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Community and Q&A

Spider Insulation

Jack Woolfe | Posted in Energy Efficiency and Durability on

I’m looking for advice on how DIY-friendly dense pack Johns Manville Spider insulation is compared to dense pack cellulose for a 12″ wide wall in a cold climate? In other threads —

— GBA advisor Michael Chandler has made the case that dense pack Spider insulation could be as good as or even better than cellulose (at least in a North Carolina climate). What was unclear to me from reading these threads was whether dense pack Spider insulation (not the glue-added variety) could be purchased and installed by a homeowner? Can dense pack Spider insulation be blown in by the type of blowers available at Lowes and Home Depot? It appears that dense packed Spider is less dense than dense pack cellulose (2.2 lbs / cu ft vs 3.5 to 4.0 lbs/cu ft) Does the lower density also mean less pressure is needed to install it properly?

Somewhere in the above threads someone stated that the insulative or air infiltration-blocking abilities of Spider insulation were overstated by Johns Manville. Does anyone know of independent analysis of Spider insulation? I also understand that typical fiberglass insulation has a decreased R-factor at lower temperatures. Is this true for Spider insulation as well? If so, by how much?

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  1. User avatar GBA Editor
    Martin Holladay | | #1


    "Installation of the Spider Custom Insulation System must be done by a certified contractor who has completed a course developed specifically to reduce defects and optimize the performance of the product by Johns Manville."

  2. Jack Woolfe | | #2

    Your linked reference refers to the non-dense pack, binder-stabilized version of Spider insulation.

    A new product by Johns Manville offers builders the opportunity to install fast-drying, high R-value insulation with reduced risk of mold and noticeable improvements over traditional cellulose insulation. Spider Custom Insulation System is a formaldehyde-free spray fiberglass that dries up to six times faster than standard spray insulation. Fast drying times allow the insulation to be applied and stay in place without shrinking or settling.

    What I referenced in my query was to a non-glue stabilized version of Spider which is blown into cavities like dense-pack cellulose is. Note in the "Performance Advantages", below, that it is a "dry installation (no binder required)".

    It also says "Works with most types of blowing machines designed to process fiber glass insulation." What I want to know if "most types" means only the high-pressure blowing machines that commercial installers use, or whether you can also use the lower-pressure DIY-type machines available at Lowes and Home Depot.

    • Won’t support mold growth
    • Won’t rot or decompose
    • Noncorrosive
    • Noncombustible
    • Won’t hold moisture
    • Won’t lose R-value with age
    • Primarily composed of sand, a
    rapidly renewable resource
    • Low dust, no itch, won’t burn skin or eyes
    • Made without formaldehyde
    • Works with most types of blowing
    machines designed to process
    fiber glass insulation
    • High coverage per bag
    • Dry installation (no binder required)
    • Uniform dense-pack cavity fill
    • Won’t settle
    • Contains no plastic, cardboard or other
    noninsulating debris
    • Minimal chemical additive content; contains
    no acidic fire-retardant chemicals
    • Safe to use and install (contact JM for
    Fiber Glass Health Indemnity
    Agreement information)
    • Contains at least 25% recycled glass
    (5% pre-consumer, 20% post-consumer)

  3. User avatar
    Michael Chandler | | #3


    The dense pack Spider install is very similar to many other blown-in-batt systems. The scrim (open weave spun-bond poly-olefin netting) is stapled up with an air-stapler approx 1" o.c.and the insulation is sprayed dry through holes punched in the scrim. Voids less than 2" or so are stuffed with Batt insulation first as those are challenging to fill with a blower. I know that some owner-builders roll glue onto the studs and plates before stapling up the scrim but I've personally never seen that done, (sounds like a good idea though but i'd still run the staples 1" oc)) During the blowing of the insulation you would take the same dust mask type precautions as spraying any fiberglass. I always have it done by my insulation contractor, largely due to the labor involved in stapling up all the scrim and the speed with which a trained crew can get that done. My crew is made up of plumbers and trim carpenters so my labor costs are higher than what I pay my insulator.

    In most cases it is cost effective to hire an insulator to do the install but there is a lot of price and quality variation in the quotes we are getting back on this stuff, the dry blow being much less subject to price gouging than the damp spray. JM makes a density tester for the damp spray application but I'm not sure how you would test density on the dry-blown behind scrim method, perhaps you could hire out the scrim install and then do the dense pack yourself to assure proper bag count. I personally trust to my good relationship with my insulation contractor. He does use a much more robust blower than what you would rent a HD to scatter cellulose on an attic floor.

  4. Jack Woolfe | | #4

    Thanks for the detailed description of how you install your dense pack Spider insulation. It's not surprising that an insulation contractor would use the higher pressure blower so he could get the job done quickly. However, it is much easier for me to find a lower pressure blower than it is to find a high pressure blower. So I was wondering if I could still get the job done, albeit slowly, with a lower pressure blower?

  5. User avatar
    Michael Chandler | | #5

    Sorry but I really can't answer that question definitively. My intuition says that the low pressure blower would not get the job done.

  6. Carolyn Smith | | #6

    Is the JM spider insulation suitable for an unvented cathedral ceiling in Atlanta, GA? We have decided against open or closed cell foam. Too many ??? on how it would affect health blown onto the ceiling in a house that is occupied. The JM Spider seems attractive to us because they say it doesn't settle/no dust (like cellulose). Our rafters/roof deck are visible from inside. We are thinking of putting up nailing strips on the rafters, having them put up the scrim and fill it with about 5" of JM spider, then finishing with Tongue and groove nailed to the strips so rafters still visible. Thoughts on this?

  7. User avatar GBA Editor
    Martin Holladay | | #7

    There are several problems with your plan.
    1. If you choose an air-permeable insulation like Spider, you MUST include a ventilation chute between the top of the insulation and the bottom of the roof sheathing. Ventilation is required by code for air-permeable insulation products. This could be a site-built chute using 1x1 sticks and thin plywood, or it could be a commercial product like AccuVent.

    2. You MUST include a tight air barrier on the underside of your insulation. That means taped drywall, with all electrical penetrations made airtight. Tongue-and-groove ceiling boards are not airtight and should never be installed unless you install taped drywall first.

  8. User avatar
    Michael Chandler | | #8

    5" of Spider, even at R-3.6/inch is still only R-18. Better would be to run 2" Hunterpanel polyiso @ R-12.6 and $26/4x8 sheet over the top of the roof sheathing followed by another layer or two of the same to get to R-25.2 or R-37.8 and then go over with OSB screwed through to the rafters with the special screws and mega-washers made to go with the Hunterpanel by ABC supply and put a new roof on.

    If you already have or can't afford a new roof on the house you could run two layers of R12.6 poly-iso between the rafters as you suggest and seal the edges to the wood with a good elastic caulk like big stretch or one of the acoustic caulks that will accommodate the seasonal movement of the wood but the labor cost would offset a lot of the gains in not replacing the roof and you would have all those thermal bridges and potential leaks at the wood to foam transitions. Atlanta is hot. a roof with no thermal bridges would be a huge advantage, and the poly iso is nasty to cut and would be miserably dusty to install overhead, whereas you could just lay it down and screw it from above in a small fraction of the time and move right on the a new roof and new flashings etc. setting you up for keeping water out of your house much more dependably than trusting to the existing flashings that may not be trust worthy.

    Consider the value of the roof you have and the quality of the flashings before you go to heroic measures to preserve them. At least look at the price of re-roofing the house with four or six inches of foam and compare it to the cost of doing the work from the inside.

  9. John Brooks | | #9

    I don't have a constructive solution to recommend.

    Unless it is a very simple low slope roof... I have to think that adding 4 to 6 inches on top and rebuilding the soffit and fascia is NOT going to be a "piece of cake" ..
    but I could be wrong

    have you actually tried this yourself?

  10. User avatar
    Michael Chandler | | #10

    I haven't gone over an old house roof this way but we've insulated old house walls and done post and beam and new pole barn roofs with wood ceilings this way .

    But look at the time involved in the idea she is proposing to run nailing strips along the sides of the rafters, scrim and blow between these strips and the roof and then nail in lots of little 14 1/2" boards between the rafters to get the wood ceiling she's hoping for. That there is a huge amount of work.

    Another option would be to fill between the rafters w/ 14 1/2" strips of foam and caulk them in place and add the nailing strips but the labor there is huge as well and working overhead with poly-iso would be majorly miserable and extruded poly styrene only moderately less miserable.

    If concern about hitting the rafters with the screws is an issue you could easily have someone spot from the interior to provide guidance on missed screws ("one inch to the east") or strap the roof with 2x4's @ 27 1/2" centers and then fill with 24" strips of 1 1/2" foam and go over that with two layers of foam in a fraction of the time needed to insulate and panel from below.

  11. User avatar
    James Morgan | | #11

    Carolyn, is this a 'deck house' by any chance, that orphan child of the sixties and seventies? Are the rafters closely spaced at 16" or 24" on center or wider, maybe 8' apart? If the latter these roofs are often almost impossible to upgrade with additional insulation from below because the window heads come so close to the ceiling deck. If this is the case Michael's solution of a roof overlay is really your only option. The eaves detail is important - to avoid a clunky over-thick fascia we've projected the roof overlay 8 - 12" past the existing fascia to create a staggered roof edge. If carefully done this will preserve the proportion and elegance of the original detail. If the roof overhang is wide you can stop the insulation at the exterior wall line and continue the extra roof depth with 2x furring.

  12. User avatar
    James Morgan | | #12

    If it is a deck house by the way you won't have to worry about hitting the rafters with those long screws. That t & g roof sheathing will be at least 2 1/2" thick and will provide a secure fixing anywhere.

  13. John Brooks | | #13

    James & Michael,
    I said I could be wrong ....and I think I was.
    I think James' tweaked suggestion sounds promising.

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