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Unvented Flat Roof with Cellulose

ErinBowman | Posted in General Questions on

Hi there!

I’ve been getting quotes for attic insulation in my Baltimore rowhouse and am being told that cellulose is not a good choice because “humidity can wear away the cellulose within 7 years”. My home has a flat roof and a very small attic space– the insulation will need to be blown in. I’m in climate zone 4a.

Is this assessment of cellulose accurate, or could it likely do fine (even with the type of roof and the region’s climate)?

If cellulose isn’t a wise choice, what do you recommend as an alternative? The contractor is recommending fiberglass, but I’m hoping for something more green.

I appreciate any advice you may have!

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  1. Expert Member
    KOHTA UENO | | #1

    Can you clarify what the design intent in this roof/attic assembly is? I.e., are you thinking of it as an unvented attic, with a cavity completely filled with cellulose? That is definitely a risky assembly:

    BSI-043: Don't Be Dense—Cellulose and Dense-Pack Insulation

    As fantastic as the dense pack approach is for walls, it is a pretty dumb approach for unvented cathedral ceilings and flat roofs. And, no matter how hard some of us work to try to stop people from going ahead and doing it, they persist on doing it. First, it is a building code violation. Second, it violates the physics.

    Or is the intent to create a vented assembly? That can be done with a combination of eave vents, cupolas/doghouses, etc.

    Insulating Low-Slope Residential Roofs
    Unless you’re careful, your low-slope roof can end up with damp sheathing

    Here’s some advice from Joe Lstiburek, a principal at the Building Science Corporation: “If you have an airtight ceiling, and you have an air gap of at least 6 inches between the top of the insulation and the roof deck, and if you have perimeter air coming in at vents at the soffit or fascia above the insulation, and if you also have ventilation openings near the center of the roof through some kind of cupola or doghouse — not just a whirlybird turbine vent — there is nothing wrong with your roof assembly,” Lstiburek told me recently. “You can build a 2 foot by 2 foot doghouse that sticks up a few feet, and put in some rectangular vents. If the ceiling is airtight, then the makeup air comes from the outside. That’s the least expensive way to do things.”

    Lstiburek continued, “The problem with this type of roof is that it is rarely executed correctly. Usually, architects don’t want to provide any ventilation around the perimeter. Or the architect won’t provide a deep enough truss to get enough insulation. If you just have a few whirlybird vents and a leaky ceiling, the whirlybirds will suck moisture-laden air out of the building and the roof will rot.”

    Also, this column seemed relevant:

    How to Insulate a Low-Slope Roof
    The owner of a 1920s Baltimore row-house looks for the best insulation strategy

    1. ErinBowman | | #2

      Thanks so much! My roof is a "rubber torch roof". To my knowledge the roof is unvented... it is a townhome with other homes attached on the right and left. I've attached a few photos here of the attic.

      1. severaltypesofnerd | | #3

        How old is the roof? Your other class of options is foam on top, a fire barrier, then a new TPO roof.

        Looks like you have plenty of depth.

        What's the construction date of the townhouse? Has anyone else in the row insulated?

        1. ErinBowman | | #4

          Construction date is 1922, new roof was put on in 2017 so won't be doing that for a while. I think other folks have insulated, but not with a concern for green building techniques.

      2. Expert Member
        Dana Dorsett | | #7

        Is it possible to crawl in to that attic space?

        It can be moisture safe to do a hybrid approach with a few inches of closed cell foam directly applied to the underside of the roof deck, with the remainder being cellulose. In zone 4A as long as at least 30% or more of the total R is closed cell foam the cellulose stays pretty dry, and the roof deck is protected. If those rafters are full-dimension 2x10s (true 10" nominal cavity depth) installing 3" of HFO blown closed cell foam (R20-R21) with 7" of cellulose blown in netting (R26-ish) you'd have R46 total, with foam-R ratio of R20/R46= 43%, which is PLENTY of dew point margin. With only 2" of HFO blown foam (R13-R14) and 8" of cellulose (R30) you'd be at about R43, which would be at the bare-minimum ratio of only (R13/R43=) 30%.

        Loose blown foam on the attic floor would fit, but would all but guarantee high moisture content in the roof deck, along with a lot of moisture cycling in the cellulose which causes unusually high and rapid settling compared to how it behaves in a properly vented pitched-roof attic (the "humidity can wear away the cellulose within 7 years” issue the contractor was warning about.)

        Unlike open blown cellulose none of this is cheap. In my neighborhood the 3" of HFO blown foam would run north of $4 per square foot, and 7" dense-packed cellulose in netting is also >2x the cost of the 13-15" of open blown cellulose it would take to hit R49 in a better vented attic. Re-roofing and installing a continuous 6-7" of 2lbs density roofing polyiso (R34-R40) might not be a lot more expensive, and would meet code min on a U-factor basis by not having thermally conductive rafters passing through the insulation layer, and the roof deck would stay dry & rot free, since it would be fully inside the thermal envelope of the building.

    2. Jon_R | | #5

      Re #1:
      Since passive venting of flat roofs is often impractical, it would be interesting to explore active ventilation - with attic/vent pressure matching the interior such that there is no warm/moist air leakage from the building into the attic. Perhaps only running when interior and exterior dew points and attic %RH indicate it will be beneficial.

      1. severaltypesofnerd | | #6

        Are you thinking of some mini heat recovery ventilator system perhaps, just for above the insulation?

        That might work, particularly if supported by a smoke or blower door test first, to find the ceiling leaks, to as there will never be a perfect pressure match.

        1. Jon_R | | #8

          I suppose one could use heat recovery, but air above attic floor insulation is largely unconditioned - so little loss if it is replaced with outside air.

          Agreed, a perfect pressure match is theoretically impossible. But it could be very slightly positive (attic > interior), meaning no stack effect driven air movement from the interior to the attic space. Without this leakage bringing moisture into the attic space, it's not clear the attic needs deliberate exit vents at all.

          While passive attic ventilation is the norm, in special cases, powered attic ventilation could be the most cost effective solution to a roof that works moisture wise.

          Note: this is not at all similar to often criticized 2000+ CFM powered attic exhaust ventilation that creates negative attic pressure.

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