by your condo association.

    1. pvahid01 | | #47

      Thank you Martin!!

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