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Community and Q&A

Johns Manville Attic Pro and Insulweb?

user-6736376 | Posted in Energy Efficiency and Durability on


I have been researching hard on the idea of using blown-in insulation in my yet-to-be cathedral ceiling. And I do plan to properly design and build it to prevent any water problems. I initially decided to go with Green Fiber insulation. But the Johns Manville Attic Pro product seems to be getting rave reviews from DIY’ers like myself. Although I haven’t come across anyone brave enough to put it between rafters. Would this stuff work as well as cell insulation when using Insulweb?

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  1. GBA Editor
    Martin Holladay | | #1

    It's certainly possible to install blown-in fiberglass behind InsulWeb, and it's also possible to get better performance if you dense-pack the insulation.

    Are you planning a DIY installation with a rented (borrowed) blower? Or do you intend to hire a contractor for this work?

    If you're hiring a contractor, trust the contractor.

    If you're doing the work yourself, you need to (a) contact Johns Manville to determine whether the insulation manufacturer approves of your installation method, (b) find out whether the blower you intend to use is powerful enough for a dense-pack installation, and (c) make sure that you know what you are doing.

    You may also want to consider the use of a different Johns Manville product: Spider insulation. Here is a link to an article with more information: Getting to Know Spider Insulation.

    Finally, I assume that you know that building codes require, and building scientists recommend, that you provide a ventilation channel between the top of the fiberglass insulation and the underside of your roof sheathing.

  2. user-6736376 | | #2

    Thanks very much for your reply. My roof is like a typical barn. Metal sheets setting on top of purlins running across the rafters. The rafters will be 2x12. I had been told that I could use furring strips on the rafters to create a vent channel. That was IF I had sheathing on the roof. But since I do not, then I was told that the air channels under the metal and between the purlins would be enough to stay well aired out. Meaning I could just put something right against the underside of the purlins. I had thought about using rigid foam with foil facing up to protect it from water drips. But would like to figure out something that is cheaper and still prevent moisture in the rafter bay.

    As far as code goes. Where I live there is no code. You mentioned dense packing the fiberglass. It seems that the Johns Manville stuff does not have to be dense packed. Hence the reason they claim you can get R70 right on top of 1/2" drywall. They specify thickness rather than density. This appeals to me over the usual blown-in philosophy. I have read up on the Spider insulation. And while it seems neat. I don't regard it as a DIY product which to me defeats the idea of learning to be self reliant. If I am being naive in any way, I certainly appreciate your advice. Thanks!

  3. GBA Editor
    Martin Holladay | | #3

    I hope you haven't built this building yet. You really need solid sheathing -- so that you have somewhere to install the roofing underlayment to catch the drips due to condensation.

    Trying to catch the drips and prevent leaks by installing strips of rigid foam between the rafters is doomed to failure. The water will enter the cracks between your rafters and the rigid foam, soaking the fiberglass insulation.

    I described the mechanism for these condensation problems in a recent Q&A thread. You might want to read that Q&A thread before you finalize your details. Here is the link: I have water coming in the house in spring, coming through the drywall and light fixtures.

  4. GBA Editor
    Martin Holladay | | #4

    Fiberglass insulation can't be dense-packed when it is installed on an attic floor. However, if you want to install blown-in insulation in a closed cavity, you want to dense-pack the insulation, for three reasons: (1) to reduce air leakage, (2) to improve the insulation R-value, and (3) to prevent settling or slumping that leads to voids at the top of the cavity.

    For more information on insulating a cathedral ceiling, see this article: How to Build an Insulated Cathedral Ceiling.

  5. user-6736376 | | #5

    Ok. Now I understand why they specified thickness. Because its intended for attics. So that makes sense. Density in an enclosed cavity. As far as the roof goes. That article you referred to seems like a gray area to me. There was no definite conclusion to the problem they were having. But it sounds like it wasn't implemented the same way I am thinking of doing. I can understand the "potential" of water getting past the rigid foam. But if it does, then I did a poor job at sealing it. But assuming the rigid foam is indeed a bad idea, It seems that others have had success with implementing this same design using the furring strips to add the vent channel below deck and above insulation. Provided all sides of the rafter bay are sealed air tight. But this is basically the same thing I am contemplating. But without the furring strips. I would gladly use something that could more reliably block the water. Like say putting asphalt across the rafter tops before laying the purlins in place? I am for whatever works with the least amount of cost and labor. Isn't everybody? Thanks.

  6. GBA Editor
    Martin Holladay | | #6

    Yes, you can use rigid foam to create ventilation baffles. People do it all the time. Here is a link to an article about that topic: Site-Built Ventilation Baffles for Roofs.

    That said, ventilation baffles aren't the same thing as roofing underlayment. Roofing underlayment is required by code and recommended by experts. For a durable installation -- one that is actually constructable by workers climbing on your roof -- you need to install the roofing underlayment (asphalt felt or synthetic roofing underlayment) on solid sheathing (OSB or plywood).

    You won't regret installing the solid sheathing. The sheathing + underlayment will catch the condensation and keep your insulation dry.

  7. Expert Member
    ARMANDO COBO | | #7

    I would advise to use plwd/osb sheathing and rigid foam on top of the sheathing... its a lot less labor intensive and much better protection against condensation. You then can install WRB on top of the rigid foam, then your 1x4 purlins and metal roof.
    Dense packed insulation in 11.25" has an R39 value, that doesn't leave you room for any air space, which means you must be in climate zones 1-3 for correct amount of insulation (your location will help). If you are in higher climate zones, you must install even more insulation. Besides, code and basic building science tells us not to install air-permeable insulation against the roof decking.
    Just because it maybe correct that you don't have code inspections in your municipality, it doesn't mean you are not regulated by a County or State, and you really shouldn't try to achieve code minimum.

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