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Community and Q&A

High-Density Fiberglass Batts vs Dense-Pack Dry-Blown Cellulose

N3y2cNDw2m | Posted in Energy Efficiency and Durability on

I had no idea until I stumbled across a website last night discussing super-insulated residential construction that such a product as Owens Corning high-density fiberglass batts actually existed. I’d assumed that fiberglass in all of its forms was pretty much an antiquated material. I’d like to better understand the pros and cons of using high density fiberglass batts as an insulation component of an exterior wall (or cathedral ceiling) as opposed to a similar thickness of 3-1/2 lb dense-pack dry-blown cellulose.

What are the benefits of one over the other? In Passivhaus envelope construction, does one have a clear advantage? Is one likely to better retain its insulative properties over time? Are there any inherent red flags with either?


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  1. user-1012653 | | #1

    You still have the same issue with high density batts as you do with any batt...that is its only as good as the install. Batts are not easy to get around electrical boxes, pipes, wires, windows, etc. There are far more gaps vs a blown product, either cellulose or fiberglass. "new fiberglass" products are getting to be fairly impressive vs the typical batt and other old fiberglass products. For example Spider blown fiberglass can have an r value of around 4.2 when netted and blown inside a wall cavity. All insulators I have talked to MUCH perfer blowing thick amonts of fiberglass vs cellulose. It is also lighter then cellulose (both a pro and con depending on who you ask) so getting r60 and 70 in the attic can be supported by 5/8" gyp. The new blown fiberglass products, such as the Spider has very similar air ratings as does cellulose. Typically cellulose has been superior at reducing air movement and circulation inside the cavity.

  2. user-626934 | | #2

    The only benefit of fiberglass batts over cellulose is it's up front cost....and that's only with a standard sub-optimal install. A batt installation that actually follows the manufacturer's directions should cost every bit as much as blown-in cellulose.

  3. Billy | | #3

    I agree with Jesse on the dense-pack Spider blown fiberglass. It is very fine like cotton, not like the thick fiberglass strands you see in batts. Although cellulose can be pretty great and has benefits such as using recycled content, if (or when?) you get water inside a wall in a tight house you will be better off with wet fiberglass than wet cellulose.


  4. N3y2cNDw2m | | #4
  5. N3y2cNDw2m | | #5

    That's very cool. I definitely like the idea of using this as the primary insulation in the exterior walls (along with an inner wall assembly filled with Roxul ComfortBatts.) Do you have any idea how thickly it can be blown in without being a nightmare to work with? I've only seen references to the stuff blown into 2 x 4 and 2 x 6 wall cavities. Could it be effectively be blown into 12" deep wall cavities (in place of dense-pack cellulose) in a Passive House envelope without compromising any of its R-value?

    Any idea how expensive this stuff is and how readily available it may be?


  6. user-1012653 | | #6

    around here, cellulose and blown fiberglass is the same cost now since cellulose had a jump in price.
    I work with a lot of subs, and every insulator I have asked the question to hands down prefer the fiberglass. Reasons are its lighter, much faster to blow, way way less dirty and dusty (I have been to job sites during the blow and there is nothing floating around in the air, netting holds it all), transporting is easier since it is packed for more dense into packaging, etc. They blow thick walls all the time around here.

  7. dankolbert | | #7

    Litawyn - where are you? We're lucky to have many talented insulation contractors in the state.

  8. N3y2cNDw2m | | #8


    The building site is in Union, about 20 miles west of Camden/Rockport.


  9. Billy | | #9

    Here's a photo of Spider fiberglass blown in a ceiling as densepack with Insulmesh netting. There's about 3 inches of closed cell blown against the roof sheathing and rafters, some strapping, and the Spider blown against that. The walls received the same treatment but with 2 inches closed cell in the walls. It is warm and quiet.


  10. N3y2cNDw2m | | #10


    That's pretty damn cool. How thick is the Spider material in your ceiling? I'm curious to what depth you could blow that stuff in a ceiling cavity before it would potentially cause problems with the Insulmesh.


  11. Billy | | #11

    Those are 2x10 rafters and the strapping is 3 inches placed on the rafters. 2x blocks were used as spacers to reduce thermal bridging and reduce stress placed on the drywall from the rafters twisting with changes in weather. Generally it is better to strap across the rafters but that didn't happen here.

    There are other areas of the house with 2x12 rafters and strapping and the insulators didn't complain about the depth being an issue.

    Your question is a good one -- I don't know if there's there's a recommended limit for densepacking the Spider. Check out the JM Spider website

    The Spider is very light -- see chart in this data sheet.

    This is the manufacturer's comparison of Spider to cellulose. I would love to see how the cellulose manufacturers respond to these claims.

    And the "third party test results" supplied by the manufacturer:

    I can attest the Spider is a really nice product. Riversong probably would argue that fiberglass doesn't provide the "hygrothermal benefits" of cellulose but I ain't going there! Time will tell.


  12. dankolbert | | #12

    J - get in touch with I&S Insulation - they're right in Wiscasset.

  13. N3y2cNDw2m | | #13


    That's awesome. Thanks for the recommendation. I'll give them a shout this week.


  14. dankolbert | | #14

    Good. And in general, I think there really shouldn't be any need to use foam in a new house, except sub-grade if you need to. If you're really shooting for passive house levels, you should think a lot about the chemical composition of the materials you use - I'd avoid fiberglass and spray foam.

    I wrote a piece a few years ago about some of our framing details for dense-pack cellulose - e-mail me at dan at kolbertbuilding dotcom and I'll send you a copy if you want.

  15. N3y2cNDw2m | | #15


    You'd avoid "fiberglass and spray foam" in all of its forms? I'm lost here. You're opposed to the Spider blow-in fiberglass afterall? What about the high-density fiberglass batts?


  16. dankolbert | | #16

    Cellulose seems to me to have many many advantages on many fronts. Its low toxicity to the occupants and environment is one. I try to avoid spray foam unless there's a compelling reason to use it, and I can rarely come up with one above grade.

    I don't know much about dense pack fiberglass - I know it's more common on the west coast. But I wouldn't use it in place of cellulose, especially not on a new house.

  17. N3y2cNDw2m | | #17


    I just sent you an e-mail. I understand the low toxicity factor that plays into you going with the cellulose, but should I assume that the Spider material therefore has a higher toxicity? I guess I assumed the stuff was pretty benign, but I've not really looked into it that far.


  18. dankolbert | | #18

    I don't really know enough about the Spider product to say.

  19. Billy | | #19

    Here's some information on Spider versus cellulose.

    I planned to use cellulose in my home and switched to Spider instead. I didn't want to worry about mold if the insulation ever gets wet. I was surprised at how fine and wooly the Spider is -- much different from the fiberglass in bats. It is white and looks and feels like cotton balls (but of course it's not cotton).

    In the end it may be a personal choice weighing all the various factors.


  20. N3y2cNDw2m | | #20

    Awesome, Billy. Thanks for this, as well as for the rest of the links you posted.


  21. Billy | | #22

    Uh-oh I forgot about that thread but can't forget Riversong.

    Let's not go there! Martin was right to shut it down.


  22. GBA Editor
    Martin Holladay | | #23

    Here's a link to an article I wrote for Fine Homebuilding on the fiberglass-versus-cellulose debate: Blown Insulation for Attics: Fiberglass vs. Cellulose.

  23. N3y2cNDw2m | | #24


    It appears you've got to be a GBA Pro member to access your article.


  24. GBA Editor
    Martin Holladay | | #25

    Many users of our site are GBA Pro members. The income we receive from your subscriptions is one way we pay our bills (including my salary).

    Of course, there are many other free resources on the Web, including this Q&A column. I certainly encourage readers to make use of any free resources they can find.

    Finally, for those who can't afford a subscription: local libraries are a great resource. Many local libraries subscribe to Fine Homebuilding.

  25. user-917907 | | #26

    ...Spider blown fiberglass can have an r value of around 4.2 when netted and blown inside a wall cavity. All insulators I have talked to MUCH perfer blowing thick amonts of fiberglass vs cellulose. It is also lighter then cellulose (both a pro and con depending on who you ask) so getting r60 and 70 in the attic can be supported by 5/8" gyp. The new blown fiberglass products, such as the Spider has very similar air ratings as does cellulose.
    Answered by Jesse Lizer

    Spider insulation can not be loosely blown into an attic. Apparently it's too light and fluffy and doesn't settle properly. It must be blown into a cavity -- wall or enclosed rafters, etc.

    Edit: There are two types of installation procedures. In one the Spider insulation is dry, without a binder. That type cannot be used loose in an attic. The other method has a binder added as the Spider insulation is blown. I don't know if that type is permitted in an attic. Possible not, as the fibers are still very porous and might allow too much air convection.

  26. Billy | | #27


    That's right, Spider is not generally loose fill so you would not blow it into an attic floor. Martin's article applies to loose fill fiberglass and cellulose.

    In an attic you could densepack Spider under Insulmesh. Perhaps it could be sprayed in with the binder, but I'm not sure what the manufacturer recommends. When they spray it with the binder they slightly wet the Spider fiberglass so it compresses to some extent. I've seen it applied this way in wall cavities without Insulmesh and it appears to be pretty dense and likely would be pretty resistant to airflow -- much more so than loose fill or batts. Third party testing would be useful.


  27. Bill_Hul | | #28

    I have blown both products and have put together a technical document that references these smaller diameter fiberglass products on our web site

    The higher the density of the batt, the harder it will be to fit around the myriad of wires, pipes and uneven framing cavities found in buildings. This means a greater potential for convection around the batts and within the gaps between them and your air barrier. Blown in products will always outperform batts because of this reason. Denser materials also block air better, and I find it more difficult to gauge density with fiberglass than with cellulose insulation, the firm mattress feel for dense pack at 3.5 lbs/cuft.

    The hydrophobic nature of the fiberglass makes me nervous from a building science perspective since any moisture getting into this assembly will end up condensing on the exterior sheathing during the winter, thus reducing its long term durability. The hygroscopic properties of the cellulose make it much more forgiving when moisture is involved and has proven itself since the 1970's.

  28. N3y2cNDw2m | | #29


    I will believe that Dense-Pack Dry-Blown Cellulose offers a higher R-value per inch and better dense pack performance than the JM Spider Spray-In Fiberglass product (sprayed in @ high density = >1.8 lbs/ft³) when the testing is conducted by a neutral testing service and not by CIMA (Cellulose Insulation Manufacturer's Association.)

    That's like commissioning the American Beef Council to conduct an objective study about the dangers of pork consumption.

    Just for comparison's sake, check out this study. I'm curious to hear your thoughts.


  29. Bill_Hul | | #30

    Hello Litawyn,

    In the summer of 2010, the cellulose industry hired Building Science Corporation to conduct side by side full scale wall air leakage testing of cellulose and blown fiberglass at their facility in Waterloo Canada. This week of testing proved that cellulose installed at 3.5 lbs/cuft had consistently lower air leakage rates than the blown fiberglass products installed at 2.0 lbs/cuft when both products are installed by the same tube insertion method. It was during this time, that we noted that the density drop of is much more steep in the blown fiberglass products, likely due to their smaller, lower mass fibers which allowed more air leakage through these lower density areas. I notice that the Spider literature does not report the cellulose air leakage numbers at its dense pack densities of 3.5 lbs/cuft.

  30. N3y2cNDw2m | | #31


    As with many disputable products debated in these forums, I'm guessing there will always be "proof" for and against them by "independent, unbiased sources." I'm going to try to talk with a few folks who actually have had the JM Spider material used in the same capacity I'm envisioning and see what the pros and cons have been. I'll keep you posted.

    Thanks for the additional info.


  31. user-509810 | | #32

    I read the report (very long) you suggested and on page 12 it says the following about ocSPF:

    "It should also be noted that field experience has reported some examples of high moisture content
    roof sheathing with full-depth ocSPF roofs in climates in Zone 6, with occasional reports in Zone 5
    and Zone 4C. However, Zone 4A should be a safe location for such a system except for high
    interior humidity or low solar heating of the roofing."

    It appears they are saying that moisture should not be an issue for my climate zone 4A. I would just have to do a full 8 inches of ocSPF to fill the cavity.

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