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One or two courses of rockwool insulation over exterior sheathing?

ethant | Posted in Green Building Techniques on

I am trying to weigh the cost and benefits of one vs. two courses of exterior rockwool insulation applied over sheathing, assuming equal overall thickness. It seems that in terms of overall labor one course of 4 inches of rock wool insulation would be easier to install. It seems that two courses of 2 in of rockwool would necessarily be twice the labor. I am curious as to the benefits of staggered seams on two courses of rockwool as compared to one course of 4 inch rockwool. Does anyone have any thoughts on this matter? thank you.

While I have your attention, I have heard mention of thicker profiles of rockwool beyond what seems to be the standard one, two, 3, + 4 available from standard insulation suppliers. For example there is the 6in cavityrock. Has anyone had experience with these thicker applications (6″, 8″???) of exterior rock wool insulation?

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  1. jberks | | #1

    Hi Ethan,

    I am no pro, but I am about to install roxul insulation exterior of my sheathing.

    To my understanding, people will stagger the seams of foam board insulation for the purposed of air sealing. Using Roxul, I'm going to guess you're not intending to use it as air sealing since its not a good product for that. Thus, I don't think staggering the seams is necessary compared to the extra labour you'll encounter of doing 2 ply.

    You'll have to consider logistics of the install. Things like weight, if you're using a boom vs scaffolding, size of crew etc. I'd imagine putting up 4'x8' sheets of rockwool would be your quickest option compared to two sheets, things like doing all your cuts around windows and doors twice. how you're going to hold up or fasten the first sheet against the wall while you place you're second sheet and strapping, etc.

  2. ethant | | #2

    I guess the advantage of one course of rock is ease of construction, but the downside is that as far as I know, the thickest possible board is 3", at R12. So to achieve R40 wall (Pretty good home) I need 8+" of dense pack cellulose. It's pretty ridiculous to frame walls out of 2x8 and they're not even 8" anyway. So that gets me into creating a double stud just to allow for one course of rock wool on the exterior which in turn makes me think why the heck not just build a double stud?

  3. brp_nh | | #3


    I've noticed your stipulation of an R40 wall in another post of yours and it seems to be turning your wall decision process into a difficult one. It can be tough to reach that value because each approach involves complications involving the expense, materials, and more.

    What is your goal? If it's PGHish or net zero, I'd suggest dropping R40 as a requirement and focus on an affordable/buildable wall that's ~R30. You can get there with a reasonable amount of exterior insulation or a medium thickness double stud wall. Or if you really want to hit R40, go with a thicker double stud wall.

    R30 is a solid value in climate zone 5A for an energy efficient house or even to hit net zero with an appropriately sized solar PV system. And there are lots of other areas (*) to concentrate on for energy efficiency vs driving yourself crazy over an R30 vs R40 wall.

    (*) These areas would be air sealing, windows, occupant behavior, and all sorts of other stuff that affect energy use.

  4. ethant | | #4

    Brian, thanks for the get-out-of-jail-free card! I thought I'd cribbed the R40 wall from the pretty good house specifications. It was the 5-10-20-40-60 rule. I've lost track, TBH.

  5. jberks | | #5


    I think you can get 6" comfortboard from Roxul, You'll have to get it ordered in. There are other wool insulation companies, like Knauf. Give them a call. Both these companies are up here in Canada though.

    I plan to use 2" board on my exterior, and 1.5" is what most suppliers carry. So I have to get it ordered by the skid.

  6. ethant | | #6

    Jamie, I've found cavityrock, which is dual density, comes in 6". More info here:

    I haven't heard much discussion of it here, but it might be a good solution for a 6" roxul application.

    Info from Roxul for cavityrock: Dual Density - 6.2 lbs/ft³ (100 kgs/m³) outer layer and 4.1 lbs/ft³ (65 kgs/m³) inner layer.

    I'm not sure how this dual density would affect installation. Has anyone here used cavityrock?

    Comfortboard: 439psf (21kPa) @ 10% compression.

    Can't find the data for Cavityrock.

  7. brp_nh | | #7

    Ethan, R40 is the target wall value for the PGH concept. I'm just trying to point out that you can still have a very energy efficient (including net zero) house without R40 walls.

    If you have the money, material availability, and expertise/labor to hit R40...then go for it. If trying to hit an R40 wall is complicating your house build, then scale it back to something more buildable and ~R30.

  8. Expert Member
    Dana Dorsett | | #8

    Ethan: Brian isn't crazy- he's really dead-on!

    The PGH 5-10-20-40-60 definition is pretty close to the sweet spot in US climate zone 6, where the inventors of the concept all live & work. You live in zone 5. In US climate zone 5 something like 4-8-15-30-60 is closer to what makes sense.

    Refer to the "whole-assembly R" values by climate zone in Table 2, p10 of this now classic document:

    Even though it was not intended as such, that pretty much the recipe for to what it takes to hit Net Zero Energy with 15% efficiency PV that fits comfortably on the roof using cold climate ductless heat pumps with an HSPF of 10/ SEER 16 for heating & cooling, which was pretty much where things stood in 2009 when that was drafted.

    The BA-1005 table 2 standard for zone 6 would be:

    ...5.5-10-20-35-75, which isn't too different in performance from the...
    ... 5-10 -20 -40-60 PGH definition.

    The BA-1005 standard for zone 5 would be:

    ...4 - 7.5 -15- 30 - 65, not very different from the...
    ...4 - 8 - 15 - 30 - 60 WAG just pulled off the top of my head.

    Fast forward to 2017, on the cusp of 2018:

    Solar panels for residential use are now typically 20% efficient (and only 25-30% the installed cost of what it was in 2009), and cold climate ductless HSPF of 12-14 are now available. You don't even need to hit the numbers in Table-2 to hit Net Zero with a PV array that fits comfortably on the roof, but you might want to anyway to be able to have enough additional PV to charge your commuter car(s).

    You really don't need 5-10-20-40-60 in zone 5 to have a REALLY comfortable and low-energy use house. The money spent on hitting 5-10-20-40-60 instead of "only" 4-8-15-30-60 would probably be better spent on the PV for the EV charger.

    An R5 window has to be a triple pane, but U0.25-ish (R4) windows can be had with double panes with low-E on surfaces #2 & #4, which is substantially cheaper than triple pane construction.

    R8 under the slab is dirt cheap if using 2" reclaimed EPS, you might even go for R12 if it's cheap enough.

    An R15 basement is now IRC 2015 code minimum. Lots of ways to get there.

    An R30 wall could be 2x6/R20 cellulose studwall 24" o.c. with some attention to advanced framing concepts to keep the framing fraction low enough, and 3-4" of rock wool on the exterior.

    An R60 vented attic could be an 18" energy heel truss to accommodate an 18" initial blow, ~16.5-17" settled depth cellulose.

    When going beyond that level of performance it's important to have clarity about your rationale for the additional expense, especially when using higher $/R materials/assemblies such as rigid rock wool or triple pane windows.

  9. ethant | | #9

    If I was a drinking man I'd buy you a beer. I feel like I just got let off the hook, big time.

    One more bit of info that I'd like to share. This is a spreadsheet I have been working on for a while, and of course it comes with the caveat that it is very unscientific and only based on my ability to get cost estimates.

    The estimate for the dense pack is for installed cost, which throws off the numbers a bit. But I hope it is illuminating because it doesn't show wood fiber as being all that noncompetitive (with rockwool).

    I have my own reservations about wood fiber insulation (like prejudice against MDF cabinets, IKEA furniture, etc), but I have yet to figure out if these reservations are unfounded. It seems, intuitively, that a rock fiber board would be more immune to the various problems (water, pests, fire, etc.) that the world throws at buildings.

    In any case, I think I'm settled on 2x6 framing with outboard insulation, maybe rockwool, maybe wood fiber. I saw Mento over Wood fiber, which at the time seemed like overkill, but now seems prudent, if for nothing else but peace of mind.

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