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Community and Q&A

Rigid foam roof deck / loose fill insulation — ratio risk

Jfantasia | Posted in GBA Pro Help on

I have a question regarding risk of exceeding recommended ratio of fibrous insulation to exterior rigid foam.   The house is a 1917 home in Juneau, Alaska (Climate zone 7, marine).  There have been numerous errors of repair and the section of interest includes the original roof deck (1917), 2nd roof (~1950) and a current rigid foam and roof deck added atop that.  The rigid foam roof deck is in place (2 layers of 3″ EPS (6″ total~R27)) and I am thinking of  blowing in loose fill fiberglass before closing up the small access remaining between original roof and 2nd roof.

The question is in regards to the risk associated with exceeding the recommended ratio of air permeable fibrous insulation to rigid foam.  The situation is challenging for two reasons: 1) Access is limited and loose fill is the only practical option to add additional insulation.  The only reasonable access is from the gable ends of the room where a section of ceiling is open and a hose can be fished in to fill the cavity.  In order to keep the insulation in contact with the underside of the sheathing, the cavity must be completely filled.  2) The cavity tapers from 0″ to 20″ (safe to high risk ratio of insulation).
A sketch is attached showing the confusing layup.  I am quite pleased so far with the addition of the rigid foam (4″ are being applied to the walls as well).  The question, is the risk too high at the deepest point to warrant the addition of loose fill insulation?

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    The risk is greatly reduced in a home that has an airtight ceiling equipped with a vapor retarder.

    If you can verify that there aren't any ceiling leaks (ideally, with a blower door), and if you use vapor-retarder paint on the interior, you won't have to worry.

    1. Jfantasia | | #2

      Hi Martin, Thank you for the quick reply and sorry for the informality. My name is Justin Fantasia (I'll have to update my profile, longtime reader, first time posting).

      Unfortunately the assembly is not going to lend itself to additional air/vapor ceiling at the ceiling. The ceiling is the existing exposed original T&G deck(the original roof deck-roof 1) and the plan is to leave it exposed. On top of that deck is the original 30# felt underlayment (in ok condition) that was left in place when the original cedar roof was ripped off and a new roof frame was built on top of the original (roof 2) circa 1950. This left the cavity in question, the tapering 0"-20" space due to different pitch roofs. On top of roof 2 is the synthetic underlayment, 6" EPS, 2x sleepers (1.5" vent), 1/2ply,udl and composition shingles.

      So in terms of air sealing at the ceiling, the T&G is there to stay and is surly not very tight. (But has the 30# felt on top of it for what that is worth). I will eventually run a blower door but at the moment it would simply inform me that the rest of the house in various on-going state of renovations is a giant hole. The sequencing of events is not ideal but is reality of budgets and time.

      Ultimately the question is: leave this area of cathedral ceiling insulated with solely the rigid foam at roof deck (bringing the area between roof 1 and 2 fully into the conditioned envelope) or try to reach a more desirable R-value by adding loose fill. This area is over the living room and probably comprises 25-30% of the ceiling area over the home. The whole thing is cut up but less challenging to reach the underside of rafters to add batt insulation below insulated roof deck. So from an overall performance standard I would hate to risk the condensation potential in an area that will not be easy to monitor/inspect once closed. But.....more cheap insulation sure is tempting.

      Any advice or alternative are hugely appreciated.

      1. GBA Editor
        Martin Holladay | | #3

        This is a judgment call. On the plus side, dense-packed cellulose reduces air leakage and helps buffer moisture when condensation begins. On the minus side, your ceiling is as leaky as a sieve, and if there are any hidden air pathways from your "attic" to the outdoors, then the stack effect will guarantee that a huge amount of moisture is directed to the underside of your upper level of sheathing. (That would be bad.)

        I'm not going to be providing advice. You're going to have to weigh the pluses and minuses and make your own judgment. Good luck.

  2. Expert Member
    Peter Engle | | #4

    Justin mentions using 2x sleepers on top of the rigid foam insulation to create a vent channel. If this is vented top and bottom, that should eliminate condensation on the outer sheathing layer. That leaves the inner sheathing. He shows it as 1x sheathing with a layer of synthetic underlayment. What kind of synthetic? If it is relatively vapor permeable, I think he's in good shape, or at least at relatively low risk. The 1x sheathing is just as vapor-open as his T&G ceiling. The synthetic underlayment should provide a pretty good air barrier to stop bulk air/vapor movement, and the EPS insulation is also relatively vapor-open.

    This is certainly a non-standard situation, and the physics says you might have trouble. I think if you manage indoor humidity, your risk is relatively low.

    1. GBA Editor
      Martin Holladay | | #5

      There are three layers of roof sheathing.

      The lowest layer consists of tongue-and-groove boards.

      The middle layer consists of another layer of board sheathing, covered by synthetic roofing underlayment and rigid foam. This is the layer of roof sheathing that is at risk of moisture accumulation.

      The top layer consists of 1/2-inch plywood with a ventilation channel underneath. I agree with you that the top layer of sheathing is not at risk of moisture accumulation.

      1. Jfantasia | | #6

        Thanks Martin and Peter,
        The conversation is helpful and reinforces my concerns. My instinct has been to take the safer path; leave the cavity empty, keep the "attic" space in the conditioned envelope, take the r-value penalty in this area, and focus on other areas to achieve overall better performance. The house has a long history of science experiments that we are trying to improve upon including some cool old "Kimsul insulation", household tinfoil nailed up beneath the drywall in one 4x8 area, invisible headers, some mercury beads rolling around between layers of flooring (it is an old miners home after all) and a snow machine burial ground out back.
        I'd like to error on the less experimental side of things as we right the sinking ship.

  3. Expert Member
    Dana Dorsett | | #7

    Temperate Juneau has 11.5 months of heating season, averaging only 2 (two) annual cooling degree days (base 65F), and due to the comparative wintertime warmth, higher than typical indoor humidity than zone 7A/B locations. It would really be a marine zone 7C location, if a definition for that zone existed in the DOE.

    That makes cheating the ratio more hazardous than in locations where there is a true drying season. If dense-packed with cellulose at the 20" deep section the EPS would be less than 35% which is less than half the IRC prescriptive ratio for zone 7.

    I live in zone 5A, and have a 99% outside design temperature cooler than Juneau's, but I also have only 8 months of heating season, with 4 months of at least some warmth to dry out the roof deck. That doesn't happen in Juneau.

    Let's not steal defeat from the jaws of victory here- the risk outweighs the reward.

    1. Jfantasia | | #8

      Dana--THAT is the issue with everything building related here in Juneau. We simply don't do drying very often, relatively low design temps, but lots of heating degree days. I'll quit asking if I can cheat, but I think I asked in a place where I knew I would get a responsible answer.

  4. Expert Member
    Peter Yost | | #9

    HI Justin -

    IF you cannot get an exceptional air control layer in this roof assembly, don't do the fill insulation.

    And finally: don't forget about what your wintertime interior moisture will be: 25% - 45% RH at 68F? how much moisture is in the space in the winter is an important determinant of dewpoint but even if I could keep the interior RH at 25%, I still would not cavity fill this assembly.


  5. Expert Member
    Peter Engle | | #10

    I'm convinced. I had not noticed the marine climate. Long periods of damp weather are not a time to push the envelope.

    How do you cool the place during the 2 cooling degree days?

    1. Jfantasia | | #11

      Oh man those cooling degree days are rough. We generally go into the basement where the sun can't scorch our pasty white faces and the air is cool and damp. But if we stay subsurface too long we start to question our moisture control measures in the basement...and that makes us we have to go back up to get our vitamin D pills.

      Thanks fellas

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