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Rigid foam minimum for 2×8 stud wall

CTSNicholas | Posted in Green Building Techniques on

Zone 5 – helping with a wall design. They are using 2x8s for studs to fit in R-30 mineral wool. I am suggesting rigid foam, EPS or XPS, but I have only read that 2×6 wall needs R-7.5 But, this is a 2×8 wall, and I don’t think R30 is traditional. So Will 2″ of XPS or EPS be sufficient? Don’t really want to have to go with two sheets and achieve 2.5″ of rigid foam if 2″ is sufficient with R30 mineral wool on the interior.

Is there any way to make 2″ work? Such as a smart membrane installed before drywall?

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  1. GBA Editor
    Martin Holladay | | #1

    Here is a link to a GBA article that will answer all of your questions: Combining Exterior Rigid Foam With Fluffy Insulation.

  2. brendanalbano | | #2

    According to this article:

    You want 27% (or more) of your R value in the rigid layer in zone 5. Which I think means you'd want a minimum of R-11.1 rigid + R-30 cavity (11.1/41.1 = 0.27).

    Hopefully someone else with more experience chime in regarding your smart vapor barrier, but that seems like a reasonable approach if you can't redesign the wall to have the appropriate ratio of continuous to cavity insulation.

    If you do go the smart vapor barrier route, are you able to include a service cavity on the inside? This way your smart vapor barrier doesn't have as many penetrations. Your sandwich (from structure inward) is something like:

    - 2x8 wall with cavity insulation
    - Smart vapor barrier
    - 2x3 horizontal strapping (with cavity insulation if you want)
    - Gypsum board

  3. CTSNicholas | | #3

    Appears to be prime only link.

    I read that I need 27% of the R Value to be on the outside for zone 5. Is that 27% of what is installed in the wall cavity? I can try to just use that formula...but wondered if simply 2" of rigid foam is sufficient and if a membrane (normally recommended with mineral wool) is a good idea or not.

  4. Jon_R | | #4

    You can use 27% of the entire wall R value. Would be nice if there were adjustments for smart vapor retarders and the permeability of the foam, but I haven't seen it.

    Note there is an advantage to using staggered seams in two layers of foam.

  5. CTSNicholas | | #5

    So, I need 27% of R30 or do I have two unknowns, such as (R30)73% + (X)27% = 100%?

    The only reason I have found for said minimum is because the interior side of the sheathing would be warm, and the rigid foam would prevent the sheathing form drying if condensing happens. Thus, the foam must be of high enough R value to keep the sheathing always above condensing point. However, the more insulation I have on the interior cavity would mean that the sheathing is slowly getting less and less effected. Right? And if I have a smart membrane, that will prevent the humid warm air from escaping into the wall cavity...yes? I feel like there is a standard but it has not been detailed enough to account for various insulation types inside the cavity and definitely not with a smart membrane versus traditional poly or no barrier at all (which I read is recommended if the exterior has foam, which is a barrier)

  6. Expert Member
    Dana Dorsett | | #6

    The R30 must be no more than 73% of the total R to be able to use a Class-III vapor retardency on the interior, which means you would need a hint more than R11 on the exterior. The more cavity insulation, the colder the average temp at the structural sheathing becomes. The more exterior insulation, the warmer the average temp at the sheathing. Warmer==drier.

    From a long term R-value point of view there is no advantage to using XPS over EPS, since as it loses it's climate damaging HFC blowing agents over a few decades it's performance declines to that of EPS of similar density. (R4.2/inch)

    So, 3" of EPS/XPS (= ~R12.6 over the long haul), or 2.5" of polyiso (= ~R12.5, derated for temperature) would be the MINIMUM necessary to mitigate the risk. More exterior foam is safer.

    If you're going to cheat the exterior R and stay at 2", go with rigid rock wool (R8), which still allows high drying rates toward the interior, and use half-perm paint on air-tight drywall, or air-tight smart vapor retarder under the air-tight drywall.

    A smart membrane isn't an air barrier unless you detail it as an air barrier. But it is still a (water) vapor retarder whether air-tight or not. Air leaks from the interior are an order of magnitude more problematic than vapor diffusion.

  7. CTSNicholas | | #7


    Such a disappointment to not be able to have 2x8 AND 2" of rigid. Anything over 2" rigid is just more bothersome with the extra detail. So maybe 2x6 interior 2" rigid or just the 2x8. It would be cheaper to have 2x8 and have R-30 while 2x6 would fit R-23 in the cavity and about R 8.4 on the exterior. Though it would help with thermal bridging, I'm not sure that's convincing enough for them.

  8. GBA Editor
    Martin Holladay | | #8

    There is no good argument in favor of 2x8 studs. For more information, see How to Design a Wall.

  9. CTSNicholas | | #9

    Another prime link ;) The reason for 2x8 in their case is they already have 30 bundles of the mineral wool set for 16" width 7.25" deep. Which would fit perfect in 2x8 framing. A 2x8 wall with staggered studs is good, but AFAIK a cellulose filled wall will not perform the same as mineral wool in a thickness comparison.

  10. Jon_R | | #10

    Thus, the foam must be of high enough R value to keep the sheathing always above condensing point.

    Simple math shows that the minimum foam levels do not prevent condensation - they reduce it. Even when the foam is sufficient, exfiltrating air will often condense in the wall and where the water goes from there may be a problem.

  11. GBA Editor
    Martin Holladay | | #11

    If you are interested in reading a GBA Prime article:

    1. GBA offers a free 10-day trial offer allowing you to read GBA articles for free. Here is the link to sign up:

    2. Once the 10 days are up, you can order a one-month GBA access for $15. That's as much as one or two panels of rigid foam. If you are a builder, and need to make business decisions (or provide informed advice) or a homeowner spending $200,000 on a new house, it's money well spent.

  12. brendanalbano | | #12

    If you must use 2x8 studs with mineral wool for budget reasons due to already owning the insulation, this guy seems to have a system he is advocating:

    It's pretty much in line with Dana's recommendations, and the service cavity makes detailing the smart vapor barrier as an air barrier easier.

    Just be aware that it is somewhat non-standard, so you won't have the wealth of resources to rely on compared to the conventional approaches (either 2x6 + rigid or double stud).

  13. CTSNicholas | | #13

    The house is already a modest size and the idea is to keep the walls efficient but not too efficient so they take away from the house. I think we could agree a 2x6 Wall with 2" XPS/EPS is the best bang for buck, yes? While if using pre-existing insulation (special ordered, not really refundable) then a 2x8 wall would be best bet, or 2x8 with 3" of XPS/EPS? I'm thinking the foam on exterior would at least make the thermal bridging on a 2x8 much less. Maybe it could be 2x8 plates but 2x4 and 2x3 studs to have an air gap, but still maintain 16" O.C. for the mineral wool?

    Just throwing out ideas for you guys to analyze.

  14. brendanalbano | | #14

    2x8 with 3" rigid is a great high performance wall. It's just not the most cost effective way to achieve that performance if you were starting from scratch. But it sounds like it will work, it's got a high R value, and it doesn't require fussy interior membranes because the sheathing is kept warm enough for your climate.

    A 2x8 wall with no rigid has a similar or slightly lower R value than a 2x6 + 2" rigid once you account for the thermal bridging of the studs, and has potential moisture issues, requiring an air barrier and smart vapor retarder on the interior. If you wanted to squeeze a few more Rs out of this wall, you could put a little bit of mineral wool in the outside, as Dana suggested, which will boost the insulation while not inhibiting drying, but until you hit the R values Dana referenced, you'll need the interior membrane, which is added cost.

    A 2x6 with 2" rigid wall is high quality and durable, but doesn't quite get into "high R value" territory until you throw a little more foam on the outside. Starting from scratch, 2x6 @ 24" on center + as much exterior rigid as you can afford is a standard and well tested approach, but since you aren't starting from scratch it isn't necessarily the right approach for you.

  15. Jon_Lawrence | | #15

    If you need to stick with the 2x8 studs, you could use Neopor EPS which will give you about R-11 at 2" - at about a 25% premium to regular EPS. That should round up to 27% exterior/total. If you can use 2x6 studs, you can still use the 7.25" ComfortBatts. Just place them in stud bay and using a long serrated knife cut them back to the edge of the stud. They literally cut like butter. 2" of Neopor or even regular EPS with 5.5" of Roxul will get you where you need to be on exterior/interior R-value percentage. And you will find lots of places to use the 1.75" Roxul leftovers in a new house.

  16. Jon_R | | #16

    You can also avoid wood sheathing and use something else for racking. Then 2" is low risk.

  17. CTSNicholas | | #17

    I have 2x6 studs and mineral wool in my home. I wanted to install XPS on the exterior but did not due to: time, budget, and talking with HVAC who suggested if I use 2" foam anywhere, I should use it on my basement versus the exterior of the house. It made sense because my basement is exposed on 2 walls.

    I did have to use the Smart Membrane, and I didn't have any problems with it. I taped the seams, and did some detail work around electrical boxes. I don't know what the inside of my wall is like now, but I hope it's dry! I have about R23 before thermal bridging is calculated.

    I know I could do better, and I want to for any future projects or builds. This is one of them. I think my Andersen 400 windows are my weakest link in my wall, and they will be for this future project if they are used again on the 2x8 house build. Yes? I think we can shave their insulation down that is already on hand to fit in a 2x6 stud cavity. Then, 2" of rigid foam on the exterior. That sounds like it will be better than a 2x8 with nothing on the exterior. This means no smart membrane on the interior, even though it is mineral wool insulation then? Because the foam will prevent any drying, and therefor the membrane would not allow *enough* drying to the interior then?

    Option 2 is to follow through with the 2x8 wall, 16" o/c (per current insulation & a straighter wall with more points to support 1/2" drywall + hang all the cabinets and such), with a smart membrane, and if budget allow install 1.5" of mineral wool (above or below the house wrap) to prevent some of the negated thermal bridging, 1x3 furring strips, and then siding?

    EDIT: Checked locally with stores and online with places nearby. 2.5" Mineral Wool by Owens is the thinnest stuff available and only comes in 24" x 48" for biggest size. Is that what you were mentioning or a different product? Sounds like that would be a lot of work!

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