GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Roof insulation

cr0ntab | Posted in Green Building Techniques on

Hey All,

I recently came across this article:

Can Unvented Roof Assemblies Be Insulated With Fiberglass?

I happen to be in CA, in climate zone 3B.

I really like the idea of that system but I wanted to get some feedback on how to do most easily copy a DIY version of that.

The box netting that owens corning provides is pretty neat, it has stable tabs that are used to create different thickness levels of insulation.

I figure if I’m going to do it i’ll go all out and do 12″ of insulation (their drawings show thats R-49, but not sure how accurate that is)

The roof rafters are 2×6’s

The alternative thought was to just use multiple layers of Rockwool (

I’ved used this stuff elsewhere on the property and I love how easy it is to work with, so I tend to lean towards it over fiberglass.

The first layer between the rafters, the second layer to act as the thermal break beyond the rafters.

Lastly, to more easily attach the “sandwich” to the rafters use a 1″ layer of rigid foam ( with long screws.

Anyone got any creative ideas?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    Q. "Anyone got any creative ideas?"

    A. My first reaction is to say, "Detailing a cathedral ceiling is no place for creative ideas. What you want is a boring, tried-and-true method with zero creativity."

    You have several hurdles to adopting the approach described in my article, "Can Unvented Roof Assemblies Be Insulated With Fiberglass?" The first hurdle involves the building code. The Owens Corning system (at least until the most recent version of the residential building code, the 2018 IRC, was adopted) is only approved for houses with tile roofs. So unless your house has a tile roof, check with your local code official before proceeding.

    Most jurisdictions have not yet adopted the 2018 IRC. For those who have, you may want to consider a roof assembly that includes a vapor diffusion vent at the ridge. For more information on vapor diffusion vents, see this article (including the comments section): "Burying Ducts in Attic Insulation."

    Unless you are willing to install a vapor diffusion vent at the ridge, I don't recommend using fluffy insulation like fiberglass or mineral wool to create an unvented roof assembly. In most climate zones, this approach is still illegal.

    For more information on all of your options, see "How to Build an Insulated Cathedral Ceiling."

    1. cr0ntab | | #2

      Hey Martin,

      I should have been more clear in the first post.

      My home does meet the criteria for the Owens Corning system, zone 3B and concrete tile roof.

      Additionally, as I understand (which admittedly, isn't all that much) the current 2018 IRC I also meet the criteria there as well. Section/Table R806.5

      Lastly, the insulation would be in the attic, I don't have any cathedral ceilings in my home.

      I like the Owens Corning product, but I fear that it will be costly to install. I'm working on getting some quotes for it regardless though.

  2. Expert Member
    Dana Dorsett | | #3

    If rafters & roof deck, it has to follow the prescriptives for a cathedralized ceiling, even if it's just the attic ceiling, not the living space ceiling.

    With a concrete tile roof the roof deck is inherently top-ventilated and the average roof deck temperatures will be higher than roofs with less thermal mass (that cool more quickly into a clear dry-air sky). An interior side air barrier is still needed, but it doesn't have to be anything exotic. The interior side air barrier still has to meet local fire-codes if left exposed to the attic.

    1. cr0ntab | | #4

      Hey Dana,

      Thanks for the clarification regarding cathedralized ceilings.

      I found these instructions for the system:

      The installation calls for (page 5) sealing gaps in the roof deck transitions, roof sheathing, roof penetrations and eaves.

      Would this be the Owens Corning equivalent of an air barrier or would that be the job of the box netting?

      I really like the idea of the Owens Corning system as it seems rather DIY friendly (even if I can't get the actual Owens Corning parts)

  3. cr0ntab | | #5

    Hey All,

    So an update on this. Owens Corning says their system is for new construction only and can't be used with a retrofit, pity.

    So I'm off looking for other solutions.

    To restate the conditions:
    Southern California
    Climate Zone: 3B
    California Building Climate Zone: 10
    Concrete Tile Roof

    California has title 24 requirements and I found a PDF (attached) that lists some options for a high performance attic (HPA)

    I'm looking at option B (CEC-400 - Page 248/PDF page 260) which is below roof deck insulation.

    Copy and pasted here (TABLE 150.1-A is on Page 254/PDF page 266):

    (c) Prescriptive Standards/Component Package. Buildings that comply with the prescriptive standards shall be designed, constructed, and equipped to meet all of the requirements for the appropriate Climate Zone shown in TABLE 150.1-A. In TABLE 150.1-A, a NA (not allowed) means that feature is not permitted in a particular Climate Zone and a NR (no requirement) means that there is no prescriptive requirement for that feature in a particular Climate Zone. Installed components shall meet the following requirements:
    1. Insulation.
    A. Roof and Ceiling insulation shall be installed in a ventilated attic with an R-value equal to or greater than that shown in Table 150.1-A meeting options i through iii below.
    i. Option A: A minimum R-value of continuous insulation installed above the roof rafters in contact with the roof deck and an additional layer of ceiling insulation located between the attic and the conditioned space when meeting Section 150.1(c)9A; or
    ii. Option B: A minimum R-value of insulation installed between the roof rafters in contact with the roof deck and an additional layer of ceiling insulation located between the attic and the conditioned space when meeting Section 150.1(c)9A; or
    iii. Option C: A minimum R-value of ceiling insulation located between the attic and the conditioned space when meeting Section 150.1(c)9B.

    The thing that stands out to me is: "between the roof rafters in contact with the roof deck"

    From everything posted here there is supposed to be an air gap between the roof deck and the insulation. Is that line a California title 24 specific thing or???

    I've also included another PDF that illustrates things a bit more clearly, Option B starts on PDF page 36 for the "Decoding Attics" pdf.

  4. GBA Editor
    Martin Holladay | | #6

    A few points:

    1. California's building energy code (Title 24) is unique. It does not resemble the building codes used in other states.

    2. Apparently California has developed a set of options for something called a "high performance attic" (HPA) -- solutions to the ducts-in-the-attic problem. The HPA approach is something found only in the California code.

    3. Whether or not you can install fluffy insulation in direct contact with the underside of your roof sheathing without causing moisture problems depends on many factors, the two most important of which are (a) the climate and (b) whether or not you have a vapor-permeable roofing like concrete tiles (which are common in some parts of California).

    4. Even in the non-California states where the International Residential Code applies, there are safe roof assemblies that include fluffy insulation in direct contact with the underside of the roof sheathing. These assemblies are safe when the builder also installs an adequately thick layer of rigid foam above the roof sheathing. For more information on this type of roof assembly, see "Combining Exterior Rigid Foam With Fluffy Insulation."

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |