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fall50 | Posted in General Questions on

I need to address the insulation situation specifically above the 2nd floor bathroom. (Pic is not the greatest) The bathroom is located in the octagon portion in the attached picture. I can access this area via the walk up attic, bit it’s tight quarters, fun times.

The joists are 2×10’s which run “into” this octagon area. I would presume I should cut square’s out of some left over XPS and foam them in place similar to what I did at my last house which was a cape cod. At my previous house I did this behind kneewalls, though unclear if this is a waste of time as the walk up attic is not finished. Hopefully that makes sense

Because I am dealing with 2×10’s I would assume R 30 unfaced FG? Would I then lay another layer of R 30 perpendicular to the joists? Because the roof tappers down and I have minimal depth towards the exterior wall , the 2nd layer of insulation may be in contact with underside of the roof deck. Will this be a problem?

Does insulation perpendicular to the joists need to be faced FG or does it not matter? I will do airsealing around the electrical penetrations and have already foamed around the Panasonic fan before doing the insulation.

While blowin would be an option were talking about 25-30 sqft and I just cant see going through the hassle of trying to rent a hopper etc.

I am in Zone 6 B

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  1. fall50 | | #1

    See attached pic

  2. Expert Member
    Dana Dorsett | | #2

    Hopefully you have soffit vents on the octagon?

    There needs to be at least 1" clearance between the fluff and the roof deck. The ceiling below and any access hatches/doors need to be weatherstripped & air tight.

    Unfaced batts are fine- just sculpt them as well as you can for a near-perfect fit, no gaps,compressions or voids.

  3. GBA Editor
    Martin Holladay | | #3

    Your description is confusing, so I'll attempt to re-phrase it.

    You want to insulate the ceiling of a second-floor bathroom. I'm guessing that there is no insulation there at the moment.

    You will be working from above -- that is, from the attic.

    If I've got this right so far, here's what I advise:

    1. Investigate whether there are any soffit vents or ventilation openings near the eaves. If there are, install wind-washing dams and ventilation baffles in these areas. For more information on this work, see Site-Built Ventilation Baffles for Roofs.

    2. Perform air sealing work. For more information on this topic, see Air Sealing an Attic.

    3. Install R-49 of blown-in cellulose. For more information on this work, see Borrowing a Cellulose Blower From a Big Box Store.

  4. fall50 | | #4

    Hi Dana.

    No sofit vents exist in this area or on either side of the octagon, instead have a small window in each gable end with a screen on it. It should also be noted no roof vent exists in the octagon, only on the main pitch of the roof which is visible in the attached pic

    While it's not visible from the exterior or the attached picture, when I walk up into the attic the roof deck is cut out where the ocatgon is located, which allows access to this area (and what I will crawl to install the FG) I would assume this access point also acts as a passage way into the larger attic space for any warm/moisture to escape

    I assume I need to lay another layer of insulation over the insulation placed between the joists. Does this layer also need to be R-30 for a total of R-60? Or should I go with lower R Value.

    Martin- I just noticed your response. Blowin-Cellouse is not in the cards for this space I will need to go with FG.

  5. Expert Member
    Dana Dorsett | | #5

    IIRC this roof originally had skip sheathing and cedar shingles, now covered over with plywood & an asphalt shingle/felt layup(?)

    With skip sheathing & cedar it was inherently vented, but now that it's clad with plywood it needs a venting scheme to protect the sheathing from interior moisture drives after it's insulated and running colder.

    The alternative to a vented attic is to insulate at the roof deck with a 2" shot of closed cell foam on the underside of the sheathing, and R30+ of fluff, with a smart vapor retarder on the interior side (or 2" of closed cell limiting the fluff to R13, no vapor retarder, which would be pretty crappy performance for you climate zone.)

  6. GBA Editor
    Martin Holladay | | #6

    I disagree with Dana. Most attics are dry. If you have moisture in your attic, it's worth tracking down (and sealing) the air leaks in your ceiling that are responsible for the moisture.

    Venting is rarely an important priority. For more information on this issue, see All About Attic Venting.

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