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Community and Q&A

Mineral Wool Underdeck

LawrenceMartin | Posted in Green Building Techniques on


I am working on a design for a residential addition that includes a roof deck over a living space below. We would like to add external insulation to the top of the deck to reduce thermal bridging. I have enclosed a detail rather than describing it here. My concern is that the mineral wool insulation sandwiched between the DensDeck and OSB may not be able to air dry through the EPDM layer and could create an opportunity for mold and rot. I am wondering if we should just use EPS in this one location. 

Thank you in advance.

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  1. Expert Member


    I'm sorry I don't have an answer to your question, but looking at y0ur section a few details worry me.

    - To insure water makes its way to the gutter, the detail relies on flashing tape. If the tape fails the water would move under the metal drip-edge - which is presumably fastened through the roof membrane, meaning water might have a route straight into the roof.

    - The drip edge being carried down the interior face of the gutter precludes using most common gutters, which have horizontal straps running from inside to out near the top.

    - The gap between the fascia and siding below can easily admit water. It needs some lapping of the materials, or the rain-screen may take on more moisture that it is shedding.

  2. Expert Member
    BILL WICHERS | | #2

    What climate zone are you in? The more typical roof assembly would be with continuous rigid foam in place of the mineral wool in your drawing, and you have to meet a certain ratio of rigid foam to interior fluffy stuff. That ratio depends on your climate zone. Assuming you fill the entire interior rafter cavity with dense pack cellulose, it's very unlikely you'll have the right ratio of insulation types here. The permeable mineral wool MIGHT also be an issue here, but I've never worked with an assembly quite like this. Rigid foam of any type is much less vapor open than mineral wool.

    It would be better to run the EPDM over the top of the metal flashing and omit the tape. This is similar to how ice and water shield would be installed. Don't trust tape here.

    I'd be nervous about the screwed in post for the railing. I prefer threaded metal fasteners and brackets here.


  3. LawrenceMartin | | #3

    Thank you, Malcolm and Zephyr7 (Bill).

    I actually was using the flashing tape to secure the metal drip edge, because I don't want to have any penetrations into the EPDM membrane. But, based on Zepher7's comment I'll probably forego the flashing tape and have the metal flashing screwed to the substrate and put the EPDM over that. I'll have to look more closely at my gutter options it looks like though.

    I am in a dry Climate Zone 5 around Denver, CO. I'm was not familiar with the ratio of fluffy to rigid but have now started reading Martin's article on that topic and I may be too low on my rigid insulation.

    I'll update the screw detail. I actually use a wire railing kit and I think they have threaded metal fasteners.

    1. Expert Member
      Akos | | #4

      You can check out here for ratios:

      Your roof looks pretty complicated, are you in a fire are thus the densdeck and mineral wool?

      A much simpler approach is to build a vented roof assembly with slightly larger I-Joists. There very little thermal bridging with I-joists, so the exterior rigid insulation doesn't add all that much to the assembly R value. You do need a larger vent space because of the low slope (our code requires 2.5"). Sometimes getting venting sorted out cost more than a bit of extra rigid, lot of it depends on the complexity of your roof.

      If you do want to go with the combination of fluffy plus rigid, you need a solid air and vapor barrier at the interface. With rigid foam, this comes built in, with mineral wool you need to add one above your plywood deck.

      You would still have to watch the ratio as per the link above, so with 11 7/8 TJI that will be a LOT of rigid. If you can step your rafters down to 9.5 I-joist or even better 2x8, much more doable dense packed. You can stay with the larger I joists but than you need smaller batts that are pushed up against the deck.

      I would also check with GP if you can use the densdeck for a rooftop deck when installed over rigid mineral wool.

      I'm starting work on an EPD project and strugled with similar gutter detail. The one you show is pretty common from most EPD manufacturers. My solution is to have a drip edge mechanically fastened to the deck as per Bill's suggestion, the EPDM glued to this and wrapped over the edge. The gutter sits behind this drip edge. Similar to here:

      1. LawrenceMartin | | #5

        Thank you, Akos. You bring up several good points. I agree, I was overthinking it and making it way too complicated. We would like to be foam-free on this build, but I am thinking I prefer foam for this specific application. That detail you linked is very similar to one I did in my own house. My house had an open deck under the outside edge and the upper deck was covered so I just left off the gutter. I think I will try and talk him into a flash-and-fill using closed-cell foam and cellulose in the correct ratios. Thank you for the link.

        1. Expert Member
          Akos | | #7

          Generally for I would vote for designing SPF out of assemblies. If this is just a small area, a bit of flash and batt is definitely simpler and better approach.

          Rooftop decks attached to bedrooms are rarely used (I personally think they are useless for anything except to pass code), better option is to eliminate it or change it to a small Juliet. This would would make things a lot simpler/cheaper. Usually this is an argument that you won't win though.

          1. Expert Member
            MALCOLM TAYLOR | | #8


            I agree about the usefulness of those second floor decks accessed from bedrooms. They fall into the same category as balconies on most high-rises. Their main attribute is visual appeal.

  4. creativedestruction | | #6


    You should identify and detail an air barrier. Ideally this would occur at the structural roof deck beneath the mineral wool or foam to keep moisture from condensing on the underside of the EPDM. It needs to tie in continuously with the wall air barrier.

    1. Expert Member
      BILL WICHERS | | #9

      If the ratio of rigid foam to fluffy stuff is too far skewed towards fluffy stuff, the moisture will just condense on that secondary air barrier inside the insulation.

      Condensation occurs when moist air comes into contact with a surface or object that is at a temperature below the dew point of the moist air. That means you need to keep the first condensing surface above the likely dew point, and you need to put the air barrier at the warmest point to block warm, moist air from getting into the assembly well before it reaches anything cold enough for the moisture to condense out as condensation.


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