Skylight Insulation // Vancouver Island BC, CZ 4C
First time posting on GBA, I must give everyone here a giant thank-you for all the effort put into this website and the wealth of information it provides.
We are in the process of resheathing our existing cathedral roof and replacing the concrete tiles with a standing-seam metal roof. We took the opportunity to reframe rafters/headers for two 36×36″ curb-mounted skylights, and we are now realizing how cautious we should be with insulating the lightwell and 6″ curb. The skylights are near the ridge of this 15ft cathedral ceiling, so getting up there frequently to wipe off moldy corners is high on the “to avoid” list! Our plan is for this to be our forever home, so we are more than happy to put in the effort needed to create a bright, healthy, low-maintenance, and low-risk home.
Roof Assembly: (8:12 pitch)
– (new) 24 gauge ProLok standing seam metal roof
– (new) Full Peel & Stick Roofnado HT Underlayment
– (new) 1/2in Plywood Sheathing
– (new) Vertical 2×4 strapping laid flat for ventilation gap
– (existing) Horitontal 1×4 strapping (previously used as battens for the concrete tiles)
– (existing) 2×6 rafters 24OC
– (new) R22 Rockwool ComfortBatt Insulation
– (existing and just tightened up) 6-mil Poly interior vapor/air barrier.
– (existing) 5/4″ Tongue and Groove Cedar Interior Ceiling (potentially also main structural roof deck?)
– (existing) Exposed Structural Beams/Trusses, about 55″ OC
We have been researching and asking various people about adequate insulation requirements in this climate zone to mitigate condensation and mold growth in the lightwell. We have received all kinds of conflicting advice, going from “nothing needs done, just slap drywall on the framing, and you’ll never have issues” to “you must insulate both the exterior of the curb and the interior of the lightwell w/ XPS or you will have condensation and rot issues”.
We have noticed how an active (and inspiring) member of this community named Malcolm Taylor lives nearby and seems very experienced with our local cold and humid climate, so we are hoping he or others experienced with skylights in Climate Zone 4C could provide some guidance on the amount of insulation needed for a condensation-free skylight and lightwell for this assembly.
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Replies
Rapha,
I'm just down Island from you, near Jordan River.
Much of the condensation typically comes from the glazing itself and air-leaks where the skylight sits on the curb, so air-sealing there pays dividends. Occurring high up on the ceiling, skylight shafts are where moisture air in the house accumulates. Controlling interior humidity is also very helpful.
There are some pretty elaborate solutions to insulating, which might make sense if your roof was similarly high R-value. My own experience with skylights in existing houses in our climate is that they do well with a 1" of XPS on the inside of the shaft extending from the top of the curb down to the ceiling.
I think that approach represents a good compromise between losing too much space in the shaft and preventing moisture problems.
Hi Malcolm, thank you so much for the quick reply and it's great to be in contact. I have surfed the Jordan River point a couple of times in recent years; what a great place.
For the last couple of years since we bought this property, I often find myself looking for answers on this forum and more often than not, I somehow end up finding the specific answer I'm looking for in one of your comments.
Back to the skylights, we are debating between Columbia for their 2 in. max curb thickness, which would allow us to add 0.5 in. of XPS or Polyiso on the exterior of the curb, or Velux for their European tech & engineering. Is there a manufacturer or model you usually gravitate towards that works well in this climate? We are planning on following your recommendation to install 1" of XPS on the interior of the lightwell.
Rapha,
I don't have a preference. Both are good enough products you shouldn't have a problem with either.
If you do go with Velux I would stick with their curb-mounted models. The skylight then becomes a separate object that can be easily removed for repair or replacement without disturbing the roof - something that's particularly useful when you are using metal panels.
Malcolm,
Thank you kindly for the guidance on this. We will be doing curb-mount and 1in XPS on the whole inside of the light shaft. Could polyiso also work well there? Or is its sensitivity to moisture a deal breaker?
Is there a chance we could hop on a phone call sometime in the next week or two? I have a few questions regarding the local green building scene and also a couple of other climate-specific questions. I'm happy to send my info to Martin so he can put us in touch.
Rapha,
Martin has been retired for some years now. I have a website that although I have retired and stopped paying for it won't go away. You can contact me through it: https://malcolmtaylordesign.wordpress.com/