Blog Post

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.

Photo: Alex Wilson
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 re-building) 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 spray nozzle coats the fibers with an acrylic binder as they exit the nozzle.


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Photo: Alex Wilson
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.

Because the fibers pack tightly and install at relatively low density, a lot of insulation can be loaded into a truck.

Photo: Alex Wilson

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.

As fiberglass is sprayed into the wall or ceiling cavity through a 4" hose, excess is vacuumed up and returned to the truck through a 6" hose.

Photo: Alex Wilson
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-pack or damp-spray cellulose.

Acoustic performance is similar; both work very well at blocking noise. According to Johns Manville, Spider installed in a 2x4 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.

A special "scrubbing" tool trims the insulation even with the inner face of studs and rafters.

Photo: Alex Wilson
For our installer, Kent Burgess of Burlington-based Environmental Foam of Vermont (an insulation contractor who 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 Travers 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.

Cost and performance

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.

After trimming the insulation and cleaning up, the job looks great. A variable-permeability vapor retarder will be added on the insulated ceiling--thus the prep at the closet partition.

Photo: Alex Wilson
Kent says the price of installed Spider averages about $1.50 to $1.65 per square foot for a 2x6 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.

Published July 22, 2013

Wilson, A. (2013, July 22). Getting to Know Spider Insulation. Retrieved from

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May 5, 2022 - 7:11 pm

@Alex Wilson: "Borate has very low mammalian toxicity—which is one reason we have liked it from a human health standpoint—and I suspect borate-treated cellulose would make an attractive nesting substrate for mice."

I assure you, you are 100% correct in that suspicion. Mice and pack rats love to nest in cellulose insulation.

October 13, 2020 - 11:36 am

I wouldn't consider either cellulose or fiberglass to be a vapor retarder or air barrier. I relied on a separate membrane (weather resistive barrier, or WRB) to serve as an air barrier, and I did not install a separate vapor retarder. The strategy is that if you can block air flow through the envelope, you control the vast majority of moisture migration, and allowing for drying potential on both sides of the air barrier allows any moisture to dissipate. The WRB I used is a high-performance membrane made by the German company ProClima: Mento 1000.

October 13, 2020 - 11:08 am

what about vapor barrier? cellulose is vapor retarded but fiberglass not.
Not having vapor barrier cause a lot of problems with condensation,
How this product prevent condensation?

February 12, 2020 - 11:00 am

How can this product be used with a PBR or standing seam roof of a pre-engineered metal building?  What would be the R-value within the purlin depth?

August 16, 2019 - 10:45 am

It appears that Spider insulation (called Spider-Plus Blow-In on the Johns Manville website) is still being made, but I don't know how the product has changed since we installed it some years ago. I'm surprised that the company doesn't have a way to locate trained installers.  -Alex Wilson

August 15, 2019 - 4:23 pm

Seems to me that SPIDER would provide a link or finder by zip code of trained installers "near me" but nooooooooo not even Google seems to know, certainly not the out of touch (more ever year) BBB and in our locale, SPIDER not even seen as a clue in ads.


FIX this

March 28, 2019 - 4:47 pm

Elizabeth, this is a really good question and one that isn't easy to answer. In very dry climates where wetting the insulation is extremely unlikely, installing Spider in an unvented roof application may be fine, but I doubt it will provide as dependable an air barrier as spray foam—particularly flexible spray foam that can move with expansion and contraction of framing members. When I used spider to insulate a cathedral ceiling in my Vermont home, I opted to go with continuous roof venting and both soffit and ridge vents. I also installed a variable-permeability vapor retarder (DB Plus from Pro Clima) on the underside of the rafters. The latter was especially important where the roof was essentially unvented because of dormers.  A friend did a WUFI analysis (a program that assesses moisture migration) on our roof and advised us to use the DB Plus vapor retarder; I wonder if that would have been needed had we used spray foam? Spider will air-seal better than fiberglass batts, but not as well as SPF. Like you, I try to avoid foam.

If there is someone in your area with building science expertise, you might want to consult them.

March 27, 2019 - 10:19 pm

If you use the Spider insulation to insulate a finished attic space do you need to vent it? I don't want to use spray foam but right now we are being told this is the only way we can avoid having to add soffits and vents on the roof.

August 19, 2015 - 8:05 am

Cynthia, yes that's correct; there is formaldehyde in mineral wool (urea-extended phenol formaldehyde). I'm not as worried about that as many others. I've examined test data from Roxul batts, and the measured formaldehyde emissions were at or below background levels, but I believe there is more of the formaldehyde binder in rigid mineral wool, such as Roxul Comfortboard. I understand that the production temperatures drive off most of the residual formaldehyde in the insulation.

Nonetheless, the industry is working to figure out how to replace that with a biobased binder, as has been done with fiberglass. The higher processing temperatures with mineral wool apparently preclude a simple switch, as was done with fiberglass.

I'm hoping to use mineral wool insulation in a writer's cabin that I am soon to build, and my plan is to use some sort of galvanized, expanded-metal lath to protect the underside of the insulation under the floor—specifically to keep rodents out. (The cabin will be on piers.) I will similarly protect the top and bottom of the rainscreen, though probably with a finer screening. Periodic inspection of the wood siding for mouse and squirrel entry will then be necessary. I could use fiber-cement siding for greater protection from rodents, but will probably stick with wood, knowing that I'll have to keep an eye on it.

August 19, 2015 - 6:38 am

Alex-Sorry- I meant to say sprayed- fiberglass. Thank you for your answer in any case, for both foam and fiberglass. It's the first time I've heard that borates might not discourage rodents. Invasion of rodents is a major problem with our home. No basement and up on concrete piers, made from an old barn frame, so hard to seal out critters. They pose a major human health threat. I understand , from looking as the MSDS, that mineral wool has formaldehyde in it? (Reference : At least this brand.....

September 25, 2015 - 5:36 pm

Cynthia, All wood has formaldehyde. And as far as I know, Canada does not allow added formaldehyde in their building products, Roxul included. Ask for no-NAUF (no added Urea-Formaldehyde).

August 18, 2015 - 10:12 pm

Cynthia, Spider is a spray-fiberglass product, not a spray-foam, but neither material could be considered rodent-proof. In our home, rodent entry at the basement level should be pretty well blocked by Foamglas insulation (which is the most rodent-proof of any insulation material), but rodent entry could occur higher in our house—perhaps through soffit vents or a gap in the metal screening on the rainscreen. So far, no problems.

To your point, I'm not convinced that borates will keep rodents out. Borate has very low mammalian toxicity—which is one reason we have liked it from a human health standpoint—and I suspect borate-treated cellulose would make an attractive nesting substrate for mice. Mineral wool may be better at excluding mice than fiberglass, because the density is significantly greater.

August 18, 2015 - 9:29 pm

Since sprayed foam doesn't require boric acid, how does it prevent rodents from entering a building or burrowing into the fiberglass? They certainly have gotten into our fiberglass batts big time.

April 9, 2015 - 1:10 pm


I reviewed your story on "Formaldehyde-Based Foam Insulation Back from the Dead". I appreciate the balanced and thorough review.

I was looking for some additional clarity on using it between walls. Here are the characteristics:

- The structure was built around 1950.
- Brick exterior.
- Thin composite wood backing, possibly masonite
- 4.5" dead air gap, no insulation currently
- Wet plaster lath interior.

The article says Tripolymer can be used, but there are better solutions, can you expand on that? A few years has gone by too, maybe things have changed? I am specifically interested in understanding this fr the C.P Chemicals product.

Do you recommend against it for this application?


April 4, 2015 - 11:45 am

I'm doing an interior gut on a 1920s house with siding installed directly onto the 2x4s. Is Spider safe for that application?

April 4, 2015 - 1:02 pm

Joe, This could be somewhat of a problem when the siding needs replacement. Removing the siding will likely damage the insulation, because it will stick to the siding to some extent. Siding is exposed to the weather, so had a shorter lifetime and usually needs replacement before other wall components.

It would be preferable to remove the siding, install a layer of sheathing on the outside of the 2x4s, then create a rain screen with strapping or a mesh product of some sort, then reinstall the siding. If you were going to all that work and you're in a cold climate, you might decide it's also worth adding a layer of rigid insulation on the exterior of the sheathing.

April 4, 2015 - 1:57 pm

Thanks Alex. Replacing the siding is really not an option. Without the opportunity to create a drainage plane I shy away from spray foam in walls (I'm in Atlanta by the way). It's tough to get a Grade I batt installation so I am hoping Spider might avoid the moisture trap problem and give me a little higher R-value.

November 6, 2013 - 9:38 pm

You can locate certified installers through the Contractor Locator tab on the Johns Manville website. Click the box for Spider.

October 27, 2013 - 8:32 pm

Would you know of any installers for spider insulation in Pittsburgh, Pennsylvania?

July 31, 2013 - 4:26 pm

I presume the photo at the bottom is of a Spider project and not foam as stated in the caption.  

Also, can you comment on the air leakage difference between cellulose and Spider?  I know cellulose is not an air barrier, but do know from personal experience on hundreds of retrofit projects that cellulose dramtically drops the air leakage rate.