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Insulate rafter bays under a flat roof, or just seal them?

Nick Van Kleeck | Posted in General Questions on

The roof of my guest wing now has exterior foam with a minimum value of R-33.   I am ready to start on the next phase of retrofitting the envelope.nIn this phase I have to decide how (or even whether) to get insulation in the 5 and a half inch bays above the ceiling, between the rafters/joists in this miniature, non-accessible “attic”.  This attic is currently vented, and I must convert it to an unvented attic to get any value out of the external roof insulation.

The most important objective is to stop the movement of outside air into the space to that the cold roof becomes a warm roof. It would be great to add additional insulation of course but that’s secondary to stopping the air movement. I’m planning to wrap the fascia and the soffit vent with rigid foam and seal the whole assembly. So one option would be to leave the bays empty, but that means relying solely on the integrity of my sealing job. 

Since I can’t visually inspect this “attic”, I have some concern about whether the wiring is sound enough. Given what I’ve seen in attics around town, I have no trust that there isn’t an open junction box or worse.  I also don’t know if this concern would be allayed by blowing in fiberglass rather than cellulose. I did buy an inspection camera/flashlight tool (which I’m struggling to get to work) just to help satisfy myself that there’s no knob and tube wiring up there. 

I have access to the bays for a hose from one side only, by removing the fascia, and the length of the bays I would be trying to fill is 12 feet.  Another option is to “drill and fill” through the ceiling. Although I’m not eager to do it, I’m willing to demo and replace the ceiling if that’s the only way to do a good and safe job.  

As usual, my thinking proliferates to fill the void of my lack of experience, so any and all suggestions are welcome.

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  1. GBA Editor
    Brian Pontolilo | | #1

    Hi Nick,

    Where is your house located? Also, have you read this?

    1. Nick Van Kleeck | | #2

      Oops - I am in the low desert of Arizona, zone 2A I believe. And I have read the article you linked, but it's probably been a year so I'll review it again.

      I'll try to focus my questions more. I would like to blow in insulation into the space between the rafters. I just don't know 1) how concerned to be about bad wiring, since I can't inspect it, and 2) Whether it's physically possible to fill the 12 foot x 14" x 5.5" bays from one end - I'll have to push a relatively small diameter tube into the space, and I'll be filling it blind unless I drill some inspection holes in the ceiling.

      1. GBA Editor
        Brian Pontolilo | | #3

        Thanks for the info Nick.

        Unless I'm missing something, seems like you can kill two birds with one stone. Cut holes in the ceiling for inspection and then use them to fill the cavities. If you are concerned with shoddy wiring, I'd look into that before insulating.

  2. Nick Van Kleeck | | #4

    Brian -

    Thank you - that's an idea I hadn't thought of. And your idea stimulated another - I could tear down the ceiling, fix any wiring issues, insulate between the rafters and maybe even leave the rafters exposed when I'm done depending on how they look.

    If I'm willing to insulate from the inside, I can seal up the soffit vents now and see how much the newly insulated roof improves the performance by itself, and tackle the ceiling at my leisure.

    Even if I miss have a few air leaks into what's supposed to be an unvented attic, it's hard to image that anything inside it will be cold enough to cause any condensation given the insulation on the top side.

    1. GBA Editor
      Brian Pontolilo | | #5

      Yes, that's the idea behind the ratio of exterior to interior insulation, to keep the sheathing warm and not create the potential for condensation.

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