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Review my conditioned attic insulation design

lazukars | Posted in General Questions on
I have a unvented attic and it was originally insulated incorrectly. It violated building code 2015-IRC-R806.5. Originally, the attic sheathing was sprayed 2″ of closed cell foam and the remainder of the 12″ rafters was filled with cellulose. The problem is the 2″‘s of closed cell foam did not meet the 2015-IRC-R806.5 R-20 code requirement for this under sheathing application. Needless to say, I had the cellulose ripped out in order to address the closed cell code violation. But before the insulation contractor comes back out to fix the problem I’d like to get my new insulation plans reviewed. Hopefully, this mitigates more mistakes. Any feedback is greatly appreciated.
Attic Specs:
• Attic has 2″ x 12″ rafters.
• The attic is not livable and is used solely for utilities
• It is a conditioned space

Cleveland, OH ( climate zone 5 )

Insulation Design:
• The rafters will be filled with ~8″ of closed cell foam. That’s around a R-value of R-52. I understand it would be much cheaper to combine the spray foam with cellulose, but after the first insulation mishap, I’d rather have all closed cell foam at this point.
• 1.5″ of mineral wool will pressed up against the 8″ of closed cell foam insulation with insulation supports. I’m doing this to add a thermal barrier over the closed cell foam.  R316.5.3. addresses this requirement.  The 1.5″ thick mineral wool is in this approved list. This adds another ~R-6 to the entire attic R-value.
• The Total Attic ceiling R-value is around R-58, which complies with R402.1.2 for overall R-value requirements in climate zone 5, which I am in.

Pertinent Codes:


• 2015 R316.5.3. : Addresses thermal barrier for the closed cell foam.
• 2015-IRC-R806.5 : Closed cell foam thickness requirements for under roof sheathing.
• 2018 R402.1.2 : Ceiling R-value total requirements



• What is the best way to spray foam the interior eaves of the Attic? Should the eaves be completely filled with spray foam while at the same time being applied to the top plates where the rafters reside?
• What is the best way to air seal the eaves in the attic? Currently there is rigid board blocking this area off to the outside. However, during cold winter nights, I can feel cold drafts coming through the eaves in the attic? I’d like to have my insulation installer go back over this area and air seal it again. Any suggestions for him?
• Does all of the insulation in the rafters need to be covered by a class-III vapor retarder? I ready this somewhere, but not sure it’s true.
• With 8″ of closed cell foam, will it be impossible find potential roof leaks? Is there anything that can be done to help with this concern?
• Do you suggest a better way to insulate the attic? The attic already has 2″ of closed cell foam on the roof sheathing.
• Are the 2018 codes in effect now? Most of the codes listed above are from 2015.

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  1. MattJF | | #1

    Did you experience moisture problems with the prior setup? I might have just thrown up something like membrane and a moisture meter to monitor things.

    Don’t do that much closed cell foam:

    Add another 1.5-2” of closed cell for at least r-20 total. Use a HFO blown closed cell foam to reduce the global warming impact. Instal r-30 rockwool batts. If the attic contains utilities like a furnace or could be used for storage you will need a thermal barrier (dry wall or approved paint).

    Eaves should sealed by continuing foam from the roof down over top plates to the ceiling drywall edge.

  2. lazukars | | #2

    @Matt F

    Thanks for the help. Much appreciated!

    Yes, I was getting a decent amount of condensation in my attic with the 2" closed foam and 10" dense packed cellulose. As of today, my contractor removed all of that celullose in preparations for correcting the closed cell foam thickness problem.

    As for your suggestions, all of what you said makes sense, and it looks like the rock wool is an approved thermal barrier as well, which is great. It needs to be at least 1.5" thick. See All I have in the attic is an air exchanger, so I'm not sure I would even need a thermal barrier, nevertheless, it seems like a good idea to have one.

    The one thing I'm confused about is a vapor barrier. Is there a specific code that says vapor barriers must be installed over the insulation in unvented attics? I'm having a hard time finding information on this.

    Thanks Again,

  3. MattJF | | #3

    Make sure you understand the difference between an ignition barrier and thermal barrier. Review the following to determine which one you need. If it is a walk up attic with significant amounts of floor you will need a thermal barrier, although enforcement is spotty.

    I am not aware of mineral wool being approved as a thermal barrier in the US. I think some thickness was recently approved in Canada though, so they may be working on US approval.

    No vapor barrier needed or required, although if the inspector demands one, use something like Membrane. Do not use poly sheeting.

    Curious what you saw with you moisture issues? Did you have mold growth on the foam? Or did you find water in the winter? I am mostly asking to make sure there isn’t something else going on. While not to code, I would think there would be a decent chance of you prior setup surviving in your location. Particularly with the air leaks you described, which would drop the indoor humidity in the attic.

  4. Jon_R | | #4

    RockWool writes:

    "ROCKWOOL COMFORTBOARD™ 80 have been approved and certified for use as a thermal barrier in front of thermoset and thermoplastic insulation in the US meeting the requirements for NFPA 275 Part I and Part II. The minimum thickness of ROCKWOOL insulation required to meet this is 2” [50 mm]. The US thermal barrier listing is currently held with Intertek; Design Listing 38968 [See Appendix B]"

    Might be easier/cheaper just to add gypsum cover.

  5. lazukars | | #5

    @Matt F

    Thanks again,

    I did not know there was a difference between a thermal and ignition barrier. That's interesting.

    > Curious what you saw with you moisture issues?

    I noticed condensation on days where the temperature was less than 15 deg F. I cut small slits in the Tyvek in random places within my attic, reached my hand up through the dense packed cellulose, and felt the surface of the closed cell foam. Some rafter bays were dry but a majority of them had some condensation on the face of the closed cell foam insulation. I'd say the amount of condensation was equivalent to a slightly foggy window in a car. The more alarming part is that towards the eaves the problem was worse. And if it was really cold the condensation on the foam turned to frost by the eaves.

    I discovered part of the problem was due to the cellulose not being pressed tight up to the face of the closed cell foam. All that was holding the cellulose up to the foam was the tyvek, and that Tyvek can only be held so tight to the rafters. There was natural sag there. My guess is that if the Tyvek was covered with a think sheet of plywood, and the cellulose was repacked, a lot of the problem would have been eliminated. However, I'm anal about code and it would bug the hell out of me to leave the 2" of closed cell foam in place when I knew that doesn't meet the R-20 minimum code for this application.

    Thank You!

  6. lazukars | | #6


    I also have conduit running from my basement to my attic. This conduit goes up through the exterior walls and enters the attic through the top plate. I'm guessing this is some of the cause of the condensation and draft problems I was feeling during the winter.


    Should the opening of these conduits be sealed off with some kind of fireblocking and/or insulating foam. or is there something else that's better?

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