# Help with skylight insulation? Can rigid foam be exposed in attic or does it need covered?



## Sg109 (Mar 21, 2021)

We have two Velux skylights just installed by roofers and they left interior work for us to do. They are installed in a sloped/half vault room with minimal accessible attic space.

























Our plan is to add 2x4 bracing to create the skylight shaft and then add plastic vapor barrier to the 2x4. Behind the vapor barrier we would place unfaced fiberglass batting and then on the other side of the batting would be XPS foam board attached to the trusses. Then tape and spray foam seams/any openings. Green (mold resistant) 1/2” drywall would then go on the interior against the vapor plastic. Similar to this photo I found online.








My question is can the XPS foam be uncovered? In the image it shows it as uncovered but the foam instructions say it needs to be covered with 15 minute (assuming 1/2” thick drywall).

or does anyone have better suggestions on how to insulate these things? We live in Reno Nevada zone 5 (for R-rating). Thanks! We are doing this like right now so timely suggestions welcome!


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## Chris616 (Dec 31, 2019)

Considering the probable working conditions in the attic, creating the “exterior walls” that make up the skylight well would be infinitely easier working from the “outside in”, standing on a ladder, instead of what you’ve described.

There is no real advantage to the rigid insulation, since you have the space to put in an equivalent R-value of fiberglass batt insulation, except that the rigid insulation can be fixed in place and used to contain the fiberglass from falling away from the vertical surfaces. I don’t know what is normal practice in your climate, but if you’re putting in a vapour barrier behind the drywall, then I’m doubtful that it is recommended to have another vapour barrier on the other side of the fiberglass, creating a place where if moisture somehow gets in that it cannot dry out. Note in the diagram that you posted there is no vapour barrier indicated behind the drywall.

I suggest to dispense with the XPS board and just use fiberglass. Figure out a way to contain the insulation from falling away from vertical(ish) sides of the skylight light well. One way would be with ½” plywood fastened in place X inches away from the light well (X being the thickness of fiberglass that you want to install). Once that is in place then you can install the fiberglass up against it, then the vapour barrier, then drywall, doing everything from a ladder.

Chris


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## SLSTech (Jan 19, 2021)

Fiberglass insulation needs to be covered by an air barrier & I generally recommend foam for this. Plus you will get better R Value per inch with foam. Should it be covered with a thermal barrier - technically yes but unless you have fuel burning appliance or use attic for storage most inspectors don't worry about it or you can ensure you are fine by meeting the exceptions found in 316.5.3 / 316.6 of the IRC.
FYI Chris - most locations in the US do not use, nor should they use vapor barrier - retarders yes, barriers no
Based on the pics - I would run the foam all the way down to the drywall aka over the framing lumber to ceiling - better airseal & less chance of condensation / other issues


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## Chris616 (Dec 31, 2019)

SLSTech said:


> FYI Chris - most locations in the US do not use, nor should they use vapor barrier - retarders yes, barriers no


Thanks. I suspected that for a climate where the house would be air-conditioned.



SLSTech said:


> Fiberglass insulation needs to be covered by an air barrier & I generally recommend foam for this.


Can you expand on that? Are you just referring to fibreglass around a skylight well, and not on the attic floor, like in the diagram above? If so, why are those two situations treated differently, since they are both separating conditioned interior space from unconditioned?

Chris


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## Windows on Wash (Aug 30, 2011)

Needs a foil covering at a minimum.


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## SLSTech (Jan 19, 2021)

Chris616 said:


> Can you expand on that? Are you just referring to fibreglass around a skylight well, and not on the attic floor, like in the diagram above? If so, why are those two situations treated differently, since they are both separating conditioned interior space from unconditioned?
> 
> Chris


Chris - only place fiberglass can be left uncovered is attic floor mainly due to "ventilation" should be happening above it (thus baffles being required). The knee wall area (skylight) would be exposed to said ventilation / air movement thus cutting down on it's performance. 

WOW - no that is not required. If they did want / need to install a proper barrier over the foam it would be an "approved thermal barrier of not less than 1/2-inch (12.7 mm) gypsum wallboard, 23/32-inch (18.2 mm) wood structural panel or a material that is tested in accordance with and meets the acceptance criteria of both the Temperature Transmission Fire Test and the Integrity Fire Test of NFPA 275."


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