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Skylight Curb Construction

Zins | Posted in General Questions on

I’m looking for ideas on how to limit thermal bridging in skylight curbs.

Zone 5, Large 1500 sq ft 1:12 roof. There are a total of 6 skylights (Velux triple pane) 16″x60″ each. For a few reasons I can’t change the size or number of skylights.

Roof assembly will be EPDM, 6″ISO, air barrier, deck, 8″ of fiberglass snug to the deck, gap, vapor barrier, drywall.

Due to 6″ of ISO, the curbs will be 2x10s. I can put some rigid foam on the sides of the curbs, but not much.

What are my options to improve this?

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  1. gusfhb | | #1

    If they are like the fixed curb mounts that I used you can make a small triangle that does not interfere with the overlap of the frame and glue the EPDM to that. Frankly doesn't do a whole lot. You can hide some behind the trim, as you cannot tell from the floor that you are covering some of the glass, but perhaps at only 16 wide that would not look nice.

  2. Expert Member


    You could insulate the outside of the curb below the skylight frame, either by adding foam board to the 2"x10" curb, or framing the curb out of 2"x4"s like very short knee-walls with a short 2"x curb above. Both of those would make flashing the skylight more complicated than it usually is.

  3. gusfhb | | #3

    Have you bought the skylights yet?
    You could buy them wider. I assuuuume the 16 inches is because of rafter spacing.
    Wider skylight, wider curb insulate inside
    Same curb, insulate outside, spacers at the screw holes, or 'L' shaped curb sides to catch screws
    No one will be the wiser

    1. Expert Member
      MALCOLM TAYLOR | | #4


      That sounds like a good, simple fix.

  4. matthew25 | | #5

    I don’t think this is an approved use case but what about the ThermalBuck product used for window and door jamb extensions? One immediate problem I see is that ThermalBuck only goes to 4” of thickness so your 6” of polyiso will still need some sort of wood curb for part of it, but perhaps the ThermalBuck can cut down on the wood thermal bridging? Just a thought, it may not work.

  5. gusfhb | | #6

    I am only familiar with the mounting of Velux FCM [fixed curb mount] skylights

    With them, the mounting screws go in parallel to the roof, while you lean on the frame to compress the gasket which seals between the top of the curb and the glass.

    I think the Thermalbuck are not intended to hold screws. While you could have something inside to catch the screw, several inches of softish material with a constant load from the compliant seal at a right angle to the screw would eventually make them loose and leak air if not water.

  6. Expert Member
    BILL WICHERS | | #7

    I would get as much exterior insulation as you can, which will help with condensation issues on the interior of the skylight wells. I speak from experience here, unfortuantely! I would form some aluminum flashing to cover the opper part of the foamboard, possibly tying that into the roof membrane if it's close enough (instead of just forming a drip edge). I would prioritize keeping water out of the skylight framing, then have insulation for condensation control as the second priority. Be very careful to thoroughly airseal the interior too, so that no moisture can sneak up into the assembly from inside too.


  7. gusfhb | | #8

    With EPDM you pretty much flash with rubber, right up the side of the curb, and if it is watertight it is airtight. With the dimensions the OP has given there is not a whole bunch of space between the top of the membrane and the bottom of the skylight frame.

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