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1 inch or 2 inches of closed cell in cathedral ceiling, then Rockwool?

Jon951 | Posted in General Questions on

I’m trying to determine if it is wise to have 1 or 2″ of closed cell foam applied to my cathedral under roof areas, then install Rockwool. House located in Adirondacks, New York State…climate zone 7-8.¬† Considerations:
1. Roof and soffit areas are not currently cut for venting
2. Concerned installation of Rockwool alone and venting soffit areas and cutting roof at ridge for venting will lead me to having to frame air gap space in bays with lumber (Styrofoam proper vents not trusted…fear they may collapse). Proper vents will get crushed if using two batt thick 5 1/2″ Rockwool …tried this once before. Seeking at least R46 performance).
3. Membrain install expensive and laborious…I believe inch or two of closed cell foam will eliminate the need for this vapor barrier which I have used in the past with good results.


  1. user-2310254 | | #1

    Probably not wise since you need R-30 (zone 7) or R-35 (zone 8) for the foam layer. But I wonder if you are in climate zone 5 or 6. What is your ZIP?

  2. Jon951 | | #2

    I have to conform to a R46 standard.....12946

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Jon.

    One or two inches of closed cell spray foam is not enough for a flash-and-batt cathedral roof assembly in your area. I suggest that you read this article. It should answer all of your questions including how much interior vapor control you will need for the assembly: How to Build an Insulated Cathedral Ceiling

  4. user-2310254 | | #4


    Essex County is Zone 6 (see So you need R-25 (51%) in the foam layer and the balance in air permeable insulation (for the total R-49). In addition to Brian's link, you might want to read this one:

  5. Expert Member
    Dana Dorsett | | #5

    First, there is no location in the Adirondacks that would be DOE climate zone 8 (not even Marcy's summit.) At altitude it could be zone 7, but it the warm edge of zone 7. Zone 7 starts at 9000 HDD annually (Saranac Lake qualifies at ~9400 HDD), but zone 8 begins at 12,600. I don't see anything even approaching mid-zone 7:

    So, designing for zone 7 might be rational, but not zone 8.

    >"I believe inch or two of closed cell foam will eliminate the need for this vapor barrier..."

    Is that "faith-based building science or something? :-)

    A couple inches of closed cell is enough to be protective of the roof deck in zone 7, but there will be frost/ice build-up in the rock wool over the course of the winter, which will melt & drip as warmer temperatures arrive.

    A stackup of 6" of EPS above the roof deck (seasonally R27, up-rated for temperature) or 5" of roofing polyiso (seasonally R25, down rated for temperature) with R15 rock wool on the underside of the roof deck would meet code performance on a U-factor basis, and provides adequate dew point control at the roof deck layer on the warm edge of zone 7.

    There are multiple vendors of reclaimed/used and factory seconds foam board in upstate NY to take some of the financial sting out of that approach:

    >"Membrain install expensive and laborious…"

    How is detailing MemBrain any more laborious than detailing polyethylene vapor barriers?

    1. Jon951 | | #6

      Roof is installed on this build, so anything above the roof deck is now not possible without taking huge financial hit. So...what's the best move here?

      1. user-2310254 | | #7

        What type of roof structure do you have? Is it trusses or dimensional lumber? If dimensional, how deep are the rafter bays?

      2. Expert Member
        Dana Dorsett | | #8

        In climate zone 7 as long as at least 3/5 of the total R (60%) is closed cell foam at the roof deck there won't be much moisture accumulation in the rock wool.

        Using 4" of HFO blown closed cell foam you'd be at R27-R28, and would be sufficient for dew point control on R15 rock wool, delivering a total-R of R42-R43. With 2x8 rafters the rock wool would would be 1/4" proud of the rafter edges. Compressing it flush with sheet-rock reduces the R-value to about R41-R42, which is fine. If the rafters are deeper than that, support the rock wool with something else, say, PERFORATED radiant barrier (about 5 perms) side-stapled to the rafters, which will add about R1 to the performance of the stackup as long as it's facing an air gap of 1/2" or greater. Don't use bubble pack or unperforated foil type radiant barriers for this application, since they would create a moisture trap.

  6. Jon951 | | #9

    Rafters are 2x12 dimensional lumber, so I could go with 4" HFO, and a stackup of 5 1/2" R23 Rockwool product. to get me approx. a total of R51 max. Or...4"HFO and fiberglass which would yield R47. I would imagine the fiberglass insulation would have to be installed unfaced and then it would be held using the perforated vapor barrier you mentioned previous.

    1. Expert Member
      Dana Dorsett | | #10

      >Rafters are 2x12 dimensional lumber, so I could go with 4" HFO, and a stackup of 5 1/2" R23 Rockwool product.

      Not in zone 7 you couldn't. In zone 6 it works. At 8487 annual HDD Lake Placid 12946 is still generically the cool edge of zone 6, but if your site is several hundred feet above the weather station (at 1940 ft) you could still be zone 7.

      That would be only ~55% of the total R as foam, which isn't enough for zone 7. It has to be at LEAST 60%. The IRC prescriptive is R30 out of R49 total, which is (30/49=) 61.2%.

      Going with 5" of HFO blown foam (R34), + R23 rock wool would almost work from a dew point control perspective (59.6%) but 5" HFO blown foam + R21 fiberglass (61.8%) would be closer to the code-prescriptive ratio.

      HFO blown foam is north of a $1.25 per square foot per inch in my neighborhood. At some point it may be cheaper to re-roof and go with used or factory seconds roofing foam above the roof deck if the roof lines are simple enough. This place in Fort Plain NY has factory-blem foam at reasonable price points:

      There are other deals & dealers out there.

      A couple layers of 3" foil faced polyiso gets you to code-min on a U-factor basis, skip the cavity insulation.

  7. Jon951 | | #11

    Roof architecture is complex and house sits at 1955 ft. elevation, so it looks like its going to be 5" foam and R21 fiberglass. Is HFO critical to this install, or other types of CC foam acceptable?

    1. MattJF | | #13

      Please use an HFO foam as the blowing agents have significantly less global warming impact than HFC based foams.

      At 5" HFO foam may actually end up on par or even cheaper than HFC based foams because they can put it down in a single pass.

      Look for contractors using the following products:
      Gaco OnePass low GWP
      Lapolla Foam-Lok 2000 4G
      Demilec Heatlok HFO High Lift

      There may be others that meet the criteria you need as well.

    2. Expert Member
      Dana Dorsett | | #14

      >"Roof architecture is complex and house sits at 1955 ft. elevation, so it looks like its going to be 5" foam and R21 fiberglass."

      1955' is only 40' higher than the weather station, not 400' or 1400'. You're still solidly in zone 6, not zone 7. So 4" of HFO blown foam + R23 rock wool works.

      >"Is HFO critical to this install, or other types of CC foam acceptable?"

      At 4" typical (climate damaging) HFC blown foam is only going to run R24, which is a bit too close to the margin for comfort unless there is also a smart vapor retarder. Another factor is that goods blown with HFC245fa MUST be installed in lifts no greater than 2" per pass, with a cooling period in between. This is both a fire-safety and quality issue- the amount of heat generated during the curing is enough to start fires if it's layed on too thick, and when it's heated that much internally there are shrinkage & cracking issues, as well as internal char pocket issued.

      The industry standard HFC245fa blowing agent is an extremely powerful greenhouse gas, about 1000x CO2 @ 100 years. If the US had signed on to the Kigali amendment to the Montreal protocol it would have been banned for this application as of 1 January 2019.

      Goods blown with HFO1234ze run about R27 @ 4", can be safely installed in a single pass, and have a global warming potential about 1/500th that of HFC245fa.

  8. Jon_R | | #12

    The proper R value of open cell spray foam meets code at less cost. So does rigid foam, but you want some spray foam for air sealing.

    Be conservative on the R values (since published values are too high for the very long term) and R ratio. If you ignore this and cut it close, then consider cellulose (vs fiberglass).

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