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Air sealing skylights from the interior after construction is complete

aunsafe2015 | Posted in General Questions on

I recently moved into a house that’s about 10 years old and has 4 skylights.  I had a blower door test completed recently.  Results came in at 4.5 ACH50.  The 4 skylights appeared to be a major source of air leakage.  So much so that you could actually hear the air coming in and feel a draft around them, standing 5-10 feet below them.

I’m not going on to my roof to try to fix them from the outside, and I’m not planning on hiring somebody to do that.  And unfortunately they are in a cathedral ceiling so I do not have attic access to them.  The best I can do is get up on a ladder and caulk any gaps.

Would trying to find and caulk gaps just be a waste of time in this situation, or might it actually yield an improvement?  Assuming not a waste of time, any potential hidden issues I should be aware of?  Or should my goal simply be to find the gaps and seal them?



  1. Expert Member
    Michael Maines | | #1

    The skylights I'm familiar with are deck-mounted Velux. They don't have a gap like windows and doors do that you can foam; the only air-sealing is usually a peel-and-stick membrane lapped from the roof deck onto the sides of the skylight frame. You may not have the peel-and-stick, or it may be done sloppily enough to get air leaks. If your home is like most, your interior air control layer is probably only drywall? If so, simply caulking it to the skylight frame would help. Ideally you'd have a separate air control membrane that you could tape the skylight frame to.

    1. aunsafe2015 | | #3

      Thanks for the response. Yes, interior is just drywall. I'll get up on a ladder soon with some caulk and take a closer look.

  2. Expert Member


    There is no disadvantage to air-sealing any gaps between the skylight and the drywall surround you can find.

    My bet is the skylights were just dropped on and fastened no air-sealing at all. The best and easiest way to seal curb-mounted skylights is to run a gasket on the top of the curb before the skylight is mounted (never use caulking as some manufacturers suggest, as it precludes removing the skylight in the future). I've done that in retrofits, and it is quick and easy, but it does involve working from the topside, so if you aren't comfortable with that I'd do as you are suggesting and see what improvements you can make from a ladder.

    1. aunsafe2015 | | #4

      Thanks for the response. The blower door tech suspected exactly the same thing -- that the skylight was just fastened onto the curb with no air sealing, or very poorly done air sealing.

      How exactly is the skylight typically fastened to the curb? It is as simple as simply removing these 8 screws (2 in each side, see attached picture) and lifting it off? I actually have easier access to them than I realized, from a roof deck, so maybe installing a proper gasket is in my future...

      1. Expert Member
        MALCOLM TAYLOR | | #6


        Usually the screws are on the inside of the frame, but I can't think of what other purpose those would have.

        Skylights can demonstrate the power of pressure imbalances and exfiltration. I've seen framers drop them into place, then go down the ladder to fasten them from inside, and as soon as they opened the front door, the skylight popped off the curb and down into the yard. I had a great view of it all - as I was the framer.

      2. jberks | | #7

        Yes those are the screws on the sides holding them in place. You can unscrew them and the skylight should be able to lift off.

        I am used to velux curb mounted skylights, which yours look similar, and they should have a polyurethane gasket integrated to create the seal. When installing, you apply some top pressure on them creating a seal, and screw them on the sides just like your photo.

        That gasket is flexible and handles expansion and contraction and keeps a seal.

        I can fathom that over the years, if you had a dimensional 2x lumber that perhaps some shrinkage of the wood over time might contribute so some leakiness. Or perhaps some debris got on the buck during install.

        I suggest taking them off to inspect them, the gasket and the buck, and reinstalling. Push down on the skylight frame a little before screwing to get them a little tighter. Just be mindful to reposition to get new screwholes. Cause if the screws go back into the old holes in the wood, it'll just sit in the same position as before.


  3. drewintoledo | | #5

    Just a thought, ever think about AeroBarrier?

    1. aunsafe2015 | | #8

      I have thought about that, but we already live in the house, and I'm a little skeptical of it from an indoor air quality perspective. I wouldn't rule it out completely, but it's way down on the list of things I'd consider.

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