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Community and Q&A

Mineral Wool Exterior Insulation, Humidity and Windwashing

LucyF | Posted in Green Building Techniques on

Every time I think I have my insulation strategy set, I learn something new that makes me question it. That’s what you get for reading the internet.

Anyway, I am planning on 2 x 6 walls with dense pack cellulose with a smart vapor membrane like Intello Plus, an insulated 2″ service chase. Outside of the cellulose would be plywood, housewrap, then probably 2 inches of mineral wool exterior insulation (Roxul Comfortboard is my plan) then strapping, then Hardiboard. I am in zone 3 mixed-humid in the Piedmont of the Carolinas.

I’ve been reading things here for about 3 years, so I have what I think is a very reasonable wall for our climate that is also fire resistant and insect resistant. This week I started reading about limiting humidity in the South by using a less vapor permeable house wrap like Pro Clima’s DA membrane ( to prevent inward vapor drive during the long cooling season.

The other issue suggested by this source is that if you have an airtight membrane on top of the exterior insulation, the insulation would be more effective because it is air sealed on all sides. I thought about that for the cellulose, but I did not think about it for the exterior insulation.

What do you guys think about using a membrane on top of the mineral wool? Sounds like a real pain to detail around windows, etc. I know Lucas Durand is building his house with exterior insulation. I am curious what kind of membrane he will use exterior to the mineral wool.

I guess what I am asking is – from a performance perspective, how much do I lose in terms of insulation effectiveness by NOT covering it with a membrane?

Thank you as always. Lucy

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  1. user-1137156 | | #1

    Take a look at Greenguard DC14 placed under your Hardiboard and maybe the strapping. DC14 is 1/4" thick fan fold XPS with drainage channels on both sides, it has a perm of about 1 so it effectively blocks solar driven inward moisture. What are you insulating the inner cavity with? Is the interior strapping horizontal? What is your lowest expected winter outdoor temperature?.

  2. LucyF | | #2

    I am trying to avoid foam if I can. That's one of the reasons I've chosen a mineral wool product.

    Between the studs will be dense pack cellulose. I will probably use mineral wool in the service cavity. I will use horizontal interior strapping. I have to figure out the exact details of the horizontal strapping - I've heard of ripping 2 x 4s for that strapping. So it's 1.5 inches deep. I don't know of a mineral wool batt insulation will fit in that gap. Maybe I need a 2" space to put 2 layers of the Roxul Comfort Board in there. Also I think electrical boxes fit better with a 2" space.

    The lowest temperature I've seen here in the last 10 years is 10 degrees. We get in the teens for several nights/winter.

    Our air conditioner season can extend from March to November. March 2, 2012 we hit 81 degrees. More common though is April to October for air conditioning.

  3. LucyF | | #3


    I didn't go with double 2 x4 walls because I thought that building details would be too much for me. It turns out the details for exterior insulation can be challenging in their own right. I've never built a house before, but I am going to starting in April. In order to make it a little easier for me to actually be involved in the building, I've ordered a kit from Shelter Kit. It's already paid for and is being cut now.

    (Don't worry, I have good support - the house is for my brother who is cabinet maker, I've got a great carpenter who is going to help me and a good buddy who is my contractor. I've been educating all 3 for the last 3-4 years about what I've learned about energy efficient building.)

  4. user-1137156 | | #4

    I asked about your expected temperatures because a double 2x4 wall with the sheathing on the outside of the inner wall and 2" Roxul board between walls may be a good option. it's the same over all thickness, actually thinner by the thickness of the exterior furring, and eliminates both strapping and furring with the same thermal performance and the same thickness. If you used 3 1/2" Roxul bats in both walls you would have a slightly better total r value. If you used aligned studs the first condensation surface would have r 15 on the inside and r22.63 on the outside. Translating, you would have condensation on the sheathing's inner surface when out door temperatures fell below -5 f if your indoor temperature was 70 and indoor relative humidity 35 % (dew point 40 f) The plywood or OSB is itself a "smart" vapor retarder with about 1 perm dry and more as it gets moist. With Tyvec or other vapor open house wrap on the outside you would all but eliminate any "wind washing". Total cost should be very close but you avoid ripping strapping and finding thinner bats for the inside. . The condensation problem in summer is reversed and you have r22 on the outside and r15.63 on the inside which means you'd have condensation at 70 indoors when the outdoor temperature was 90 and dew point of 82 a relative humidity of 75 %. Since some summertime condensation appears inevitable I'd definitely use plywood not OSB. Both summer and winter condensation situations are improved if more of the r value is outside the condensing surface, perhaps dropping to 2" bats on the inside which of course reduces total r value correspondingly. BTW 2" Roxul bats are available as either " safe&sound" or" AFB" and are actually intended as sound control but it's the same stuff & r 4/inch. Since the outer wall would be erected unsheathed you could eliminate the rigid Roxul and use r22 bats in the outer wall fitting strips of 2:" batt behind studs along the way trading some material cost for labor. Another option would be use a layer of 2" Roxul for metal studs which would end up having seams centered behind each stud. Or even better space the wall 3" apart and use 3" metal stud roxul then 3 1/2" wood stud between the studs for a very slightly thicker wall than your original with the strapping on the outside. Combining r26 on the outside with r8 in the service cavity is about as good a ratio as can be had.

  5. user-1137156 | | #5

    With strapping over exterior insulation and an indoor service cavity you are building greater complexity than double walls as I've described. The trick to doing double stud walls is to lay out plates for both walls at the same time, setting the inner wall's plates aside till it is framed.

  6. user-757117 | | #6

    To answer your question, I'm using Typar over the exterior as a WRB and as a "windbreaker".
    I chose Typar because, like you, I wanted something a little less vapour open than the usual Tyvek to mitigate inward vapour drive in the summer (not that my climate is as hot or humid as yours, but it can sometimes get pretty sticky in the summer).
    I'm not sure I understand exactly the sequence of the wall profile you've described, but it sounds like you are already specifying a membrane in your assembly, so I hesitate to suggest you need another...
    Do you have any drawings or sketches?

    Windwashing of exterior mineral wool boards...
    Good question.
    Until recently I would have assumed that mineral wool boards (high-density as they may be) would need something to prevent windwashing...
    But after reading the comments here, I'm not-so-sure...
    I noticed you were also a commenter to the same blog, so I assume you know what I'm talking about.

  7. LucyF | | #7

    I chose my particular wall assembly because I had already done a retrofit on my house with polyiso exterior insulation so I have some familiarity with that process. And I really like the idea of a smart inner vapor retarder - the Intello Plus membrane where it cannot be penetrated. I also like dense pack cellulose which we used on a rental house I own. So I have some experience with that as well.

    I had planned to use polyiso on the exterior of the house we will be building, but with the availability of Roxul Comfortboard I've decided to go with that due to all the bugs in the South and to avoid a petroleum product.

    Shelter kit has just added double stud walls as an option to its kits this year, but it seemed like an alien concept to me so I did not choose it. You're right about the complexity though.

    I struggle with building terminology. Here is a description of the wall assembly as initially planned from the inside dry wall to the outside Hardieboard.

    Drywall > Service chase horizontal 2 x 4 with roxul insulation > Intello Plus vapor retarder > 2 x 6 studs with dense pack cellulose > Plywood taped or using a Prosoco product to seal the seams of plywood > Tyvek > 2" Roxul Comfortboard > Furring strips with insect screen > Hardieboard.

    So now I am deciding if I need to move the Tyvek layer outward of the Roxul Comfortboard and use something like a less vapor permeable membrane (such as Pro Clima DA membrane or Typar even) to limit inward vapor drive as well as prevent any windwashing.

    Thank you for your advice.

  8. user-1137156 | | #8

    With the material kit on the way and paid for it's definitely too late to change! I well know how you are feeling right now, and how you will soon feel. Though it was about 40 years ago I remember well the overwhelming feeling when they delivered the lumber package for the first house I built. The thought that I've got to carry it all piece by piece and build a house out of it was very intimidating! Good luck and above all stay safe! I agree you want a lower perm but not a vapor barrier WRB to reduce inward moisture. And you want to locate it outside all the insulation . You'll still want drying to the outside, on average. I still think a double wall with a 2 1/2" space filled with r10 "metal stud" Roxul comfort bats with the sheathing on the outside of the inner wall would be better and save some money. and actually be much easier to build. Just think of me as a stubborn old man advocating double walls.

  9. user-1120647 | | #9


    I would leave the tyvek under the roxul. Inward vapor drive will not be a major factor as you have a good ventilation gap behind your cladding and you don't have a reservoir cladding (like brick or stucco) to store moisture.

    Leaving the tyvek behind the roxul will make it easier to install and detail at windows, and make it a more durable air barrier as it will be protected and supported from both sides.

    Wind washing does not appear to be an issue with IS (at least in cold weather).


  10. LucyF | | #10

    I do think that would be easier to build that way. I also think our building inspectors will like it better as well. South Carolina is not exactly at the forefront of green building technology. I think our energy codes are still from 2006.

    Also, we would like to hear more about your research. It sounds very interesting.

    Anyone advocating an advanced building practice like double stud walls cannot not be thought of as an old man. Stubborn, maybe, but not old. Thank you for commenting.

  11. user-757117 | | #11

    Thanks for the extra detail...
    I think you're doing fine with building terminology.
    I generally hesitate to comment if I'm not sure about what's what, and on top of that I'm a little "out of my element" thinking for your climate zone.

    When I think about cooling dominated climates, I tend to simply reverse all the standard guidelines for heating dominated climates.
    For example:
    In a heating dominated climate, the exterior of an assembly should be at least five times as "vapour open" as the interior.
    So in my mind, in a cooling dominated climate, the interior of an assembly should be at least five times as "vapour open" as the exterior...

    Instead of wanting the sheathing to be "warm in winter", you may want the sheathing to be "warm in summer".

    Maybe someone with more "southern experience" could chime in here...

    Despite the rainscreen cladding I think inward vapour drive is still a concern for your climate.
    With a taped plywood air barrier located at the exterior of the wall as you described, I think you have not just a great air barrier, but it is in the most "climate appropriate" location and will also provide good vapour retarding performance to boot.

    On the interior side of your assembly some type of air retardant membrane or ADA is probably a good idea.
    Whatever it is should be at least five times as "vapour open" as the plywood to allow for drying potential to the interior - in this case (considering the plywood at ~0.5 perms dry) the "Intello Plus" membrane at 0.17 perms dry may be more impermeable than ideal.

    Jumping back out to the exterior side again, you still need some type of WRB, whether Tyvek, Typar, or felt.
    The WRB can be located either on top of or under the exterior mineral wool boards - whatever works best for your flashing details.

  12. albertrooks | | #12


    A couple of notes for you:

    In the service cavity I'd suggest using a 4lb mineral wool. It's still R4 per inch, will be less expensive and easier to work with than a higher density comfoboard. For horizontal battens try ripping a 2x6 in half. We stock a Heco 130mm T drive screw for this. It's a 1/3 thread so it drives in one quick motion and will compress the batten to the frame. I like screws because they will help straighten the wall.

    Using mineral wool as an exterior rigid insulation layer applied with a screw and a batten has been done in Central Europe for many years. If you use Roxul Comfoboard then according to most European builders with application experience I've talked too, the boards density is high enough to omit an overlaying WRB. Place a WRB behind the MW at the sheathing.

    MW will not shrink. The quality of the application will remain permanent and you don't have to worry about joints opening due to shrinkage like most foam boards.

    Sounds like you're building a very durable assembly. Nice work.

  13. LucyF | | #13

    Thank you everybody.

    Albert, how fat is the 4# mineral wool? Can I compress the 4# mineral wool to fit in a 1.5 inch space? Will the drywall guys be happy about that? Remember these are drywall guys who have probably never seen mineral wool insulation.

    Where are you using the Heco 130mm T drive screws, inside for the service chase or outside for the rainscreen? I did not see that size screw 130mm on your website, Small Planet Workshop.

  14. albertrooks | | #14

    Hi Lucy,

    You can get the 4lb density MW in many thicknesses. Just find a good distributor near you. I've done well by starting with the Roxul rep in my area.

    Yes... We don't have the 130mm size up on the web yet. We will get more of the new items up in the next few weeks. Feel free to call if you'd like. We've got a new shipment arriving next week.

    It's possible that you could find something closer to you, but it can be hard to find a 1/3 thread AND a T drive at 130mm long in the same item. It's a good combination. Most screws like this are a 2/3 thread. Meaning the threads cover 2/3'rds of the length. The upper threads are in the way of compressing the 2x3 onto the wall framing. They fight the compression and add load to the drive.

  15. albertrooks | | #15


    I just noticed that you are thinking of a 1.5" cavity. Yes you can get 4lb MW in 1.5" thickness.

    A nail gun is probably be all you really need. to apply a flat 2x4.

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