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Community and Q&A

Velux skylight curb uninsulated?

aztecrsf | Posted in Energy Efficiency and Durability on

GBAs, 

We’re ordering skylights for our build now, curb mount on a cathedral ceiling. Thus no light shaft other than the space between the 10″ rafter. As I look at their specs, they sit on a nominal 2x lumber on edge, and give you 1-1/8″ of extra room for waterproofing, flashing, etc. There’s certainly not much room for insulation under that. On the inside, I suppose we could put 1/2″ of rigid foam between the curb 2x and drywall, and only lose 1″ of skylight area. 

We’re in coastal San Diego, so about the most mild climate possible. But I still hate having R-0 anything where avoidable. Am I missing something, or over-thinking/over-worrying? 

Thanks!

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Replies

  1. gusfhb | | #1

    If I had this to do again I would buy the next size wider skylight and then you have extra space for insulation
    My roof deck is thick tongue and groove decking, but I think someone could come up with a mounting scheme for a 2 0r 3 inch wider skylight on a standard roof deck. I'm thinking screwing from the bottom would work.
    To be clear a wider skylight wider curb, insulation, then trim over
    You cannot tell from the inside that you have covered part of the glass
    I would not use drywall there as any condensation will be ugly

  2. Expert Member
    MALCOLM TAYLOR | | #2

    aztecrsf,

    You aren't missing anything. Hopefully the next generation of skylights will thicken their frames incorporate space for insulation on either the inside or out.

    There are several workarounds which vary in complexity. The easiest is as you say to line the shaft with a layer of rigid.

  3. aztecrsf | | #3

    Huh, wow, I'm kind of surprised by this. Especially since they're rated Energy Star.

    I guess I could make the curb out of 3/4" ply and line it with 3/4" rigid. It would still be pretty stiff and anchored well to the roofing, and the skylight itself would help keep it so. Not sure it's worth the trouble to push 6 sq ft of R-0 curb to R3.

    1. Expert Member
      MALCOLM TAYLOR | | #4

      aztecrst,

      Technicality they are right, the skylight is fairly efficient. They just don't take any responsibility for how it needs to interact with what it attaches to.

  4. gusfhb | | #5

    I would not do this, as plywood is not as stable in this orientation. Attached pic right above my head as I type. Trimmed like this allows you to slide in 3/4 foam which adds to the almost R2 of the 2x4. If you can visually take further diminishing the size of the glass, you can put as much foam as you wish

  5. Expert Member
    Joshua Salinger | | #6

    Aztecrsf,

    Most NA skylights don't address the install and even the best ones are in the R-3 range. Putting that in a R-38+ roof is going to be an energy loss.

    There are European models that have triple pane glass and also have insulated install kits. Manufacturers such as Lamiliux or Fakro are available through some NA distributors such as 475 supply. You may want to look into these.

  6. Expert Member
    Akos | | #7

    In zone 3 it is not something I would worry about. Fit in the 1/2 polyiso and call it a day.

    What I have done before is use a deck mount skylight but put it on a curb (long story but works pretty well). This would let you make much thicker curb which would leave lot more room for insulation. Doing this does mean custom flashing kit.

  7. aztecrsf | | #8

    Thanks, all, for the good ideas.

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