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Community and Q&A

Any advice for installing skylights in an unvented cathedral ceiling, other than don’t do it?

saucymonkey | Posted in General Questions on

Hi I’m getting set to install 2 velux skylights in the roof of my addition. Its a shed roof style addition that I built using 2x 12 rafters that will be dense packed with cellulose and on top of that I have a SIP of 2 layers of 5/8 advantec sheathing and 2 layers of 1.5″ blue foam.

I’m concerned about condensation issues and this will be my first attempt at installing skylights so any advice in general beyond the manufactures instructions will be appreciated. I live in climate zone 5 and yes I am now aware that 3″ of foam is not the recommended amount, but I was unaware of this before the roof went on. Would I be wise to add interior storm windows over the skylights for the winter months or would an airtight window box be a terrible idea?

Also since the roof is unvented but I have eves that overhang 2′ should I put vents in the eves with no way to vent the air up and out, or should I just box them up tight? Thanks -Chris

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  1. GBA Editor
    Martin Holladay | | #1

    You don't need soffit vents if your roof is unvented.

    If you haven't installed the roofing yet, you should consider adding more rigid foam on top of your roof assembly. Other than that, the best thing you can do to try to prevent moisture problems in your roof assembly is to pay fanatical attention to air tightness. Of course, that means no recessed can lights. Pay close attention to air sealing at the perimeter of each room, at any partition top plates, and at any penetrations like plumbing vents.

  2. Expert Member
    Dana Dorsett | | #2

    Since climate matters, what is your location?

  3. saucymonkey | | #3

    Thanks for the replies, I'm on the coast of Maine. The roof is already on unfortunately, But I do plan keep it as tight as possible. Its a simple10x20 sloped ceiling and I feel confident I can make It air tight. I just had a meeting with my insulation contractor and he talked me out of doing multiple skylights. I'm now just going to do the one venting skylight in the bedroom and a Velux sun tunnel in the windowless bathroom where there is a small vented attic space above. Originally I was going to put in a fixed skylight there with an insulated light shaft through the attic. I knew this was asking for trouble and my insulation contractor strongly discouraged the idea. I'm kind of disappointed that I've had to nix my original plans, but I'm much more comfortable with this decision.

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