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Explain Flush Mounted Lights and Insulated Ceilings please

Mike Arrington | Posted in General Questions on

New construction – electrical phase. I get why canned lights are bad – heat loss, ice dams, etc, but how are flush mounted lights better? Is it because you put a J box in ceiling rafters (attic), insulate really well, and only have a Romex sized hole to power the fixture in your thermal barrier ceiling to seal – as opposed to cutting a 5” canned light hole to seal? Or am I missing something ? Thx. 

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Replies

  1. Eric Whetzel | | #1

    Typically, if you're trying to maximize the benefit, it's a combination of the flush mount lights in a J-box with a service core or chase. This way you're keeping the light fixture (including the J-box) on the interior side of the air/thermal barrier.

    You can check out how we did ours in the second half of this blog post: https://kimchiandkraut.net/2017/08/16/ceiling-details-for-passive-house/

    We went with 2x6's to establish our core, but you can go with a thinner set-up, depending on what else you're trying to include in this space.

    Also, if you google "how to make ceilings airtight for passive house" and look at the images, you'll see how others have approached this area as well.

    1. Mike Arrington | | #5

      I think I see what you mean. I’m putting Taped Zip board as my thermal barrier on top of 2x6s in attic floor. I’m not an electrician, so can they put all of the J Boxes, wires, fixtures, etc on the interior side of my thermal barrier this way and not penetrate the thermal barrier at all? I assume flush mounts, pendants, track lights will all work with this setup. Or would I need to add 2”
      Furring strips to underside of 2x6s?

  2. DCContrarian | | #2

    With a traditional can light, the fixture serves as junction box and also contains the air sealing. If you put in a junction box rather than having the fixture act as the junction box you can air seal the junction box independently. It's a smaller hole, and it's easier to inspect and maintain the air seal.

    Note that a junction box in the ceiling has to be attached to a joist, while a can fixture can be supported by the drywall.

    For the house I'm working on now I'm going to try using pancake boxes, the only penetration through the drywall is for the wire and the box isn't part of the ceiling. I'm still trying to find flush-looking fixtures that go over a pancake box.

  3. Joe Norm | | #3

    Pancakes are not rated for more than one 14-2 piece of Romex. This makes them pretty limiting.

  4. Expert Member
    Zephyr7 | | #4

    I also recommend against the use of pancake boxes. They’re too limiting, and very tight to work in. I would recommend a “deep” (2-1/8”) box for more room. If you want a box that is easy to air seal, look at the white (usually) fiberglass “hard boxes”. These boxes have no holes in them — you punch out a thin spot where you want the wire to enter, leaving only the hole or holes you punch out for wires needing to be sealed. These boxes are much easier to air seal than the plastic or metal boxes are.

    Bill

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