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Condensation behind dense-pack cellulose at the roof?

user-1022408 | Posted in Energy Efficiency and Durability on

We have a project in which we dense-packed the 2×10 rafter bays (walk-up attic) with cellulose, behind which we used vent baffles for ventilation and over the surface we used netting.
The homeowner then placed 6mil poly over the entire surface and used temporary space heaters to keep it warm. We are now seeing a build-up of condensation at the underside of the plywood roof sheathing in some areas–but not all–which was sufficient to wet/stain the dense-pack cellulose. I’m wondering if anyone else has experienced a similar condition and/or if there is simple fix?

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  1. GBA Editor
    Martin Holladay | | #1

    My first question is, how can you see what is happening on the underside of the roof sheathing if the sheathing has a ventilation gap below it, followed by a ventilation baffle, followed by 7 or 8 inches of dense-packed cellulose?

    Did you open up the roof from the top and pull some roof sheathing?

    Did you install a sneaky high-tech camera with an attached light source?

    Did you disassemble a section of the roof assembly from below, pulling out the cellulose?

  2. user-1022408 | | #2

    HI Martin:
    Thanks for the quick response.
    We did some disassembling looking for the lowest impact investigation to start.
    I apologize for the following misinformation--the insulation is actually blown fiberglass--not cellulose.
    (Still getting used to using the proper terminology rather than the vernacular.)

    My insulation contractor and I removed a small section of the blow-in at the collar tie level and looked at the condition of the roof above the ceiling first. We were concerned that perhaps the vent baffles had been crushed or not run sufficiently long enough. That proved not to be the case. We also made a slit in the netting on the roof slope and snaked our hand in beside a rafter to feel the underside of the roof surface. We could feel "frost" at the sheathing surface…
    (The assembly, as constructed, consists of 9 1/4" of dense-pack (fiberglass) in the slopes and then 16" of loose fill at the flat slope portion above the collar ties…)

  3. GBA Editor
    Martin Holladay | | #3

    This is a classic problem -- a variation on the "cold OSB" problem.

    Since it is unlikely that you will be able to warm up the cold OSB by installing a thick layer of exterior rigid foam -- always the best solution, but a very expensive one -- you have to do the next best thing, which is to interrupt the flow of moisture.

    The moisture is piggybacking on exfiltrating interior air. You need an air barrier on the underside of your fiberglass insulation.

    Check for air leaks at the base of the rafters (where the rafters meet the perimeter of the attic floor). Then you will have to either make the polyethylene airtight -- difficult but not impossible -- or install a durable interior air barrier like gypsum drywall.

  4. user-1022408 | | #4

    Hi Martin:
    I appreciate you advice. The temporary nature of the use of the attic was "finished space" seems to be the issue. When we completed our work, the attic was an unfinished storage space (the attic floor is heavily insulated with blow-in…)
    It seems the solution is to "finish" the attic if it is going to be used as living space--meaning completing the assembly with drywall, tape, and then latex paint coatings. If sufficient air sealing is provided, the roof sheathing will dry (eventually) and the issue should resolve...

  5. jnarchitects | | #5

    You mention the homeowner covered the area with poly. I'm assuming you mean the underside of the rafters? And there is blown-in fiberglass in the rafters and the attic floor?

    I have seen a similar problem where the fiberglass installers just shoved in fiberglass batts as a way to stop the cellulose at the eaves. They had sprayed closed cell foam into the eave pockets, but it didn't fill beyond a couple of inches. As a result air leaking through the second floor ceiling had direct access from the attic floor framing out to the eaves and then up the rafter bays.

    We pulled out the fiberglass batt "stops" in the eaves and then refoamed the entire eave area with open cell foam to seal off the transition from the attic floor framing to the roof.

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