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Energy Solutions

Getting to Know Spider Insulation

Spray-applied fiberglass insulation offers huge benefits over fiberglass batts and even has some advantages over cellulose

Spider insulation being sprayed into an open wall cavity. [Click the photo to enlarge.]
Image Credit: Alex Wilson
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Spider insulation being sprayed into an open wall cavity. [Click the photo to enlarge.]
Image Credit: Alex Wilson
The spray nozzle coats the fibers with an acrylic binder as they exit the nozzle. Kyle Novak uses this mask when installing Spider. As fiberglass is sprayed into the wall or ceiling cavity through a 4-inch hose, excess is vacuumed up and returned to the truck through a 6-inch hose. Blowing Spider into the 15-inch-deep rafters, including behind framing for the built-in low closets. A special "scrubbing" tool trims the insulation flush with the inner face of studs and rafters. The scrubber has a rotating brush and spans the studs or rafters. The 6-inch hose entering the side of the truck brings vacuumed fiberglass back to a hopper, where it is mixed with virgin fiberglass and blown into the house through the 4-inch hose exiting the rear of the truck. Because the fibers pack tightly and are installed at a relatively low density, a lot of insulation can be loaded into a truck. Kent Burgess, on a very hot day, emptying a bag of compressed Spider insulation into the hopper in his truck. Kent's truck backed up to the garage, through which both hoses passed into the house. It's messy during installation, though remarkable dust-free. And it cleans up beautifully. Scrubbing the insulation back even with the rafters. (They're in a dormer.) Lots of fiberglass comes off, but it all goes back to the truck for reuse. Insulating the band joists. The exterior is sheathed with Zip sheathing, which forms the air barrier. On the exterior side of the Zip sheathing, we installed 6 inches of cork insulation. Scrubbing the walls on the first floor. The insulated wall and ceiling upstairs after clean-up. Customers love Spider insulation, because it's so neat and tidy after installation is complete.

We’ve just completed the installation of a relatively new and (at least in New England) little-known insulation material called Spider. As a reminder, the house we are renovating (really rebuilding) in Dummerston, Vermont, has provided an opportunity to try out dozens of innovative products and materials that I’ve long researched and written about in Environmental Building News.

Insulation has been a particular focus of the project, in part because some of the most common insulation materials on the market have environmental or health concerns, including halogenated flame retardants and blowing agents that contribute significantly to global warming.

In previous blogs I described Foamglas, a cellular-glass material, that we installed under the foundation slab and on the outside of the foundation walls, and expanded-cork boardstock insulation that we installed on the outside of above-grade walls spanning over the wood framing.

Here I’m covering the third innovative insulation product we used on the project: a spray-applied fiberglass product made by the Johns Manville Company called Spider.

Spray-applied insulation that doesn’t require netting

Spider insulation is installed into open wall and ceiling cavities in much the same way that damp-spray (or wet-spray) cellulose is installed. Like cellulose, it fills very well around wires, penetrations, and any irregularities in the wall cavity. It performs far better than fiberglass batts, which I think should only be considered on very small jobs where bringing in an insulation contractor can’t be justified.

The fiber insulation is sprayed from the truck and as it is blown into the wall or ceiling cavity the fibers are coated with a small amount of acrylic binder. That makes the fibers sticky (thus the name “Spider”) so they stay in the cavities. It even works in overhead cavities, where netting is required with cellulose.

As with damp-spray cellulose, the cavities are over-filled, then the excess is trimmed flush with the inner face of the studs or rafters. This is done with a special “scrubbing” or “screeding” tool, which has a wide, electric roller that spans two studs or rafters.

As Spider is installed, a second worker vacuums up the material that doesn’t stick to the cavity or is scrubbed off, and this goes back into a hopper in the truck. With the most advanced installation equipment, as was used on our project by Environmental Foam of Vermont, the recovered insulation is mixed with virgin material at a ratio that can be adjusted. For overhead blowing into cathedral ceilings, a higher proportion of virgin insulation is recommended for better adherence, while a higher proportion of the recovered insulation can be used in walls.

Comparisons with cellulose

I have long been a fan of cellulose insulation, and I have actively promoted it over the years. But spray-applied fiberglass has some advantages that I came to appreciate while working with and chatting with the installers.

While cellulose has higher recycled content (about 80% — the rest being flame retardant, usually borates), Spider has reasonable recycled content: 20% post-consumer and 5% pre-consumer recycled glass.

Spider goes in at significantly lower density: typically 1.8 pounds per cubic foot (pcf), while cellulose is typically installed at 3.5 to 4.0 pcf. For our cathedral ceiling application, we were worried that the 15” insulation depth would be so heavy with cellulose that it would cause the drywall to bow inward between the strapping.

The insulating value is slightly higher with Spider: R-4.2 vs. 3.7 to 3.8 for dense-packed or damp-spray cellulose.

Acoustic performance is similar; both work very well at blocking noise. According to Johns Manville, Spider installed in a 2×4 exterior wall, with 1/2” particleboard siding, 1/8” pressed-cardboard sheathing, and 1/2” drywall, provides an STC (sound transmission class) rating of 43, which is much higher than a comparable wall with fiberglass batt insulation and somewhat higher than a wall with cellulose.

Fiberglass is an inorganic fiber, so if it gets wet it may dry out better than cellulose — though you don’t want any fiber insulation material to get wet.

From a health standpoint, cellulose and Spider are both made without formaldehyde, but Spider doesn’t require a flame retardant, while cellulose does. While the borate flame retardants used in cellulose have always been considered safe for humans, the Europeans have recently challenged that contention, and those chemicals are being considered for addition to the European REACH program. There has in the past been concern about respirable glass fibers potentially being carcinogenic, but this concern has largely disappeared, and with Spider few fibers seem become airborne.

Spider installation is far less dusty than cellulose. I was working in the house during most of the two-day installation, and I was amazed how little insulation was in the air. I wore a dust mask, but was otherwise unprotected. My arms and eyes didn’t get at all itchy, as they do when I have installed fiberglass batts. The installers were wearing shorts and tell me that they experience no itchiness.

The contractor’s truck makes fewer trips

For our installer, Kent Burgess of Burlington-based Environmental Foam of Vermont (an insulation contractor which installs a wide variety of insulation materials, despite the name), one of the biggest advantages over cellulose is that he can fit about two and a half times as much of the bagged material into his truck than with cellulose. This is mostly because it goes in at a lower density, but I think the packed bags are also more dense. For a large job this can mean avoiding the need to return to home base to fill up with bags of material.

Kent used to install a lot of cellulose, but he far prefers Spider now. He is fairly new to Spider — having purchased equipment only last fall — so he was able to convince his mentor, Kyle Novak, of Advanced Insulation Systems in Traverse City, Michigan to make the 12-hour drive east to help out of our job. The deep, sloped-ceiling application was tricky, and Kyle’s experience would be invaluable, since he has been installing Spider since early 2006, not long after it was introduced to the market.

It costs a little more than cellulose

Kent says that Spider averages about 10% more expensive than damp-spray cellulose, but costs have a lot to do with the size of the project and the distance traveled. For a project further from his home base, using Spider can avoid the need for a return trip to pick up more material. In that case, Spider will be significantly less expensive.

Kent says the price of installed Spider averages about $1.50 to $1.65 per square foot for a 2×6 wall, or roughly 28-30¢ per board-foot, vs. maybe 24¢ per board-foot for cellulose. A quality closed-cell spray polyurethane foam (SPF) job will cost 80¢ to $1.00 per board foot for a large job, and with SPF there is the issue of how much can be installed at a time (because the curing is an exothermic reaction, and the foam heats up). Plus, Spider is a lot safer; supplied-air respirators aren’t needed with Spider, while they are with SPF.

The drawback is the cost of getting set up to install Spider. Kent has about $70,000 invested in the equipment.

In the seven years Kyle has been installing Spider he’s had no real problems. “I think it’s the greatest thing on the face of the Earth,” Kyle told me after spending a day and a half spraying the material. “It doesn’t settle,” he said, and customers love the look of the finished job.

When Kyle has gone back into houses insulated with Spider to do repairs or additions and opened up walls, he has seen absolutely no problems.

For cavity-insulation applications, Spider is a great option. Cellulose is also a great product, but for deep installations and sloped ceilings, I don’t think anything beats Spider today. Fiberglass batts aren’t even in contention.

Alex is founder of BuildingGreen, Inc. and executive editor of Environmental Building News. In 2012 he founded the Resilient Design Institute. To keep up with Alex’s latest articles and musings, you can sign up for his Twitter feed.


  1. user-1091831 | | #1

    Nice Comparison Article
    Thanks for this article, Alex. I think reusing the "scraps" by cycling it back to the hopper is an excellent way to maximize yield. I wonder though if the return hose ever has any clogging issues, since the material is now coated in adhesive after its exited the nozzle. I can just picture it slowly sticking to the hose's walls and slowly plugging up. But perhaps between the diameter of the hose and the vacuum pressure in the hose, maybe its not a problem.

  2. user-2310254 | | #2

    Fiberglass fibers in the air
    Hi Alex:

    Can you point to any sources that refute the concern many have about inhaling fiberglass fibers? I'm planning to use dense packed cellulose but wonder if borate is safe for chemically sensitive types.

  3. Alex Wilson | | #3

    On clogging the vacuum hose
    There were a couple situations in which the supply hose separated due to missing hose clamps and use of tape instead, but I didn't hear about any problem with the vacuum line. At one point, though, the vacuum sucked up a 3" or 4" wood disk (from cutting a pipe penetration). Kent immediately shut off the vacuum and had to take apart the hose to get the piece of wood out. A piece of wood in the hopper would have really gummed up the equipment. It's important before beginning an installation to make sure the entire space is clean (vacuumed) and all objects removed. That wooden disk must have been sitting on partition wall framing and hidden.

  4. Alex Wilson | | #4

    Carcinigenicity of fiberglass

    There is a fairly good--and up-to-date--review of health concerns about fiberglass on Wikipedia. I"m pasting that here; you'll have to go to the link to check the footnotes if you want to dig deeper:

    "Health problems

    The National Toxicology Program ("NTP"), in June 2011, removed from its Report on Carcinogens all biosoluble glass wool used in home and building insulation and for non-insulation products.[8] Similarly, California's Office of Environmental Health Hazard Assessment ("OEHHA"), in November 2011, published a modification to its Proposition 65 listing to include only "Glass wool fibers (inhalable and biopersistent)."[9] The U.S. NTP and California's OEHHA action means that a cancer warning label for biosoluble fiber glass home and building insulation is no longer required under Federal or California law. All fiber glass wools commonly used for thermal and acoustical insulation were reclassified by the International Agency for Research on Cancer ("IARC") in October 2001 as Not Classifiable as to carcinogenicity to humans (Group 3).[10]

    The European Union and Germany classify synthetic vitreous fibers as possibly or probably carcinogenic, but fibers can be exempt from this classification if they pass specific tests. Evidence for these classifications is primarily from studies on experimental animals and mechanisms of carcinogenesis. The glass wool epidemiology studies have been reviewed by a panel of international experts convened by the International Agency for Research on Cancer ("IARC"). These experts concluded: "Epidemiologic studies published during the 15 years since the previous IARC monographs review of these fibres in 1988 provide no evidence of increased risks of lung cancer or mesothelioma (cancer of the lining of the body cavities) from occupational exposures during the manufacture of these materials, and inadequate evidence overall of any cancer risk."[10] Similar reviews of the epidemiology studies have been conducted by the Agency for Toxic Substances and Disease Registry ("ATSDR"),[11] the National Toxicology Program,[12] the National Academy of Sciences[13] and Harvard's Medical and Public Health Schools[14] which reached the same conclusion as IARC that there is no evidence of increased risk from occupational exposure to glass wool fibers.

    Fiberglass will irritate the eyes, skin, and the respiratory system. Potential symptoms include irritation of eyes, skin, nose, throat, dyspnea (breathing difficulty); sore throat, hoarseness and cough.[15] Scientific evidence demonstrates that fiber glass is safe to manufacture, install and use when recommended work practices are followed to reduce temporary mechanical irritation.[16]

    Fiberglass is resistant to mold but growth can occur if fiberglass becomes wet and contaminated with organic material. Fiberglass insulation that has become wet should be inspected for evidence of residual moisture and contamination. Contaminated fiberglass insulation should be promptly removed.[17]

    While the resins are cured, styrene vapors are released. These are irritating to mucous membranes and respiratory tract. Therefore, the Hazardous Substances Ordinance in Germany dictate a maximum occupational exposure limit of 86 mg/m³. In certain concentrations may even occur a potentially explosive mixture. Further manufacture of GRP components (grinding, cutting, sawing) goes along with the emission of fine dusts and chips containing glass filaments as well as of tacky dust in substantial quantities. These affect people's health and functionality of machines and equipment. To ensure safety regulations are adhered to and efficiency can be sustained, the installation of effective extraction and filtration equipment is needed.[18]"

    [end of pasted content]

    Let me add that I always recommend wearing a dust mask when working with fiberglass insulation, there should be isolation (such as drywall) to keep airborne fibers out of the living space, and ducts should not be run through any loose-fill insulation.

    I had expected the Spider installation to be dusty and the installers to be wearing Tyvek suits; I was quite surprised to see the installers wearing shorts, to encounter almost no fibers in the air, and to experience no itching during the two-day installation.

  5. Expert Member
    Dana Dorsett | | #5

    Can they really hit 1.8lbs density...
    ...without netting?

    I'd previously been led to believe that open-sprayed Spider ran about 1.0-1.2 lbs density, and that it took netting or wallboard to hit 1.8lbs, due the highly-springy nature of the fiber.

    This would be an important distinction in assemblies that are not meticulously air sealed, since air retardency of 1lb Spider is fairly lousy compared to damp sprayed cellulose, or even open-blow attic density cellulose, whereas @ 1.8lb density Spider's air retardency is up there with dense-packed cellulose, and @2.2lbs+ is measurably more air retardent than dense packed cellulose.

  6. user-593033 | | #6

    Equipment costs
    Alex, does the equipment cost include the cost of the truck? $70,000 seams like a lot of money for a blower and a vacuum system even with the added cost of the adhesive injection. Thanks

  7. kimcrow | | #7

    1.8 lbs density and netting
    Dana's question is a good one! JM literature states 2.2 pcf. Would have been great to have a "test section" on site to see what kind of densities the on site crew was actually blowing.

  8. wjrobinson | | #8

    Yes, please add comments as
    Yes, please add comments as to density.


    Net-applied spider
    Ive been a big proponent of spider for many years. Weve used it in walls up to 12 inches thick behind netting up and up to 5-1/2" with the damp spray. One thing I've noticed is that it is often less expensive to net a 2x6 wall than to damp spray it due to the cost of the equipment and training for damp spray. I like the net and pack system better anyway because the residue from the damp spray on our stain grade windows can interfere with the finishing. We always use net and fill in 10"-12" thick double framed walls because it seems like there could possibly be some issues with drying time before drywall.
    To get a good pack on double framed wall we offset the studs one and a half inches so the left side of one is aligned with the right side of the one behind it. This way each bay can be packed tight without loose bleed over to the next bay but there is no loose pack between the faces of aligned studs except at window openings where we pack those spaces by hand with strips of batt fiberglass.

    There is a compression cylinder test for verifying proper density I've seen it used on the interjects but it hasn't made it to our job site yet. I really prefer spider in walls because it has no flame retardand and the really small fibers are much less itchy than conventional fiberglass. It doesn't hold water so it doesn't get heavy and settle away from the top plate. I have gone back to using cellulose in ceilings up to R-50 and cellulose over R-19 batts above R-50. I think it's less likely to get redistri uted by wind during hurricanes. I reserve spray foam for cathedral ceilings and applications where I can't ventilate effectively or really can't avoid having ductwork in the attic, etc.

    Hope this is clear - typing it on a phone is less than ideal.

  10. Expert Member
    ARMANDO COBO | | #10

    Rigid Foam
    Alex - Did you install rigid foam on top or CC foam bellow the roof decking? If so, why you installed one instead of the other? Did the spider insulation play any part on the decision for choosing above or bellow roof decking CC insulation?

  11. Alex Wilson | | #11

    No foam on most of the roof deck
    For most of the roof (with 16" truss rafters) we used no foam. The Spider goes right up to an air space formed with strips of reinforced Pro Clima Mento housewrap. The housewrap is caulked to the underside of the top flanges of the rafters and secured with pieces of wood lath. That took considerable labor, but obviated the need for a second layer of sheathing in creating an air space.

    There are two dormers on the roof that we framed with either 2x10s or 2x12s (I forget which) and added four inches of XPS foam under the roof sheathing in creating an unvented (hot) roof. As an insurance policy, we installed a variable-permeability vapor retarder that I'll describe in my next blog.

  12. Billy | | #12

    Good Article, but Incomplete
    Good article, but there is a lot more to explore when discussing Spider.

    As Mike Chandler suggests, dense-pack with netting is a good way to go with Spider, especially in ceilings.

    For blow-in on walls, it works well on top of 2-3 inches closed cell spray foam.

    For existing construction it works well to blow it in dense-pack behind drywall or plaster. This is especially true in old brick homes where you would not want to risk using cellulose in the walls.

    We used all these techniques in my home.


  13. ecdunn | | #13

    I presume this is so permeable that baffles must be installed against roof sheathing? How does this product perform compared to expanding foam between rafters, since the foam also creates a tighter structure?

  14. rockinroger | | #14

    spider insulation
    So, does Spider have a slightly higher R-value than SPF @ 4.2 per inch?

  15. user-987846 | | #15

    spider on pitched ceilings?
    I love Spider, and have been using it in walls for a couple of years. I was under the impression that it could not be installed in a pitched ceiling situation? I am curious how it held up for you? I know in my homes, I have to keep an eye on the drywall installation - pretty easy to catch a corner and lose a chunk. I have my installer bib the ceilings and blow the same insulation without the binder. I am under the impression that this does not give my quite the R-value of the actual Spider, but it holds up. I have also had issues with getting the cavities filled behind blocking. I noticed in your pics that you have similar situation next to windows for curtain rods, etc.

  16. Alex Wilson | | #16

    Spider overhead
    I was pretty nervous about whether it would stay in place, as Kyle said it would, and I pushed to get the vapor retarder and strapping up (see this week's blog) ASAP. I also talked with the drywall contractor asking them to use care not to knock into it; it is indeed fragile! I suspect that a installer skill influences the success with sloped-ceiling installations. Kyle has been installing Spider for over five years. He increased the adhesive percentage somewhat for the ceiling installation and had to dial back the amount of recycled material in the mix. You will see in the photos of this week's blog about our vapor retarder that we installed strapping for drywall 12" on-center, rather than 16". We wanted to provide that extra support to the 15"-deep Spider insulation.

    Johns Manville claims R-4.2 per inch, but I don't know what the actual installed density in our house is. (Per an earlier comment, it would have been great to install it in a test cavity that we could later go in and measure actual installed density; I didn't think of that.) I don't know what you mean by "this does not give quite the R-value of actual Spider." Our material was installed as intended and, as far as I know, it went in at about 1.8 pounds per cubic foot, which I believe is supposed to deliver R-4.2/inch.

    Roger, this would be slightly higher R-value than open-cell, low-density SPF, but lower than closed-cell SPF. Frankly, I'm surprised that the reported R-value of Spider is higher than that of cellulose or open-cell SPF. I would have guessed that it would be the same. Even if it's only R-3.7/inch (worst case), our overall R-value for the roof (with 15" of insulation) would be around R-50, allowing for some discounting for thermal bridging through the scissor-truss rafters.

  17. Jon_R | | #17

    Some installers are applying
    Some installers are applying wet sprayed cellulose on ceilings, even horizontal ones. But details are sparse.

    Does Spider R value decrease with temperature and/or radiant effects?

  18. Fargo137 | | #18

    Lots of misinformation about cellulose when I read the posts. Boric acid is used in over 200 consumer products including eye wash. Boric acid is less toxic than table salt. The weight of cellulose against ceiling drywall is a none issue when dense packed correctly, cellulose would be self supporting. The multiple benefits of cellulose with 100% boric acid far exceeds any form of fiberglass. The hygroscopic property of cellulose will perform much better in managing moisture than fiberglass. The borates also deter bugs and rodents and would kill any mold that may exist on framing members. If you want a safe, healthy and durable building use cellulose.

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