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More confused….open vs. closed spray foam for my climate zone

jpcjguy | Posted in Energy Efficiency and Durability on


I want to finish off my third floor. My house is in Richmond, VA – Climate Zone 4 – (right in the middle of the zone). House was built in 1993. The attic is walk up and rafter built with 2x8s. Here are some pics of the space:

I am looking to insulate all the rafters and bring the HVAC into sem-conditioned space. I would get a mini-split system for the 20×20 “room” that would be drywalled in on the third floor. (The rest of the attic space would be left unfinished with a fire barrier of the insulation).

I just replaced the 2nd floor HVAC with an all new unit last summer. The roof is original and will be replaced in the next month or two – this way I can watch for leaks before deciding on an insulation solution.
So I have to have R-30 according to my local AHJ. If I go fiberglass, I would attach 2x4s to the 2×8 rafters to give me a space of 10.75″ to add insulation and leave a gap for a vent from soffit to ridge.

If I go spray foam (professionally done only) I am unsure on open vs. closed in my zone. 2 of the three quotes I got said open but could do closed for more $$$ (shocker). I get my third quote tomorrow. After reading so many posts/blogs/articles I am lost on the open vs. closed for my area. We get really hot and, last winter, hit the low teens at night (unusual though).
Any thoughts?

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  1. jpcjguy | | #1

    Sorry pics did not work in original post

  2. GBA Editor
    Martin Holladay | | #2

    In your climate zone, the 2012 International Residential Code calls for a minimum of R-49 roof insulation -- so even if your local code official is fine with R-30, it would be much better if you could aim higher.

    You can use either open-cell spray foam or closed-cell spray foam in you climate zone. However, beware of deceptive sales practices by spray foam contractors. It's common for these contractors to try to convince homeowners that "R-20 spray foam performs as well as R-38" and other similar nonsense. To arm yourself against this type of falsehood, you might want to read It’s OK to Skimp On Insulation, Icynene Says.

    Here are links to two other relevant articles that you may want to read:

    Open-Cell Spray Foam and Damp Roof Sheathing

    Creating a Conditioned Attic

  3. Expert Member
    Dana Dorsett | | #3

    Using R6/inch closed cell foam where it is thermally bridged by framing is a waste of good foam, since it's performance is so severely undercut by the framing fraction. With 7" /R42 of closed cell foam in a 2x8 rafter you'd almost make code min on an R-value basis, but would still be a long way away from making it on a performance basis (a U-factor of U-0.026, which is a whole-assembly R of about R38) due to that thermal bridging.

    Open cell foam is far greener since uses about half the polymer per R, and is blown with water (a very low impact blowing agent) instead of HFC245fa, which is a powerful greenhouse gas (~1000x CO2).

    Going unvented with 7.25" of open cell trimmed flush to the rafter edges you'd be at about R27 center-cavity, and you'd have about the same thermal bridging of the rafters. It would require a class-II vapor retarder such as "vapor barrier" latex on wallboard, or a layer of half-inch OSB, or a "smart" vapor retarder like MemBrain or Intello on the interior side to protect the roof deck from interior moisture drives, but with those measures in place it would be fine. If you installed 2x6s perpendicular to the rafters 24" o.c. and installed R23 rock wool (or even R21 fiberglass) batts between them you'd meet code on either an R-value or U-factor basis. In some ways installing an air-tight layer of OSB over the open cell foam, with the 2x6 & fiber on the interior side is better, since you can then use the 2x6 bays for routing wiring, etc.

    If you wanted to "cheat" the code a bit and use 2x4 w/R15 batts that's fine too- it may even make it on a U-factor basis, despite a center-cavity R in the low 40s.

    If you build in soffit to ridge venting you can use open cell foam OR an all-fiber solution without much moisture risk. You could also get by with just 1-2" of closed cell foam and the rest fiber with minimal risk to the roof deck, but it's worth using dense packed cellulose or fiberglass if going that route. See:

  4. jpcjguy | | #4

    Thanks for all the information. Brain overload! :)
    Martin - thanks for the links - very informative. Has me pretty much saying no to OCSF because of the moisture risk.
    Dana - so attach 2x6s perpendicular to the rafters and then do two layers of insulation - 1 in the rafter bay and then another in the "horizontal" new bay I create with the 2x6s? The primary purpose is the reduce thermal bridging? Could it all be fiberglass? So R-19 in the rafter bays (6.25" thick - leave 1" for vent) and then R-19 again or R-22 in the new horizontal bay?
    I struggle with the conditioned space examples because the only part that will be conditioned is the 20x20 room I and finishing off - it will have a mini-split. How does the rest of the attic (staying attic looking) need to be - vapor barrier? airflow or not? Argh....

  5. GBA Editor
    Martin Holladay | | #5

    Q. "I struggle with the conditioned space examples because the only part that will be conditioned is the 20x20 room I and finishing off - it will have a mini-split. How does the rest of the attic (staying attic looking) need to be - vapor barrier? airflow or not?"

    A. Presumably you have an insulated barrier that separates the conditioned part of your attic from the unconditioned part of your attic. Think of the unconditioned part of the attic as the outdoors. That part of your attic needs to be vented to the exterior.

    The conditioned part of your attic needs to be separated from the rest of the attic with airtight walls and/or ceilings. These barriers need to be insulated.

  6. Expert Member
    Dana Dorsett | | #6

    In an unvented roof it's less risky if you make the 2x8 cavity fill open cell foam (installed in 2 passes for fire safety during installation quality- most manufacturers specify no more than 5.5" in any one pass). Open cell foam is a far better air-barrier than fiber insulation, and limits the risk of any potential errors or breaches of your vapor retarder layer.

    If you're going with soffit-to-ridge venting everywhere you can go all-fiberglass, but not with low-density R19s or R22s. R19s are crap- they only perform at R18 when compressed to 5.5" in a framing bay(!), and perform at R13 compressed into a 2x4 bay- they are a "fluffed" R13 (same weight per square foot), which makes them more of an air-filter than an air-retarder. They are WAY too air-permeable to use without an exterior air barrier. R22s are about as bad, performing at only R19 when compressed to 5.5".

    The only batt solutions that won't underperform their labeled value in this application are higher density versions. High density "cathedral ceiling" R21s are manufactured at ~ 5.5" and are more than 2x the density of R19 or R22s, and sufficiently air retardent that they don't lose much to wind-washing even without an exterior air barrier. Better still would rock wool R23s, which are more fireproof and at least as air-retardent.


    Going perpendicular to the rafters with your new framing dramatically reduces the thermal bridging compared to simply sistering onto the existing rafters. Assuming a 15% framing fraction for the roof (24" o.c. spacing with limited blocking), and a 15% framing fraction for the cross members the thermally unbroken framing drops to under 3%.

    Going with a vented solution you can use standard latex on wallboard as the vapor retarder, but it's still worth inserting a broad sheet vapor permeable air barrier such as housewrap stapled to the 2x8s before installing the new 2x6s. That makes nailing up the 2x6 framing slightly more awkward, but it limits the potential for air infiltration should there be any less-well fitted batts. With R23s in both the vented rafters and the 2x6 cross members you are at R46 center-cavity, but it will outperform R49 between joists due to the much lower thermal bridging. With R21 fiberglass it would be almost as good, but you'd then have to do the math on the actual framing to know for sure that you ducked under the U0.026 code max limit.

    You have some places that will need air sealing attention around vent piping & flues and the collar ties, etc. It's not possible to get a reliable air seal with lapped sheet goods where it isn't supported by framing, and you may have to install a few pieces of 1x furring and use some can-foam to make it truly air tight in those areas.

    Take the roof insulation all the way down to the soffits, don' try to air seal the partition walls and insulate only one section of the roof. Air sealing that the latter configuration is possible, but it's a bit of a fools errand, tougher than doing it all at the roof.

  7. jpcjguy | | #7

    Martin - So my original thought was to insulate all the rafters (soffit to ridge) and then just drywall the finished space with 2 doors into the unfinished side (with the HVAC). Standard closet type doors would separate the two spaces. My thought was the actual room would have the mini-split system and the rest of the attic (with the HVAC) would benefit from the increased insulation - yet not be conditioned. I see my thoughts are flawed.

    Dana - Never thought about the rock wool R23 (5.5" thick) - that would fit nicely in the space in the rafters and still give me soffit to ridge air gap. Question - would I need a baffle if going this way? Or just keep it lined up with the edge of the rafter?
    if I did the "twin R-23" , I guess I need to learn how to frame that 2x6 across the rafters (scratching head....)
    Ok - now factoring in the prices - Roxul R23 is more than fiberglass batts - Simple math for the main roof - 27 bays a side (so 54 total) each is 25ft long - that is 1350 "feet" R30 fiberglass bats are 52 a package and give me 44 feet. Roxul is also 52 a package but only 32 ft. Rough math has me getting 12 more packages of Roxul than fiberglass - that is about $636 more for the Roxul. And since I need to do this twice - about 1275 more.
    Granted the double Roxul gives me R46 - but as pointed out by Dana probably more than R49.
    So I am looking at $4576 for the Roxul - not taking into account the 2x6s, nails, etc.

    On the otherhand - the OC foam quotes I received were $6,000 (one had 5" and the other 6"). BUT that only gives me in the neighborhood of R18-R22 (3.7 R-inch).
    One of the quotes had the closed cell option (4" roof deck, 2" gable walls) for $8433.
    And I need to factor my time value if doing the roxul and 2x6s...
    I am not looking to win any awards with my house regarding insulation and efficiency but want to get the best bang for my buck. We plan on staying in the house a while but you never know.

    Back to ARGH! :)

  8. jpcjguy | | #8

    And how do I install 2x6s around the knee walls??!? If I go that route?

  9. GBA Editor
    Martin Holladay | | #9

    Your use of the word "conditioned" is confusing me. You had written that the small room in your attic was the only part that would be conditioned. But then you describe installing insulation in all of your rafter bays in order to create an unvented conditioned attic.

    Your plan to create an unvented conditioned attic is a good one. That way, you will bring your HVAC equipment and ducts inside the home's conditioned envelope. If you do that, all of your attic (not just part of it) will be conditioned.

    It may not be directly conditioned, but the temperature in your attic should closely track (more or less) the indoor temperature elsewhere in your house (as long as you install enough R-value in your new sloped roof assembly).

  10. jpcjguy | | #10

    Martin - Sorry for the confusion on my use of the world conditioned. This is all new to me. I thought conditioned meant having direct heating and cooling - hence the 20x20 drywalled room would be "conditioned". The rest of the attic, including the 2nd story HAVC system, would benefit from being in an insulated space - but not have direct ducts. My thinking was it would be easier to do all the rafter bays than the back of the kneewalls and the room framing.

  11. GBA Editor
    Martin Holladay | | #11

    "My thinking was it would be easier to do all the rafter bays than the back of the kneewalls and the room framing."

    You're right -- easier and better.

  12. jpcjguy | | #12

    So - since I am replacing my roof soon - would XPS or polyiso on top of my sheathing be an option to help minimize what I need in the attic space?

  13. GBA Editor
    Martin Holladay | | #13

    Of course. For more information on this topic, see How to Install Rigid Foam On Top of Roof Sheathing.

  14. jpcjguy | | #14

    Ok - an option then. From a DIY perspective the two layers of R23 roxul using 2x6 perpendicular to the rafters seems like the most cost effective to achieve R40+. Since the R23 is only 5.5" and my 2x8 is 7.25" That gives me a natural space of 1.75" Do I need to use a baffle then? I was planning on using the Accuvent baffle at the soffit - but do I need to take the baffle all the way to the ridge?

  15. jpcjguy | | #15

    Here is the baffle : Accuvent baffle

  16. GBA Editor
    Martin Holladay | | #16

    If you install air-permeable insulation like Roxul mineral wool, you need to have a ventilation baffle in every rafter bay where Roxul is installed. The ventilation gap must extend uninterrupted from the soffit to the ridge vent, or to the attic under the ridge vent. The only situation in which the ventilation gap can be omitted is if you have an adequate layer of foam insulation above the mineral wool, in which case you no longer have a vented assembly -- you have an unvented assembly.

    All of this is explained in my article, How to Build an Insulated Cathedral Ceiling.

  17. Expert Member
    Dana Dorsett | | #17

    To achieve adequate dew point control with foam above the roof deck in climate zone 4A the exterior foam needs to be at least 30% of the total R. The IRC chapter 8 prescriptives presume R49 as the total R:

    So, if you install 4" of EPS (R16-17-ish) or 3" of polyiso (R16-R17, derated for climate and location within the stackup), (or 3.5" nailbase polyiso or 4.5" nailbase EPS) on the exterior you could install up to about R35-R38 of fiber tight to the underside of the roof deck without much moisture risk.

    Roxul sells some high density rock wool R30s that fit 2x8 framing. If you install 3" polyiso or 4" EPS above the roof deck you'd only at about R46-R47 center cavity but it would still meet code min on a U-factor basis due to the thermal break the continuous foam provides over the rafters.

  18. jpcjguy | | #18

    Martin - I am not question a gap for ventilation, just if I have to use a plastic/foam baffle or could it just be an air space between the roxul and the sheathing.

    So I reached out to the building inspector for my county and he stated this in an email: "Minimum insulation in sloped ceilings as per N1102.2.2, 2012 Virginia Residential Code, shall be R-30 in the sloped area if the sloped area is 500 square feet or less or 20% of the total insulated ceiling area. If the area exceeds this then R-38 minimum is required."

    So I looked up the code and this is what I found: "N1102.2.1 (R402.2.1) Ceilings with attic spaces. When Section N1102.1.1 would require R-38 in the ceiling, installing R-30 over 100% of the ceiling area shall be deemed to satisfy the requirement for R-38 wherever the full height of uncompressed R-30 insulation extends over the wall top plate at the eaves.
    Similarly, when Section N1102.1.1 would require R-49 in the ceiling, installing R-38 over 100% of the ceiling area shall be deemed to satisfy the requirement for R-49 wherever the full eight of uncompressed R-38 insulation extends over the wall top plate at the eaves.
    This reduction shall not apply to the U-factor alternative approach in Section N1102.1.3 and the total UA alternative in Section N1102.1.4."

    So if I only have to have R-38, instead I could use the following:
    3½" thick batts with R-value ratings of R15
    5½" thick batts with R-value ratings of R23

    Then 5.5" would be in the 2x8 rafters and still give me my airgap. Then use 2x4s horizontally and use the R15. This also helps with thermal bridging.

    I think the EPS/XPS on the roof and the additional layer of sheathing would end up being too costly and I cannot do it myself.

    So looking at spray foam:
    Open cell will only get me (at 3.7 per inch) 26.825 R value in the 7.25" rafter cavity and this costs 6K.
    Closed cell will get me (at 6 per inch) will get me 43.5 R value and this will run me 9K+.
    If I can pull off the Roxul to achieve R-38 (with reduced thermal bride benefit) for 5-6K, this sounds like the best reasonable solution (granted my time and sweat equity - but I am used to that)

    Am I missing anything?

  19. GBA Editor
    Martin Holladay | | #19

    Q. "I am not questioning a gap for ventilation, just if I have to use a plastic/foam baffle -- or could it just be an air space between the Roxul and the sheathing?"

    A. As with many of your questions, the answer can be found in the article I linked to, How to Build an Insulated Cathedral Ceiling. In that article, I wrote, "As with all types of vent baffles, it’s a good idea to pay attention to airtight construction methods, especially if you will be installing air-permeable insulation in the rafter bays. Seal the edges of each panel with caulk, and tape the seams between panels with a high-quality tape."

    Roxul mineral wool is air-permeable. Air-permeable insulation materials need an air barrier on all six sides, so the ventilation baffle isn't optional. (If there is no air barrier above the mineral wool, the performance of the insulation will be degraded by wind-washing.) Install the ventilation baffle with attention to airtightness.

  20. jpcjguy | | #20

    I understand - thanks for all the help Martin!

  21. jpcjguy | | #21

    My head is swimming doing the math - those SmartBaffles you mention in your article are awesome and I am looking at spending 2000-2300 on those alone. Couple that with 2 layers of Roxul, 2x4s, screws, nails and tape for baffle seams, can spray foam for sealing small areas - I am looking at probably 7500 all in with me doing the job.
    In comparison - if I go closed spray foam (4") that gives me R24 - and I get the R15 roxul and stuff that in the remaining 3.25" in the rafter bay (compressing it 1/2"). I can achieve the R38 code value and that comes to 9800. But that is no 2x4s, taping, nailing, cutting, etc. So is my time worth 2300? ( or $2000 if I negotiate with spray foam guy)?
    Funny how I thought insulating the attic space was going to run me 2-4K range. My how my view has changed!!! Sheesh - wondering if the third floor is even worth it to finish off....or do I go back to insulating just the box? Might be easier after this analysis....

  22. GBA Editor
    Martin Holladay | | #22

    $2,000 of Smart Baffles is a lot.
    These baffles cost about $1.33 per linear foot.
    $2,000 buys you 1,503 linear feet.
    Maybe your sloped ceiling measures 10 feet long -- if so, that's a lot.
    So you have enough Smart Baffles for 150 rafter bays, each 10 feet long.
    Let's say you have a gable roof with 75 rafter bays on each side.
    Each rafter bay is 16 inches wide, so that makes your attic 100 feet long.
    That's a big attic.

  23. jpcjguy | | #23

    Martin - I have 27 bays each side so 54 bays total - this is just the main house - not the space over the garage. Each bay is 25 feet from soffit to ridge. That is 1350 linear feet. There are also two small dormers - 1 to the front of the house and 1 to the back. So I added 20% for those and the garage - that brings me to over 2K. I attached a pic of the house.

  24. jpcjguy | | #24

    Here is a house pic

  25. Expert Member
    Dana Dorsett | | #25

    The performance of rock wool R23s with or without exterior air barriers isn't much different as long as it's air-tight to the interior. There is some wind-washing potenetial at the soffit, but nothing like the severe performance hit you get with low density R19s & R22 in that configuration.

    Using housewrap as a continuous exterior air barrier by over-wrapping the rafters and side-stapling it 5.5" in from the inner edge to accommodate the batt can work. It's a PITA in tight spaces and more time-consuming than Smart Baffles, but it's do-able.

  26. jpcjguy | | #26

    Dana - thanks for the option. Sounds like this is quite a PITA job. On the spray foam side - interesting that my estimates for spray foaming my rafters are all open cell and average 5-6". This gives me no where near R-38 - yet none of them informed me that additional insulation would be required.
    Also - since we are in Climate Zone 4a - also considered humid - it appears that closed cell is the better choice against the roof decking and yet all the installers said open cell is fine. Which is interesting considering their thickness levels they will spray would probably be not enough to keep moisture from forming on the roof sheathing and start the rot process - or am I missing something here?

  27. Expert Member
    Dana Dorsett | | #27

    With 5-6" of open cell directly on the roof deck and no interior vapor retarders you'll get moisture cycling in the roof deck- count on it. The severity of the issue varies with the roofing color and your roof shading factors. The foam itself will have a vapor retardency of 5-8 perms, which means it'll accumulate more moisture than you'd like over the winter, but it'll dry quickly in spring. With minimal springtime roof shading and a dark colored roof it can go a long time that way- maybe forever, but don't count on it. (There are existence proofs that unvented dense packed cellulose cathedral ceilings can make it for decades in your climate zone too, but that doesn't mean they all will, which is why codes don't allow it.)

    You really need 10-11" of open cell foam (installed in two passes) to hit R38, but with a smart vapor retarder or half-perm vapor-barrier latex paint on the ceiling gypsum the moisture risk would be pretty low. At 10" thickness most half-pound foam is already under 5 perms, and much less risky to the roof deck already. But it can still accumulate more moisture than is prudent if you don't bring the vapor permeance a bit lower.

    With 2x8 rafters and 2x4s perpendicular to the rafters you'd be at 10.75" total depth, which is perfect for hitting the R38 target if filled right up then trimmed flush with the 2x4s. In my neighborhood it would cost between $3-4/square foot for that much depth & total quantity of half-pound foam, depending on how hungry the foam contractors are. (They don't seem too hungry this summer.) YMMV. A 10-11" ocSPF solution with MemBrain or vapor barrier latex on the interior layers isn't a terrible way to go if you're stopping at R38. When you re-roof you could then add exterior foam for more of a thermal break if it's in the budget, but with horizontal 2x4s you'll already be at a higher performance point than a full fill of ocSPF in a 2 x 12 rafter bay due to the lower thermal bridging.

  28. jpcjguy | | #28

    Great info Dana. I reached out Icynene and the local rep shared this with me:

    "Open cell spray foam is an excellent choice in our climate zone (4). The climatic conditions associated with Richmond are mild enough that the chances of interior condensation occurring on your roof deck are rare. This is confirmed by the 2015 IBC which does not require air impermeable insulation (i.e. Spray Foam) to have a vapor retarder if installed on the underside of structural roof sheathing in climate zone 4. Even in extreme cold weather if open cell spray foam is used, moisture that may be present on the underside of the roof deck, would be able to dry to the inside through the water vapor permeable open cell foam. Our dealers have successfully conducted this construction approach many times.

    With regards to R-Value, there are a few different ways to adhere to code. The first, as you mention, would be the prescribed thermal insulation requirements, which in this case states R-38. An alternative method would be the performance path to code adherence. In this situation the code allows users to “trade off” performance increases elsewhere—for example, high efficiency heating and air conditioning, ventilation heat recover, and air tightness—for lower R-Values in various assemblies. Doing so allows a builder (or homeowner) to optimize the use of foam for the building. Usually this requires that the building be computer modeled, using a program such as REM design, to demonstrate equivalence to Code-prescribed measures. The typical R-Value found in this performance method for a attic assembly in climate zone 4 is R-20 or about 5.5” of open cell foam.

    This is the reason why in a retrofit situation many localities will allow 5”-6” of open cell foam to pass even without a specific model being done for the building. They have seen so many models work that they understand the improved performance of an air barrier insulation like spray foam will perform as well as the prescribed assembly. It is also the reason why most installers are going to recommend that thickness. "



  29. GBA Editor
    Martin Holladay | | #29

    My thought is that Icynene has gotten a little bit more sophisticated in the way it recommends thin insulation, but that it is still peddling the same weak tea that it was peddling in 2010 (see "It’s OK to Skimp On Insulation, Icynene Says.")

    If a builder hires an energy consultant to do the performance path calculations, then the approach suggested by Icynene might be justified.

    But there is no justification whatsoever for these sentences: "The typical R-Value found in this performance method for a attic assembly in climate zone 4 is R-20 or about 5.5 inches of open cell foam. This is the reason why in a retrofit situation many localities will allow 5”-6” of open cell foam to pass even without a specific model being done for the building."

    If a local code official allows the use of R-20 roof insulation in Climate Zone 4 without insisting that the builder show evidence of performance path calculations, one might conclude that the official is either lazy or ignorant. Perhaps a more charitable explanation for such a ruling is possible, but I can't think of one right now.

  30. Expert Member
    Dana Dorsett | | #30

    The notion that R20 makes it on a performance basis is pure marketing crap. R20 may be the financial lifecycle break-even limit in cheap-energy parts of Zone 4, when using a premium product like ccSPF, at 10-12 cents per R per square foot, compared to open blown cellulose at 3-4 cents/R-ft^2. You can put R60-R80 open blown cellulose on the attic floor for the installed price of R20 ocSPF.

    A foam vendor's "That's all ya need..." argument needs to be read in that context, and should be construed to mean, "That's all that can be EASILY rationalized on a financial basis."

    At 5-6" it's easier on the installer since that's the max that can be safely installed in one lift, and it has less sticker shock than R38. But it too vapor open to be a sure thing, for the roof deck, and from a comfort point of view it's not all that great under a 140F roof deck on sunny July afternoon.

    I can believe R38 ocSPF might have a performance basis argument in a R49 code min areas (at least in some instances), but not R20 where R38 is required. Note, R38 is only a substitute for the R49 prescriptive under Virginia building codes, no the IRC, and then only in cathedralized ceiling applications. Swapping that out for R20 would be a huge step down in performance from any correctly installed R38 implementation.

    As for the IBC 2015 not requiring interior vapor retarders for air impermeable insulation tight to the deck in VA, review the "1" ocSPF + spray fiberglass" and "Full-depth ocSPF" columns for the zone 4 locations in Table 3 of Straube's WUFI simulations (third & fourth columns from the left):

    They all hit 28%+ moisture content on the north facing pitches, some for more than a month. Even at 20% moisture content the rot risk is elevated.

    Still see section R806.5, near the bottom of this page, and read it carefully:

    Yes, they only require Class-II vapor retarders on air-impermeable insulation for zones 5 & up, but these codes are based on the presumption of the full IRC prescriptive R-value found in Table N1102.1.2 on this page: (about half way down.)

    Half pound Icynene is 17 perms @ 2" (R7.4). That would be about 5.6 perms at 6", but it would still be about 2.6 perms @ R49 (the presumptive R value), which is pretty good. That is still not class-II (sub-1 perm) vapor retardency, but it's a heluva lot tighter than than 6" foam. Even at the prescriptive R49 it's not insane to use vapor retardent latex on the ceiling gypsum. Straube's simulation that showed a possible hazard was at R38 (the IRC 2009 prescriptive R-value), which would be about 3.3 perms. But that hazard level was still only about half the amount of moisture that would be getting in with only 6" foam.

    The guy is is optimistic to the point of delusion if he thinks a mere 6" of Icynene is going to going to be safe for the next 50 years without additional vapor retarder in all installations just because it's air-impermeable. It won't rot out in 5 years, but don't count on it at age 25, particularly on the north facing pitches. (But by then he will have retired, and you will probably have sold the house, eh? :-) )

  31. jpcjguy | | #31

    So I sent the same response from the Icynene rep to one of my building inspectors to get his thought and this was the reply:
    "Yes , I agree with this. I feel open cell foam is the least likely to condensate than other types of insulation. The vapor retarder installed on the interior of the assembly should stop moisture vapor from getting into the insulated cavity."

  32. jpcjguy | | #32

    I think I might do some math also - analyze my utility bills over the last year and do some modelling at breakpoints from 10%-50% to get my worst to best savings and see how long it would be to break even on ROI. I want to do this right, but 8-10K on insulation is not really in the budget. I might end up at insulating the "box" again. This way I can just increase the height of the attic floor insulation behind the knee walls and in the small peaks and call it a day.

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