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I’m wondering if you can insulate the attic roof rafters

kristinburton | Posted in General Questions on

Thanks everyone for your insight. I think the thing I’ve learned is that if your not going to finish the attic area and the walls to the finished room are insulated you don’t have to insulate the roof joist, but you do need to insulate in between the floor joist and the finished walls.

In our doormer, There are two small doors going into the attic area from a finished dormer area, and the walls that are finished have batting. The floor joist in the attic have old blown fiber glass that has settled, so we need to fluff it up or add to it.. Thanks so much everyone.

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  1. Expert Member
    Dana Dorsett | | #1

    On the floor they're called "joists", on the roof they're called "rafters".

    Yes, you can insulate at the rafters, but it's more complicated.

    The IRC 2012 Chapter 8 gives some prescriptive guidance, but it's pretty blocky:

    But it's more complicated than that. It's possible to cheat the IRC 2012 prescriptives with as little as 1-2" of closed cell foam on the underside of the roof deck + fiber insulation.

  2. Richard Beyer | | #2


    Flash and batt CC minimum is 2 inches unless the manufacturer (not the installer) put's it in writing.

  3. GBA Editor
    Martin Holladay | | #3

    I'm going to disagree with both Dana and Richard.

    1. Unlike Dana, I don't recommend that you violate building codes. If you want to insulate a sloped roof assembly on the interior with a combination of spray foam and air-permeable insulation, the spray foam layer has to meet certain minimum R-value levels, as described in this article: How to Build an Insulated Cathedral Ceiling.

    The minimum R-value of the spray foam layer is:
    R-5 foam for Climate Zones 1-3,
    R-10 for Climate Zone 4C,
    R-15 for Climate Zones 4A and 4B,
    R-20 for Climate Zone 5,
    R-25 for Climate Zone 6,
    R-30 for Climate Zone 7, and
    R-35 for Climate Zone 8.

    2. Richard's recommendation that the minimum thickness of the spray foam layer is 2 inches would only work in Zones 1 through 3 or Zone 4C. In all other climate zones, you need thicker foam.

  4. Richard Beyer | | #4


    Interesting you say this because you said this in a previous post;

    "Joe Lstiburek recommends that flash-and-batt jobs include a minimum of 1 inch of closed-cell spray foam in Zone 5. If you're in Zone 4, there is no minimum foam thickness, because condensation problems are unlikely in Zone 4.
    That said, it's hard to spray an even 1/2-inch layer of spray foam. Anyone who tries probably ends up with an uneven layer that varies from 1/8 or 1/4 inch to 3/4 or 1 inch. You might be better off asking for 1 inch -- it's likely to be a better air seal.
    Answered by Martin Holladay, GBA Advisor
    Posted Wed, 09/01/2010 - 15:39"

    Was it known failures or a simple code change that changed your opinion?

    This is what Industry expert Mason Knowles (Mr. Spray Foam himself) stated about Flash & Batt.

    "RE: Flash -n- batt.... "
    " I personally like to provide the thickness of closed cell foam that will provide the vapor retarder properties in the assembly without relying on a separate vapor retarder element over the fiberglass. 2 inches is typically the thickness most folks recommend in northern climates. Although a few like to install 2.5-3 inches in the more severe climates such as Montanna and Northern Wisconsin
    But if you can provide a vapor retarder element over the fiberglass that is installed properly (no tears, gaps, holes, etc) then the lower thickness of closed cell foam (1 inch) would work.
    Some folks are selling and marketing hybrid systems (flash and batt) in northern climates with 3/4 to 1 inch of foam followed by fiberglass that has a vapor retarder facing.
    But, I am fairly conservative on this issue, knowing how often the fiberglass and vapor retarder is not installed properly."

  5. GBA Editor
    Martin Holladay | | #5

    You are talking about walls. I am talking about roofs.

    The recommendations (and code requirements) are different for roofs, because rafters are deeper than studs; because R-values are highers for roofs than for walls; and because thicker fluffy insulation in rafter bays than in stud bays requires thicker foam R-values.

    All of this is explained in my article, Calculating the Minimum Thickness of Rigid Foam Sheathing.

  6. Richard Beyer | | #6

    Martin the men installing this product believe the same rule applies. Don't shoot the messenger. ;)

  7. GBA Editor
    Martin Holladay | | #7

    If contractors in your area are using the flash-and-batt method to insulate unvented roof assemblies, then the layer of spray foam has to follow the minimum R-value requirements that I have explained.

    If they are only installing 2 inches of spray foam in Climate Zones 4A or 4B, or in Climate Zones 5 through 8, their installations violate the building code.

  8. Richard Beyer | | #8

    Martin this is good to know because a friend of mine has flash and batt in a million dollar home and the insulator certainly did not follow this procedure. In fact, the insulator used 1.5" of open cell and then fiberglass with craft paper including the attic roof. Their reasoning was the foam would air seal and the fiberglass would bring the rest to code. The code official passed it because he said as long as the R value meets the code requirement that's all he could enforce. FYI... Code officials are immune from legal liability in Connecticut. It's been 4 years since the installation and this guy said he is spending less money on heat in his 3500 sq ft colonial than his brother in his 2400 sq ft raised ranch.

    I recall reading an article written by Mason Knowles regarding his own house in Virginia where he sprayed 2" of closed cell on his attic ceiling and filled the rest with cellulose to meet code.
    If the experts are advising this then to inquiring minds who is policing it? Certainly not the code officials.

  9. GBA Editor
    Martin Holladay | | #9

    Most states have an energy code, but residential energy codes are rarely enforced, except in California and a few areas where building inspectors are adequately educated and where municipal budgets are high enough to permit enforcement.

    For more information on this decades-old problem, see Are Energy Codes Working?

    Even in areas where energy codes are rarely enforced, however, contractors who violate energy codes are putting themselves at risk. In the event of a failure or a lawsuit, contractors who have ignored code requirements are unlikely to be on the winning side of a court judgment.

  10. Expert Member
    Dana Dorsett | | #10

    Martin: Just because I pointed to Straube et al's WUFI simulations indicating that doing something slightly different from code when using closed cell foam on the interior can be low risk doesn't mean I'm recommending it. My general recommendation on unvented roofs is to put rigid insulation on the exterior, and if OSB is used for the roof deck that it be in EXCESS of the IRC prescriptive values.


    "Flash and batt CC minimum is 2 inches unless the manufacturer (not the installer) put's it in writing."

    According to whom? This sounds like some variance that was allowed by some inspector, or some marketing argument by a foam company or something. It certainly is not in the IBC/IRC.

    Anyone contemplating a flash & batt rafter solution at less than IRC Chapter 8 prescriptive values read Building Science Corp's ba-1001 ( ) in it's entirety- it's not every long. In most of the US a mere 1" of 1.2 perms /inch closed cell foam would be sufficient vapor retardency to be protective of the roof deck. Cheating IRC chapter 8 (and chapter 7, if walls) prescriptive R-values may still leave some mold potential in the fiber insulation itself, but with 1-2" of closed cell foam (2" in colder climates) your roof & rafters won't rot away. If taking that route it's probably better to use dense packed cellulose rather than 1.8lb JM Spider (the fiber they simulated) to gain the benefit of safer moisture buffering that you would get with cellulose, and not with fiberglass. The air retardency of 3.5lb cellulose is comparable to (slightly tighter than) 1.8lb Spider (though at 2.2lbs density Spider is more air retardent than 3.5lb cellulose.)

    If you are not insulating the roof to the IRC 2012 Chapter 11 code minimum for roof you can DEFINITELY cheat on IRC 2012 Chapter 8, as long as the ratio of foam-R to total-R is maintained. eg: For climate zone 5 Chapter 11 prescribes R49 for roof-R, and chapter 8 prescribes a minimum of R20 against the roof deck if fiber insulation is to be used on the interior side with only class-III vapor retarders. A ratio of R20/R49 means about 40% of the total R must be air-impermeable insulation, so if you're looking at only R30 between the rafters you can get away with R12 foam, and up to R18 as fiber insulation. That could be 2" of closed cell foam and 5.5" of low-density fiberglass (say R19 batts compressed to 5.5", which delivers R18), a stackup that fits nicely into milled-lumber 2x8 rafters.

    Similarly, if the total R will exceed code-min, the prescriptive R value or the foam is not sufficient, and needs to be increased proportionately.

    The ratio will differ by climate zone- see both Chapter 11 AND Chapter 8 to figure out the minimum ratio for a given location:

  11. Richard Beyer | | #11

    Dana that's exactly what it is... marketing, installer and SPF expert arguments. Definitely not a code issue.

  12. Expert Member
    Dana Dorsett | | #12

    I guess that's one way to double-up on the foam volumes, when only an inch would do! :-)

    Seriously, a 1" shot of most 2lb foam is pretty close to being a Class-II vapor retarder on it's own, and plenty of protection in some climates. Some 2lb foams test at 0.8 perms @ 1" , which would in fact duct under the Class-II definition, not that it's dramatically different in moisture protection than stuff that tests 1.2 perms @ 1". From the perspective of protecting the roof deck an inch is pretty good except in very cold climates. In a wall flash'n'batt it's enough almost anywhere, and preferable to 2" if there is any foam sheathing slowing down the drying rates toward the exterior.

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