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Would it make sense to use Insulweb to dense-pack my attic floor?

saucymonkey | Posted in Energy Efficiency and Durability on

Hi, I was wondering if there would be enough of an advantage to make it worthwhile to install some elevated collar ties above the ceiling joists in my attic to give me about a 20″ deep cavity to cover in Insulweb and then dense-pack the floor with cellulose?

I’m really concerned about achieving a good air seal in my attic because I have an old house that was retrofitted with cellulose last year and I have enough warm air infiltrating the attic that there are little white fuzzy mold spots appearing on the roof sheathing. Last weekend I cut out three gable-end vents (its an L shaped house) to help get some previously nonexistent ventilation in the attic, but I’d like to do more to stop the warm air from getting in the attic space altogether.

Currently there is about 14″ of loose fill cellulose on the attic floor and it seems to me like I need a whole lot more. I should mention I’m on the coast of Maine, climate zone 6.

I’m intrigued by what I’ve read on here about installing cellulose to a mattress like level of firmness with the Insulweb and since I’m planning to have an addition I just put on blown in, I thought I could have them do this to my attic as well. I’ve never heard of this being done though and so I imagine the installers will look at me sideways when I ask them about it and I wanted to be prepared to explain why it’s a good idea, unless of course its not.

What do you guys think?

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  1. GBA Editor
    Martin Holladay | | #1

    I'm sorry to hear that you have evidence that interior air is leaking into your attic.

    Here's the bad news: the time to perform air-sealing work on your attic floor is before you install cellulose insulation. Did your insulation contractor suggest that this air sealing work be performed?

    It's possible that your attic floor isn't leaky, and there is some other explanation for the mold spots you have seen. However, if you are fairly sure that the problem is caused by air leaks, you'll need to move the insulation, section by section, so that air sealing work can be performed under the insulation. Then the insulation can be put back.

    Your Insulweb plan is not a good idea, for several reasons:

    1. It will be hard to achieve high densities under the circumstances you describe.

    2. Even when installed at a high density, cellulose is not an air barrier.

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