Skylight detail on unvented cathedral ceiling
Hi All,
I have an upcoming renovation plus addition project in coastal Maine (6A) where the architect has specced unvented cathedral ceilings with 4″ of foil faced polyiso above the roof deck (Zip panels), with a standing seam metal roof mounted to strapping screwed to the roof and rafters through the insulation. There will be R-37 worth of dense packed fiberglass in the rafter bays below the roof deck. Problem is, there are 4 skylights on one side. I don’t have any experience installing skylights on a roof with exterior insulation. What is the best detail here in terms of sealing against bulk water? Curb mount or deck mount skylight? I’m thinking it’ll have to be a curb mount but not sure if the Velux flashing kit will interface correctly with the metal roof. Or go deck mount on top of 4″ curbs to get it up to the level of the polyiso and use grace or vycor to flash the curb all the way up from the roof deck?
Any insight on this would be appreciated!
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Replies
Curb mount. Flash according to the roofing manufacturers specs.
No point in putting a deck mount on a curb.
conifer525,
Does the exterior insulation really change much? If you fill in the strapping around the skylight RO so you have a solid surface extending out say 12" as a substrate for your waterproofing, you are in the same condition if you were mounting them to a typical roof deck. Then pick either a curb or deck mount based on your preferences.
Make sure the curbs are insulated all the way up to the edge of the skylight. You don't want the inside surface of the curbs to be a condensing surface or you'll end up with interior water problems. You'll also want to ensure you do a very good job air sealing those curbs. What you want to avoid as much as possible is direct thermal bridge paths (i.e. uninsulated areas of framing or other support structure), and avoid any air gaps between insulated and uninsulated surfaces within the assembly.
I would try to put the curb in so that it's tight to that exterior polyiso, and make sure that the entire area of the curb is insulated. You can't completely prevent thermal gradients in the skylight support structure, since you are basically poking a hole through your insulation, but you do want to try to keep things insulated as much as possible.
Bill
Curb mount vs. deck mount is primarily a regional thing. In the northeast US, we almost always use deck-mount on residential projects. Your situation is unusual, though, and making a curb is probably the only safe way to do it. The manufacturer should have details available if you contact them.
As others said, if there's a curb, there's no reason to use a deck-mounted skylight, but your builder may not be familiar with the required details.