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How do you describe 6 ” spider fiberglass in the walls and energy star draft stopping?

mrives | Posted in Green Products and Materials on

I would like someone to give a clear, specific explanation of the above so a “conventional builder/architect” would easily understand this customer desire.Is spider fiberglass an eco choice? Any specific name brand best?

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  1. GBA Editor
    Martin Holladay | | #1

    "Spider" is a brand name for a type of spray-in-place fiberglass insulation. Spider insulation is manufactured by the Johns Manville Corporation of Denver, Colorado.

    The term "Energy Star draft stopping" is not a technical phrase; it has no technical definition. "Energy Star" refers to a program for building energy-efficient houses; the program is administered by the U.S. Department of Environmental Protection.

    The phrase "draft stopping" is sometimes used to refer to air-sealing work -- that is, work designed to lower the rates of air infiltration and air exfiltration in a home. Air leakage rates in different homes are usually compared by looking at the results of blower-door tests. Air leakage rates are usually reported as air changes per hour at 50 Pascals (ACH @ 50 Pa) or cubic feet per minute at 50 Pascals (cfm @ 50 Pa).

    A variety of tasks could be called "draft stopping" work. These might include the installation of gaskets under bottom plates, the use of contractors' tape to tape sheathing seams, the use of spray polyurethane foam to seal leaks at wall and ceiling penetrations, and the installation of weatherstripping on attic access hatches.

    An Energy Star home USUALLY has lower air leakage rates than conventional homes -- but not always. Many builders of energy-efficient homes complain that the Energy Star Homes program sets a low bar for energy performance. Many builders are selling homes that perform much better than Energy Star homes.


    Spider Fiberglass is a type of micro-fiber fiberglass developed by Johns Manville that can be applied to walls and also roofs as a spray-in-place product or a blown-in-batt. It's R-value is better than fiberglass batts, and is actually equal or better than open cell spray foam. In a 2x4 wall you can get R-15 in a 2x6 wall from R-20 up to R-23 depending on density of installation. Open cell foams at R-3.6/inch typically get R-20 in a 2x6 wall. it averages 20% post-consumer glass and 5% post industrial glass and is completely formaldehyde-free. I think it is reasonable to call it an "eco choice". During the installation the air born glass fibers are irritating to inhale so a dust mask must be worn when supervising the installation.

    Spider doesn't look like regular fiberglass, I've had builders with thirty years experience pick it up in their hands and ask what it is. The fibers are so small that I can barely see them with the naked eye. It looks like freezer frost or white dryer lint. This means you get more air spaces between the fibers which is where the insulation value comes from.

    It is less expensive than open cell foam and you can fish wires through it without taking down the dry wall. If you need to open a wall up to make a repair you can remove it to a trash bag and then pack it back in before closing up the wall. Unlike spray foam there is no expansive force to push low-voltage wiring out of the wall and there is no knifing the excess away and disposing of it.

    As a spray-in-place application the dry insulation is blown out of a hose about 1 1/2" diameter and two small misters coat it with a water based adhesive while it is air born that makes it stick to anything it touches (including wood windows, you need to mask them so the glue residue won't interfere with stains and clear varnishes.) A rotating scrubber rod levels and packs the face of the insulation and cleans the faces of the studs and an operator with a big vacuum sucks up the excess off the floor where it travels back to the hopper in the truck and then back with the incoming insulation and into the wall again. There is no waste. The site is left clean.

    In a blown-in-batt application a fibrous scrim is stapled up over the studs and the cavity is blown full of dry Spider with no adhesive. This has to be done carefully or you can get pressure on the back of the dry wall that can lead to nail-pops. For folks like me who are building with double stud walls and are blowing 12" at R-46 we need a moisture-free product so this is the way we do the thick walls.

    It can be sprayed on the underside of roof decks but it is very permeable to moisture vapor so it must be covered with a vapor retarder such as dry wall with a latex paint on it. I prefer to use open cell foam on my sealed attics here in NC especially for the sealing I get over the tops of my walls. (We block for the foam in plane with the exterior wall sheathing so we get a very tight seal at all wiring and plumbing penetrations on our top plates this way.)

    Energy Star draft stopping is a set of air tightening best practices published by the Dept of Energy as the "Energy Star Thermal Bypass Checklist".
    You don't need to use spray foam or Spider to take advantage of the Energy Star program and the third party inspection service it provides is an important part of all the major green building certification programs.

    I hope this is helpful, Michael

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