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Community and Q&A

Exterior rock wool insulation with batts?

user-995912 | Posted in Energy Efficiency and Durability on

Hello folks,

First, I’d just like to say how thankful I am for the invaluable resource that is GBA. I spend A LOT of time reading the blogs and Q&A and I find that pretty much any question I might have has been covered here already. Though sometimes my understanding gets a little vague regarding certain details so I am hoping for some input on a potential wall detail.

I want to build a “Pretty Good Wall”, ideally with a whole wall R-value of 30-ish, which I feel is a good target for my location in CZ 4A (Although I think we may be closer to CZ 5A here in the mountains of SW VA. Elevation 2400 ft)

I have done the exterior foam thing on a few projects (thanks to GBA) but I tend to lose sleep thinking of carpenter ants and termites in places I can’t see and I would like to limit the use of foam when I can. I try for reclaimed goods when possible but it is somewhat limited in my area.

I really like the idea of the Roxul ComfortBoard IS but availability in my area seems to be a no go from a cost and quantity standpoint.

Double wall construction with dense pack cellulose is also appealing but the installers in my area don’t give me the confidence that I need and they are very resistant to dense packing a large open void. I have used damp spray cellulose a few times and I am very comfortable for a standard 2×6 depth, as well as a 2×6 wall with horizontal 2×2 cross hatch. I am hesitant to go any deeper with damp spray but if anyone can offer any info doing this than I am all ears.

So here is my proposed wall description and an attached image(hopefully):

From inside to out:
1/2″ GWB primed and painted
2×6 24″ OC load bearing wall with 5 1/2″ damp spray cellulose in bays
1/2″ CDX plywood sheathing taped and caulked for air barrier
2×4 24″ OC non load bearing wall with 3 1/2″ Roxul ComfortBatt
WRB Typar or similar
1×4 vertical battens for rainscreen
Siding TBD

My uncertainties with this wall are in regards to the 2/3 to 1/3 rule, which I don’t really understand, as well as how this wall performs in summer with very occasional AC usage.

Any comments, critiques are appreciated,


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  1. user-1137156 | | #1

    The reason for the 1/3 -2/3 "rule" is a condensation concern when it's really cold outside. Abiding the 1/3-2/3 "rule places the first "condensing surface" where it's temperature will be above the dew point of indoor air.
    I notice that in your illustration you are showing staggered studs. The staggered studs will give a higher whole wall r value BUT at the cost of a considerably greater risk of moisture issues with condensation on the plywood at the "cold stripes" created by the outer studs. Aligned studs OTOH suffer a slightly higher heat loss but have the warm stripe from the inner stud next to the cold stripe thus substantially reducing moisture risk.. Do you absolutely need a 2x6 inner wall? if you could use a 2x4 inner wall you could space the walls 2 1/2" apart and use r 10 mineral wool in between the plywood and outer wall for considerably higher r value ( with aligned studs @20% framing whole wall r =33+)

  2. user-626934 | | #2


    I'm involved in a couple of projects in central VA that are using 2" Roxul Rockboard 80 over a 2x6 stud wall (one just finished in Charlottesville, one being framed in Waynesboro). According to the builder, the price is ~$0.80/ft2 for the 2" depth. I don't think there were any big issues in terms of order size. As far as I know, Rockboard 80 and Comfortboard IS are basically the same product....though if there's someone out there from Roxul, please chime in.

    You can see some pictures of the house in Charlottesville here -

    If you're interested, email me at semmelhack(at) and I'll track down the source.

  3. user-995912 | | #3


    I appreciate the possible lead on the board options. I am about 2.5 hours south on I-81 so this could work.

    My understanding is the Rockboard 80 is it rated for below grade applications and the Comfortboard IS is for above grade but like you say, it seems like there is not much difference.

    I'll email you soon and thanks for the link. I visited the site a couple years ago-nice to see some more recent projects.

  4. GBA Editor
    Martin Holladay | | #4

    The rule that required no more than 1/3 of a wall's R-value to be on the interior side of the polyethylene vapor barrier is an old rule of thumb that used to be used in Canada. It applied to cold-climate walls that include polyethylene.

    Your wall doesn't include any polyethylene; the corresponding material in your wall is plywood, which is vapor-permeable enough to avoid the type of problems that can occur in a wall with polyethylene in the middle.

    Although the wall assembly you suggest probably costs more to build than some simpler alternatives, I don't think it will have any moisture problems.

  5. user-995912 | | #5

    Thanks for the input, Jerry.

    When you say:

    "The staggered studs will give a higher whole wall r value BUT at the cost of a considerably greater risk of moisture issues with condensation on the plywood at the "cold stripes" created by the outer studs."

    I am guessing that the moisture in question here is interior, right? Are we concerned with vapor diffusion in this case? Assuming I have a well installed air barrier and a ventilated rainscreen, is this a big concern and would you consider this build up to be higher risk than a double wall that places the sheathing at the coldest part of the assembly?

    I am not dead set on the 2x6 load bearing wall but more often than not I find 2x4 material rather crappy.
    Honestly I am not a big fan 24" centers with wall framing. I feel it's the right thing to do from a more insulation/less lumber standpoint but to me drywall and siding don't usually look as good with 24" centers.

    I hear you on spacing the outer wall a couple inches from the sheathing to drastically reduce the thermal bridging and up the whole wall R-value, but again material availability and cost will be issues here.

  6. Expert Member
    Dana Dorsett | | #6

    FWIW: The whole-wall R of that stackup is about R15 for the 2x6 w/cellulose layer (R14 if you used doubled top plates & headers), and about R11 (assuming R15 batts, single headers & top plate). Add another R1 for the ply + gypsum, and you're at about R27 whole-wall. If you used R23 rock wool in the 2x6 instead of spray cellulose you'd be at about R29.

    If you made the structural wall 2x4 16" o.c. w/damp sprayed cellulose (R10 after thermal bridging), but inserted 2" ofType-II EPS (R8.4) between that and your R11 rock wool studwall, with R1 for gypsum + sheathing you come in at a true R30, at the same thickness. (If you used 2" foil faced polyiso instead of EPS you'd be at about R35). With this stackup the top/bottom plates and the headers all get thermally broken with R8.4 EPS. When the foam is located mid-assembly, the ants & termites are tunneling in the EPS, it means they already found a path into the wood, and ate through some borate-loaded cellulose to boot- it is not an increased termite hazard the way it might be on an exterior location.

  7. rshuman | | #7


    Partly in response to Dana's calculations of whole wall R value, wouldn't a more efficient way to build the wall be to simply attach the outer 3-3.5" of rock wool to the sheathing and skip the 2x4 framing? You get a similar or larger whole wall R value and, for this portion of the wall, save lumber and framing costs.

  8. user-1137156 | | #8

    The air inside your air barrier (the plywood) will be indoor air and have the dew point of the indoor air. With air permeable insulation that air will contact the plywood. The temperature of the plywood where the indoor air contacts it will control the moisture transfer to the plywood. The temperature of the plywood is determined by the temperature difference between outdoor temperature and indoor temperature and the ratio of r value inside to outside the plywood. if 1/3 of the r value is inside the temperature of the plywood will be the indoor temperature minus 1/3 of the difference between indoor and outdoor temperatures. For example if indoors is 70 f and outdoors is 10f the plywood will be at 50 f ( 70-(70-10)/3). This example would start to have moisture issues if the indoor dew point was allowed to get to 50 f.( a dew point of 50f implies a relative humidity at 70f of 50%. pretty high). Now take the case of a staggered stud wall. At the location of an outer wall stud the r value outside the plywood is about r4 and inside the stud is about r20. The plywood temperature will be 70- 20/24*(70-10) =20 f. An indoor dew point higher than 20f will result in moisture gain in the plywood. this means indoor humidity must be kept below 15%, which is really low! Even the aligned stud case with a 2x6 inside and 2x4 outside is less than ideal as at the studs 3 1/2" outside and 5 1/2" inside says the plywood temperature will be about 34f. meaning indoor relative humidity at 70 f must be kept below 27%. While the wall I proposed, 2x4's aligned with r10 continuous between is not quite 1/3-2/3 but puts the plywood temp at 47.5 f allowing a 70f RH of about 47%. The cost difference of thicker insulation between double walls is lower than any other way of improving thermal performance. If you don't like 24"OC frame 16" oc and use 3 1/2" spaced walls. FWIW using 3 layers of mineral wool in the cavities with 2 2x4 @16" OC walls and 2 1/2" r10 mineral wool between and a framing fraction of 30 % still gets above r30.4 whole wall assuming aligned studs and 1/2" plywood on the outer face of the inner wall. You can gain the "space" for the added inter wall insulation by eliminating the 1x4 furring and using Greenguard DC14, a 1/4" thick fan fold XPS drainage plane between the house wrap and siding, it also adds r1 to the assembly.

    I respectfully diss agree with Martin, your original wall is a disaster from a moisture perspective (see the above calculations).

  9. user-1137156 | | #9

    There are a couple of problems with you suggestion. First rigid mineral wool is about 3 times the $/r-sq ft of bats. Second the added labor of installing furring over out-sulation is near the cost of framing a wall and the cost of the long fasteners makes it more costly.. With a fully framed outer wall window attachment and flashing is traditional, reliable and well understood, not the case with out-sulation.

  10. user-995912 | | #10

    Everyone, thanks for commenting. I'll hopefully have to time in the next couple of days to discuss some of the points brought up here. In the meantime I'll be thinking about all this and pricing out a few different options.

  11. rshuman | | #11

    I was not aware of the cost differential between rigid mineral wool and bats, thanks. Regarding the furring, if I understand the design of Noah's wall correctly, that would be applied in his case as well as mine, right? That said, different fasteners would be required as you note. Finally, is framing windows in cases where out-sulation is used somehow 'riskier'? I had not gotten that sense from numerous discussions I have read at GBA but I also have no actual experience in any of this.

  12. GBA Editor
    Martin Holladay | | #12

    Jerry Liebler,
    The wooden components of all wood-framed walls experience moisture accumulation during the winter. This applies to studs, bottom plates, top plates, and OSB sheathing. In most walls, the moisture that accumulates in February begins to dry out in March and April. This is normal, and does not usually result in mold or rot.

    If this normal cycling bothers a builder, one possible solution is to adopt the PERSIST approach -- one that puts all insulation on the exterior of the structure.

    You are correct that the plywood in this wall assembly will accumulate moisture in January and February. That always happens with wooden components in walls. It usually isn't a problem, however. Hygroscopic materials like lumber and plywood redistribute and store moisture. When conditions change in March and April, these materials begin to dry out.

    If the plywood layer were polyethylene, there would be no redistribution. Moisture might puddle at the bottom plate, and that fact might cause problems.

  13. wjrobinson | | #13

    One of the first to design homes with all the insulation outside the frame....

    google; bruce brownell adirondack alternate energy

    His homes have low ACH, the frames are warm and dry, and the insulation was continuous on all six sides of a home.

    The Brownell design has been improved overtime and a study was done of his homes that included a list of further improvements

    As to the designs being discussed on this thread. They are complicated and labor intensive IMO.

    Simple is good. Insulation is good. Shoot for both.

  14. Expert Member
    Dana Dorsett | | #14

    Martin: In his stackup he putting the plywood in the middle of the R, with R15 rock wool on the exterior and R20 cellulose on the interior. A layer poly adjacent to the plywood (either side), it would be unlikely to ever reach the dew point temp of either the interior or outdoor air long enough to matter.

    With 5.5" of cellulose to buffer the moisture on the interior side it would take a MAJOR water leak to end up with enough puddling to matter at the bottom plate of the structural wall, with or without the poly. But assuming he air-conditions the interior to about 30F in summer (not likely- that's pretty nippy! :-) ) there's at least a chance of consequential condensation pooling up on the poly.

    But I agree that adding a layer of poly only reduces (however slightly) the resilience of the assembly, especially in this climate, in an assembly with a 3/4" rainscreen to enhance drying.

  15. GBA Editor
    Martin Holladay | | #15

    It sounds as if we are basically in agreement, and that you don't support Jerry Liebler's position.
    (Jerry wrote, "I respectfully disagree with Martin. Your original wall is a disaster from a moisture perspective.")

    The point I was trying to explain to Jerry was, "Plywood is not polyethylene."

  16. user-1137156 | | #16

    Martin and Dana,
    Please note this is a STAGGERED stud design! At the location of the outer stud it is r4 (the stud) on the outside and r20 of cellulose on the inside! The plywood temperature at the outer stud locations will be cold enough that moisture build up will start well above freezing outdoors even with an indoor dew point of 40f. Dana's hypothetical layer of poly on the inside will be dripping! I'd expect to see wet drywall every spring as the cellulose became fully saturated.

  17. wjrobinson | | #17

    Jerry, Martin and Dana are making good sense. All of you need to stop talking poly as poly is not in the designs and is getting this thread off track.

    Cellulose walls in our part of the country don't have moisture problems as a general rule.

    This thread needs to get to a conclusion that is useful folks. What is it?

  18. user-1137156 | | #18

    I'm not talking about poly AT ALL! This STAGGERED stud double wall is far less "resilient" than an equally complex wall that is designed with moisture concerns in mind. Because of the staggered studs the moisture behavior is almost as bad as a much simpler wall with the plywood on the outside! There is negligible benefit from placing the air barrier mid wall, there should be a "payoff" for the added complexity and there is if the design is changed to ALIGNED studs and vapor open (not foam and especially not foil faced foam) mid wall insulation is added.

  19. albertrooks | | #19

    Geez I enjoy how these threads go "polytangental" about polyethaline and such.


    Roxul Comfoboard is the same density as Rockboard 80. Both are 8lbts,ft3. The comfoboard is "stiffened" by adding more binders to make it less "squishy" when applying it to a woodframe wall. Hence the reason it's a residential product vs Rockboard 80 which is a comercial product.

    Both are R4/inch.

    Residential and comercial are two seprate chanels with seprate sales reps and distributors. A comercial distributor can tell you about Rockboard 80 availabilty but probably is not much help on a residential product.

    I suggest finding a Roxul distributor who deals with residentail products. Yoiu can probably find the comfoboard IS in your market. But... I'm a West Coast guy and could be wrong...

    I'd skip the wall with batts in an exterior applied frame. You'll still have low density batts in cavities where air will circulate and reduce the insulations effectiveness in addition to the thermal bridging of the frame. Even if the layout is staggard and just at the platres.

    Read up on how to fasten MW to a wood frame. It's not easy or quick but once done is safe and effective.

  20. wjrobinson | | #20

    Jerry, the problem with all the wall ideas is not moisture it's labor cost.

    When will developers nationwide adapt wall structures this complicated?

    Never. They won't get built to have moisture problems. End of story.

    Don't worry, be happy

  21. user-1137156 | | #21

    What is your estimate of the labor cost/sq ft of rough framed wall? To achieve r20 out-sulation one could use a 2x4 wall with r 15 in the stud spaces over a 2 1/2" layer of mid wall bats or apply 5" of comfort board which would require 8" long screws through furring. The difference in just the cost of mineral wool is about $2.50/sq ft. add labor cost to attach the rigid insulation and significant cost of fasteners. I'd encourage you to do an honest cost comparison publicly. right here.

  22. user-995912 | | #22

    I really appreciate the discussion, folks.

    Just to make it clear, I am by no means attached to this wall configuration and really was just thinking about a no foam wall that could be built with in stock products from my locally owned building supply.
    The owner is working hard to help me out and we'll see what he comes up with for the Comfortboard IS.

    Albert, I m glad you pointed out the convective issues that are still present with the MW batts. Is this a non issue with the 8 lb density board?

    A 2x6 wall with 3 or 4" of exterior MW would be a really nice wall. Could be a little $pendy and I am a little concerned with having the rainscreen battens plane out well for the siding. I have read and re read on the subject here at GBA so I know it can be done, just haven't see yet myself.

    Dana's proposed use of EPS between the inner wall sheathing and the outer 2x4 wall is intriguing as it *should* keep the foam well isolated from the Bug Zone.
    I guess a better variation would be the 2" of EPS followed an inch or 2 of Comfortboard IS. Again to keep the bugs at bay. Then long fasteners through the battens, MW, EPS, WRB, plywood and into the studs. The EPS will certainly be a more cost effective R-value vs MW-yet to be determined by how much though.

    My biggest decision then would be where to locate the WRB. Windows would be installed as innies so either in between the plywood and EPS OR between the EPS and the MW?

    I am correct to think that this latest proposed wall would offer a safety margin allowing for the use of OSB? (gasp!)

    IF I went with this assembly with the plywood/osb as the air barrier, would it be necessary to tape the EPS as well?

    Thanks a lot for any further comments.


  23. user-1137156 | | #23

    Be care full which "experts" you listen to!!!
    Dana's suggestion of foam mid wall has a serious down side, it substantially reduces the ability of the wall to dry to the outside. His suggestion of foil faced foam mid wall essentially eliminates any outward drying of the cellulose & plywood. From what I've found EPS is close in price but higher than mineral wool bats but significantly less than rigid mineral wool. I would certainly avoid OSB mid wall in any staggered stud wall. The wall I proposed at the end of post #8 would however be reasonably "safe" with OSB. The convective loss of insulating effect is a "red herring" that may apply to cheap, poorly fitted, fiberglass bats but is not demonstrate able with properly fitted mineral wool bats which perform just as well as the higher density rigid mineral wool.

  24. albertrooks | | #24


    I didn't bring this up because I enjoy just watching where things go. However some of your goals resonate with me.

    I'm helping some very large projects attach very thick (4" to 6") layers of rigid mineral wool this year with the Heco Topix Therm that we stock. It works fine but this is still a complicated assembly to build.

    For these reasons we began importing Thermacork in 2012. We are really having great luck with it this year. It's the same R4 per inch, permeable enough to dry and really easy to apply. We stock thicknesses of 1/2" to 4" and am getting ready to add lap joints for the 4" thick. A lap means that you can still use a standard framing gun to nail the lap and hold the panel until you are ready to apply battens. It won't compress so you don't have to use expensive fasteners to avoid compression or worry about the shear loads of your siding (for average applications). All this... and it's a carbon negative insulation: 40 tons CO2 sequestered at port of Seattle after manufacturing and transportation. A total win win to me.

    It's a higher cost material. Our list price is about $1.20/bf for a small project but it applies so much easier than MW that it's not hard to earn the initial cost back through installation cost savings. Especially as things get thicker.

    Our company WestCoast Associates (parent of The Small Planet Workshop) is the importer for the Western States and wholesales Thermacork to resellers like your local yard. If your yard is interested, they can contact WCA (360) 943-5272. Or- if your in the East, contact

    Here is a ad we are running with our Seattle reseller in a local builders magazine with a few pics on it. I think it's still got some corrections to be made so don't laugh at any typos (at least out loud).

  25. albertrooks | | #25


    Just to answer the variety of questions:

    I'd suggest breaking the purposes of the layers apart:

    The air barrier is best towards the warm side. Once created at a layer, It doesn't have to be re-created at another. It just needs to be complete for the whole structure.

    Bulk water needs to be stopped. Managing it as far out to the exterior as possible is better. Once created, it doesn't have to be recreated. I don't think taped foam makes a good WRB because it moves so much.. Other probably disagree.

    If it's not an air barrier or a WRB, you don't have to add anything to it like tape.

    Back to Mineral Wool. The higher the density, the less airflow in and around. 8 lb will not allow significant airflow that will affect insulating value. It doesn't need a "wind tight" layer over it. Low density batts like AFB will let some air move in the cavity. If the cavity gets "wind washed", some of the air will be pushed out which will heat or cool the cavity. There are different densities of batts available from Roxul.

    The added thermal bridging of the frame is admittedly a larger source of heat loss than reasonably installed batts that are not getting and wind washing.

    There is no perfect builder, nor perfect wall, just best efforts.

  26. user-1137156 | | #26

    Would you agree that a properly installed " WRB, Typar or similar" outside the 2x4 wall filled with MW bats should essentially eliminate "wind washing"?

  27. albertrooks | | #27


    I don't think I'd go that far. If the batts are less than 4lb density then air can move through them. If properly installed means stapled around the opening, then in a rainscreen application it leaves too many paths for wind to push or draw through these gaps.

    If the WRB is adhered to the frame at all points and joints are taped, then yes. If the batt density is above 4lb then yes.

    However, even if yes, there is still the thermal bridging of the frame. That's the significant point to "continuous exterior insulations": It breaks the thermal bridging. Seems like a lot of work to build and still have an assembly that has thermal bridging. The dew point calc is no longer valid where the frame meets the inner sheathing etc... I'd rather tackle the big issues upfront. There is just not an easy way to have a quality build in my experience.

  28. user-1137156 | | #28

    The mid wall insulation is continuous,it breaks most of the thermal bridging,but to achieve the same r value result does require a thicker wall with double stud walls! The outer stud wall is nothing more than a really strong and robust furring system that allows very conventional "outie" window installation and flashing, with added insulation to boot. In post 21 I challenged AJ to do a cost comparison .of 2 ways to achieve r20 out-sulation with mineral wool, he hasn't responded but I know the result. The double stud wall wins easily but it is 6" thick vs 5" FWIW I plan on using Greenguard DC14, a 1/4" thick fan-fold XPS drainage plane outside the house-wrap instead of a "rain screen detail". The DC14 adds r1 of continuous insulation, is easier to install and 1/2"thinner than furring, adds additional wind blocking, besides isolating the solar driven moisture from reservoir cladding (which I will have with "thin brick") while allowing the wall inside it to dry outward. I would have a real challenge if I tried to hang, on furring, thin brick at 7#/sqft outside 5"of mineral wool.but it's trivial with a double stud wall.

  29. user-1137156 | | #29

    Regarding your mention of dew point calculations: With staggered studs the temperature will be be different warmest (closest to indoors) at the inner wall's stud and coldest at the outer wall's stud. With aligned studs, if the mid wall r is greater than a stud the plywood temperature will be warmest at the stud locations and coolest midway between (the locations of the temperature extremes will be swapped for less mid-wall r). One can use these locations and assess risk at each location, my preference and what I'd call worst case, or simply use an average, knowing that the plywood will re-distribute moisture to an average (this seems to be Martin's and Dana's preference). Under some conditions the average method may be adequate but it certainly is improper with surface temperatures below freezing.

  30. albertrooks | | #30


    Im getting a but confused. I'm not AJ (who enjoys bear meat with a demiglace... Hiya AJ!) I'm Albert who wrote the above posts.

    I'm not sure if you're now talking about a classic double stud or the wall Noah detailed above. Not that it's a big deal but I was suggesting that the exterior frame will cool the interior side of the plywood sheathing which could cause some interior moisture to condense since it may be on the interior side of the air barrier. It just seems like a lot of work for an assembly that still has some significant short comings.

    AJ... That old post from 2010 still makes me chuckle. Hope your good. Maybe you'll show up to the Portland ME PH conference.

  31. wjrobinson | | #31


    Thanks for sharing old chuckles anew. Last bear burgers we had were mixed with sausage, pretty tasty, and that bear was a Maine bear. Great to have hunters for friends. Remember, eat a bear, save a tree or was it eat a bear save a windmill...

    Speaking of bears and barbecue my favorite saying for a bit last year was... "Time to barbecue" and it wasn't in reference to bear...

    When is the conference and will there be bear?

  32. user-1137156 | | #32

    In my mind what we are discussing is: alternative walls which use mineral wool to insulate outside the sheathing, which is the air barrier, on the outside of an inner structural wall. Noah's original wall used staggered studs for an outer wall and NO mid wall insulation. Leading directly to your "concern". Martin says this wall has no moisture issues, I diss-agree and say that ice will build around the outer stud lines whenever the indoor dew point is above 32f and the outdoor temperature is below 20f The alternatives that solve this issue include: continuous out-sulation with rigid mineral wool, a much more costly insulation, and double stud walls with sufficient continuous mid wall insulation. In post 29 I described how I evaluate the moisture concerns.
    I assert that: If the goal is set as r20, or above, of out sulation a double stud wall is the most economical (material + labor costs) but it 's downside is it is slightly thicker while it's upsides include greater strength and ability to support heavy cladding with simpler and well understood window and flashing details..
    None of these walls are a "classic" double stud wall which has sheathing on the outside and requires an interior air barrier in cold climates.

  33. albertrooks | | #33


    I don't know what's on the menu, but here is the conference: it'd be fun to see GBA folks. It's my first time in ME and I'm looking forward to it!

  34. albertrooks | | #34


    Sounds like a good assembly. There are a lot of ideas out there to make thick walls. I seem to work with folks doing all styles from R55 in Thermacork, 2x6 exterior foam, 2x6 mineral wool, diffusion open R40 to 50 Agepan DWD , R40 single 2x8 frame Agepan 2" THD and the list goes on... It great that there is all this creativity going on.

  35. Expert Member
    Dana Dorsett | | #35

    With 2" of polyiso between my recommended staggered 2x4 studwalls there's another R12 between stud edge and interior or exterior. The notion that any of the mid-assembly stud edges will be cold enough long enough any season to create a condensation issue on either foil facer doesn't add up.

    Sticking with 2x4 & 2x6 with only CDX would also be low risk. Even if the stud edge were cold enough under heavy air conditioning for the stud to adsorb moisture, the stud itself redistributes that moisture to warmer depths in the rock wool, which has excellent drying capacity. In winter, condensing temps at the OSB are rare & fleeting (the location in question has wintertime outdoor temperature averages north of 30F), and the cellulose fill redistributes the moisture VERY well.

    If there were poly adjacent to the CDX (as in the 1/3 rule, per Martin's "The rule that required no more than 1/3 of a wall's R-value to be on the interior side of the polyethylene vapor barrier", but in this hypothetical stackup would not be at the 1/3 point), it's similarly low risk, but simply not necessary. (Poly on the interior side would be a mistake though.)

  36. user-1137156 | | #36

    If you can find a contractor willing to install CertainTeed's Optima at the recommended density of 1.8#/cuft. , you can dramatically simplify, & no doubt cost reduce, the insulation process and still get r 4.2/" and a vapor permeable wall on both sides of the sheathing using 2 2x4 walls spaced 2" (your original wall depth). Using the same insulation material in both cavities would get you about r31.4 whole wall at 20 % framing with aligned studs and no moisture worries. or staggered studs gives r 33.8 and some moisture concern as the lowest plywood temperature will be 36f, 10 f lower than the aligned stud case with 10f outdoor and indoor 70f temperatures. Adopting the Greenguard DC14 to replace the furring but keeping the same wall thickness adds r3.1 and reduces the risk of staggered studs.

  37. user-995912 | | #37


    Thanks to John Semmelhack I have a good source of Rockboard 80 at a good price- $0.80 sq ft for 2" and $1.20 sq ft for 3" with no minimum order. I am going to stick with a 2x6 wall with damp spray cellulose and probably go with 2" of exterior MW. Whole wall of about R-23, right Dana? Not sure if it is worth the $0.40 sq ft up charge for the additional R-4 for a whole wall of R-27 but its tempting. Still short of the R-30ish wall I was hoping for but it doesn't get that cold around here and we're not talking about huge energy savings between R-23 or R27. Ah, where to draw the line?

    Thanks again to everyone for the reply's. It's been helpful.


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