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I live in MA and would like to insulate my attic, but need to know if we use blown in cellulose if that means we will lose the storage space up there.

ruthryan | Posted in Energy Efficiency and Durability on

I would like to insulate my attic, and it sounds like blown-in cellulose is preferable to fiberglass. The contractor tells me the depth will be 9 inches, but my fear is that I will lose all the storage space in the attic, as the joists are 6 inches. Is there a way to resolve this dilemma?

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  1. GBA Editor
    Martin Holladay | | #1

    If you want to use your attic for storage, it's important that the attic floor boards (or plywood subfloor) be high enough to allow for a full depth of insulation under the floor boards. If your floor boards are now installed on 5 1/2 inch or 6 inch joists, they aren't high enough.

    The solution is to raise the floor. This will require the installation of additional framing on top of your existing framing. It will also be necessary to build an insulation dam around your existing attic hatch. Once this work is done, new plywood can be installed on top of the new framing.

    The work is fairly simple, and can be done by any carpenter.

    But the way, if you live in Massachusetts (Climate Zone 5), the building code requires a minimum of R-38 insulation -- and that means between 10 and 11 inches of cellulose (settled depth). Most contractors install more than that to account for future settling. Nine inches isn't enough.

  2. Expert Member
    Dana Dorsett | | #2

    Adding 2x4s on edge perpendicular to the existing joists and adding t & g, ship-lap, plywood or OSB above that as your storage decking works fine, and has considerably lower thermal bridging than adding that putting the 2x4s edge-to-edge with the existing joists.

    While, 9" of cellulose only adds up to ~R32-R33, which doesn't doesn't quite meet the R38 code-min for attic insulation in MA (which is based on IRC 2009):

    But with the lower thermal bridging of going cross-wise with the 2x4s it will deliver nearly-comparable performance to a code R38 between 2x12 joists (with no flooring), despite the lower than code center-bay R. (R28.6 whole assembly R compared to ~R32.7 whole-assembly, after thermal bridging.) Going to 2x6 adders crossed with the pre-existing joists and giving it the full 11" would put you at about R40.7 center-cavity, and deliver a whole-assembly R of about R34.7, beating a code-min approach slightly.

    Since the labor cost is about the same (for both the additional framing and insulation) in either scenario, it's going to be worth going for the 2x6 cross-joists and filling it fully. Since it doesn't need to meet the dynamic loading of a living-space floor, putting them 24" o.c. will cut the installation cost and give you a modest boost in thermal performance to boot due to the now lower framing fraction.

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