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Community and Q&A

Non-foam exterior insulation options for a non-vented flat roof

Owenmd | Posted in General Questions on


I am building a house in climate zone 3 (SF bay Area) in a WUI and have a non vented flat structural roof, 2×12 LVLs covered with 5/8ths structural sheathing.  Above the structural sheathing  we have 2x sleepers to create slope to drain with a TPO roof covering.

For insulation we have fiber glass in-between rafters, blown in was is in our building permit but we recently received a quote for a BIB system that was $7/SF so considering batts.

I understand we need 13% of total roof R value above structural sheathing to mitigate risk of condensation on underside of structural sheathing. What I don’t understand is if it needs to be rigid and why?

I saw in a post answered by Martin Holiday that rock wool could be used because the air permeance of the mineral wool isn’t much of an issue.

The reason I ask is that we have been trying to avoid foam in our building and the Building Department approved a 2″ layer of mineral wool board (comfortboard 80) above the structural sheathing and between sloped sleepers. Sleeper go from 2″ at eves to 5″ at peak, so there was an additional cavity that we were told we had to fill, they accepted blown fiberglass. Not sure what filling this las few inches at peak accomplishes.

The insulation installers have said that unless we use some kind of foam above structural sheathing it is not to code and will not work from a  building science perspective.

Our reason for avoiding foam is mainly because we don’t like the idea of all the fire retardants foam use, even though it will be enclosed up in our roof structure. Or the blowing agents and other harmful chemicals in spry foams.

I have read that there are some polyisos that use alternates to the halogenated flame retardants and  spray foams like Icynene that use water as blowing agent and may not have fire retardants, but without really understanding all issues we would rather use alternative unless economical and building science cases outweighed chemical/environmental impact.


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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    I don't know what a WUI is.

    From a building science perspective, it's possible to install semi-rigid mineral wool on the exterior side of roof sheathing as a substitute for rigid foam. Of course, you'll need a layer of roof sheathing or cover board between the mineral wool insulation and the roofing.

    Because using mineral wool insulation in this location isn't common in the U.S., it won't surprise me if local contractors or builders (a) are unfamiliar with this approach, (b) refuse to accept the approach. If you have your heart set on mineral wool, you could try to recruit a manufacturer's representative from Roxul (Rock Wool) to provide technical support for your proposal.

  2. Jon_R | | #2

    Rigid foam insulation can act as insulation and an air barrier. Even with an interior side air barrier (they are never perfect), the latter decreases the risk of interior air flowing to the exterior, depositing moisture and returning to the interior. But this lower risk may not be significant.

    Note that 5.1.4 in below says "rigid or sheet". I don't know what the intent was for this (ie, if it allows Comfortboard between sleepers, then why not fiberglass too?). On the other hand, SF might meet the 45F requirement with no above sheathing insulation.

  3. Owenmd | | #3

    Hey Martin,

    My name is Owen (I see that for some reason my user name isn't updating, sorry).

    WUI is Wildlife Urban Interface, there are additional fire requirements when living close to wild areas.

    Yes we have cover board over sleeper.

    Are there any issues with the mineral wool being air permeable from a building science perspective? It seems to me that If the roof sheathing is taped, it would act as air barrier preventing warm moist air to further propagate up into sleeper cavity where it might then condense.

    With a 13% R Value (for zone 3) layer of mineral wool are there any issues with filling the remaining 0-3" sleeper cavity with fiberglass? can the rest of this be left empty?

  4. Owenmd | | #4

    Hi Jon,

    To me it seems that taping sheathing would keep any warm moist air from propagating into sleeper cavity, and installing enough mineral wool wool would keep sheathing above due point for condensation control.

    Yes, I agree I do not understand why it maters what kind of insulation is above the structural roof sheathing as long as it is keeping structural sheathing warm enough.

  5. GBA Editor
    Martin Holladay | | #5

    Q. "With a 13% R Value (for zone 3) layer of mineral wool are there any issues with filling the remaining 0-3" sleeper cavity with fiberglass? can the rest of this be left empty?"

    A. As long as you meet the 13% R-value minimum rule, the extra fiberglass does no harm. But from a practical viewpoint, my opinion is irrelevant, as long as "The insulation installers have said that unless we use some kind of foam above structural sheathing it is not to code."

    Needless to say, you have to convince your contractor and your local code inspector, not me, of the viability of your plan.

  6. JC72 | | #6

    Have you checked the website of the rockwool manufacturer? It may have some information which the installers can rely upon. Ultimately if the installers aren't comfortable with the material then it might be wise to just use rigid foam. There's nothing inherently dangerously about rigid foam used in flat roofs anyways. If you want to make an objective decision regarding risk be sure to consider the entire assembly vs stopping at the foam and then letting emotion take hold.

  7. Owenmd | | #7

    Thanks for your responses

  8. Owenmd | | #8

    Hello again,

    We had put this decision off and now are more pressed to make a decision. We would like to use rock wool here (understanding it is not standard) but would like to run by the reasoning of the insulation installer as I have gotten further clarification regarding reasons he thinks it would be a bad idea to use rock wool as the exterior insulation for an unvented roof assembly.

    His reasoning is that foam allows no air to exist within a board of foam, where as mineral wool (being air permeable) would have some percentage of the top rock board filled with air. While we plan to have an air barrier (taped/sealed sheathing) the argument is that there can always be mistakes and if moist warm air entered the exterior insulation layer at the roof, the Top Rock would pose a greater risk than foam, as moist air could propogate into a Top rock board and condense when it got close enough to cool TPO membrane.

    I can understand the logic here but I don't understand if this is a concern, what % volume of air would a mineral wool board have and is this significant?


    1. GBA Editor
      Martin Holladay | | #9

      The roofer is correct that air movement through the mineral wool insulation foam wouldn't be good. You can prevent air movement through the mineral wool by taking three steps:

      1. Make sure that you have an air barrier below the mineral wool. This could be synthetic roofing underlayment with taped seams or roof sheathing with taped seams.

      2. Make sure that you have a perimeter board (the same thickness of as the mineral wool) that is airtight. This will probably require caulk or tape at the seams. You don't want outdoor air to sneak into the mineral wool at the perimeter.

      3. Make sure that the mineral wool insulation has an air barrier above it. This should be easy -- your membrane roofing is an air barrier.

  9. Expert Member
    Dana Dorsett | | #10

    An alternative would be to use mid density fiberboard (asphalted or not) as "walkalble" highly moisture-tolerant insulation, which needs no exterior decking or sleepers. It runs about R2.65/inch.

    A single layer of SonoClimat ECO4 from MSL (from Quebec) plus anybody's 1/2" asphalted fiberboard sheathing would get you the minimum R5 prescribed by the IRC for zone 3.

    Four layers of 1/2" or three layers of 3/4" of anybody's asphalted fiberboard sheathing would also get you there.

    Given the high expense of rigid rock wool and the additional labor of sleepers & top decking this may ultimately be a cheaper-better option.

    [edited to add]

    It would take 3" of fiberboard to have the equivalent R of 2" of rock wool (2 layers of 1.5" thick SonoClimat ECO4= R8, or 4 layers of anybody's fiberboard sheathing.) But it wouldn't have the thermal bridging of the sleepers, and would add structure to the roof deck.

    A few years ago was tangentially involved with a rehab project on a flat roof in climate zone 5 that had two layers of 3/4" fiberboard on top of the sheathing with ~R10 vermiculite under the 3/4" plywood roof deck. Even after nearly 60 years of service the fiberboard was still in good shape. Rather than making a vermiculite mess the owner opted to build a low-slope attic over the pre-existing roof to house new HVAC, sealing up the old roof, insulating the new roof with 6" of polyiso above the new roof deck.

  10. Jon_R | | #11

    > the Top Rock would pose a greater risk than foam

    My opinion is similar. Mineral wool is mostly air and sealed cavities with free air pump from pressure changes - they pull in moisture even when you air seal them well.

    I expect that condensation on the underside of the TPO is going to drip to the sheathing. A waterproof underlayment would help.

    Maybe someone makes foam without fire retardant.

  11. Jon_Lawrence | | #12

    FHB used Top Rock in the 2018 Pro House in California. They installed a vapor permeable air barrier underneath the Top Rock and then metal roofing on top.

  12. maine_tyler | | #13

    This Old House in its latest season did a Net-Zero retrofit where they also used Roxul Board. They had quite a stack-up, however, including closed spray foam from underneath, and multiple peel and stick membranes / sheathing layers sandwiching the Roxul...

  13. Jon_R | | #14

    What about the argument that you meet the requirement with no foam or Top Rock? SF is 50F in January.

    "5.1.4. Alternatively, sufficient rigid board or sheet insulation shall be installed directly above the structural roof sheathing to maintain the monthly average temperature of the underside of the structural roof sheathing above 45°F (7°C). "

    Where did 13% come from for your design?

  14. Owenmd | | #15

    Thanks everyone for your responses, all helpful!

    Jon R. That is interesting point, if sheathing needs to be above 45 degrees for the average coldest 3 months, would I be looking at the low average monthly temperature?

    Fairfax, where we are building, has the following monthly high/low averages:
    November 64° / 46°5 days
    December 56° / 42°8 days
    January 56° / 42°9 days
    February 61° / 44°8 days

    I got the 13% I think by looking at "Table R806.5 - Insulation for Condensation Control" for zone 3C taking R5 and dividing by R38 (min R value per code for our zone). I assumed we needed at least R5 given the R value listed in table from my climate zone, but does condensation not really occur if out side temperatures are above 45 degrees of average?

    1. Expert Member
      Dana Dorsett | | #16

      That analysis of the IRC is correct. The IRC doesn't really take the 3C coastal temperate zone separately, so there is some wiggle room.

      No interest in doing it with fiberboard insulation?

      Three inches of fiberboard has the same R and more thermal mass than 2" of rock wool between sleepers, it will outperform the rock wool. The 2" sleepers would have to be built-up 2x stock or rough-sawn full dimension 2x, which are a thermal bridge through that expensive rock wool, and the thermal mass effects of the fiberboard will improve the overall performance, lowering the peak and average loads seen by the cooling/heating. It's nailable, walkable, water tolerant, and will ADD structure to the roof- it doesn't need a secondary deck above. A 2" rock wool between sleepers + 5/8" CDX solution is only 3/8" thinner than 3" fiberboard.

      The insulating and water resistant properties of fiberboard seems to get a lot more respect in Europe than in the US. In your application it seems likely to come in cheaper too, since the installation is a LOT simpler, and the material is fragile in handling than rigid rock wool- you can back the truck over the fiberboard without doing significant damage, or leave it out in the pouring rain, etc.

    2. Jon_R | | #17

      Code allows you to choose - either R5 or the 45° method. You want to use the average - which is 48° for the 3 lowest months. So unless local codes are different, apparently you don't need any insulation above the sheathing.

  15. Owenmd | | #18

    Dana and Joh, Thanks for responses, definitely filling in some gaps for me.

    Regarding the Fiberboard, the Rock wool that we are using is called TopRock DD:

    The reasons I chose Top Rock was because I was trying to avoid foam, we are in a Wildlife Urban Interface so we need fire protection and TopRock DD is non combustible. TopRock can be used without sleepers and cover board and has R Value around 4/inch. Since we have 12" LvL with another few inches for sloping structural sheathing, I thought 3" of TopRock would satisfy the 13% exterior ratio if internal roof insulation was fully filed with fiberglass (not sure if I can partially fill internal rafter cavities with rockwool or fiberglass batts as total total roof assembly is around R80: R12 exterior + R65 interior )

    Now I am trying to figure out what to do, unfortunately I just learned that my contractor ordered the TopRock and it is unlikely that it will be cost effective to return, so I may be stuck with it.

    Fiberboard would have been a good option to consider. I am actually planing to use a fiberboard on top of my slab on grade foundation to provide R5.8, it is 1.5" Gutex Multitherm (I am hoping I am choosing the right product here)

    But sound like what Jon is saying, from a code perspective, don't even need the exterior insulation, since the average 3 coldest month temperature is above 45 degrees. I don't understand details for why this is but I would take this to mean even if I used the TopRock rock wool I will not be likely to have consequential condensation issues, given my climate.

    One thing to note is that the building site is on north west facing base of a hill and in the dead of winter roof won't get any sun, this changes as move out of winter but parts of house wouldn't get full sun until spring.

    We will be air sealing sheathing plane, anything else we should consider doing? would using a WRB over roof sheeting help protection sheathing if there is condensation within the exterior TopRock? Would using a smart vapor barrier (Intello) on interior be advisable?



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