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Community and Q&A

Rainscreen with Mineral Wool Insulation

mjbroz | Posted in Green Building Techniques on

Hi all.
I am planning out my wall construction for a home in zone 6. Not much rain (15″ annually), but plenty of snow in winter (56″ annually). Hot (and mostly dry) in summer (80sF to low 100s), cold in winter (mostly lows in the single digits, but sometimes minus teens F). My house will not have much direct sun due to northern exposure and tree shading.

All that said, here is the wall I am considering:
– wall board
– 2×6 fir studs on 16″ centers, Roxul Comfortbatt (R23) between the studs
– OSB, taped and sealed for my air barrier
– house wrap for vapor barrier
– 3″ of Roxul Comfortbatt IS (R12)
– 1×4 verticle furring strips, plus rainscreen vents at top and bottom between furring strips
– cement board exterior

A few concerns I have are: the rainscreen causing a chimney effect during a fire (wildfire is a big issue out here), the exterior only being R12 in my zone, and the work required to keep the furring strips coplanar for good looking exterior siding.

This got me thinking about the following modification:
– change the 1×4 furring strips to 2x4s (still laid flat over the 3″ of Roxul)
– fill the gap between furring strips with an additional 1.5″ of Roxul (R6)

This would add a combined R4.5 to R5 to the exterior, avoid any potential chimney effect, get rid of the vents top and bottom between furring strips, and should make it easier to get the furring strips coplanar since the 2x will deflect a lot less than 1x. In addition, this still allows me to use 6″ screws (probably Timberloks or CTX lags) for attaching the furring strips because I can countersink them a bit when I am predrilling

Of course, this loses the rainscreen gap. My understanding of Roxul is it should a drain fairly well when water/water vapor gets behind the cement board. Remember I don’t get much rain, I have 2′ overhangs (for snow issues), and wind driven rain is unusual. The only concern I can come up with is the cement board will not be able to dry out as quickly, which could effect durability of it’s painted surface (I’ve seen some issues with paint peeling off of cement board). And of course I have a fair amount of Roxul to custom cut to size.

My questions are:
– has anybody else out there tried something like this
– does anybody see significant issues to this approach

Note: I have also considered just going to 4″ of Roxul on the exterior and only venting the bottom of the rainscreen (using horizontal furring to block the top gap). This might be simpler, would at least partially address the chimney effect and be a better R factor. Unfortunately, I have seen concerns about using more than 3″ of Roxul when hanging heavy siding (like cement board) on it, and of course the 6″ screws become 7″ or 8″ (depending on what I can get).

Thanks in advance for your thoughts and advice.

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  1. iLikeDirt | | #1

    I think you have a good idea. The Roxul is itself porous and drainable, so water will fall right through it. In your climate (Sounds like Northern New Mexico or somewhere else in the intermountain west) this sounds like a good approach if wildfires are a concern.

    As for your choice of cladding, it might make more sense to hit it with a simple one-coat stucco instead of painting it. Painted cementboard might not end up looking especially aesthetically pleasing, prompting you or a future owner to remove it even if it's still sound. If you do paint it, I'd recommend using silicate mineral paint anyway since the sun will do terrible things to normal exterior paint over time if my guess about the location is correct. Silicate mineral paint will last darn near forever, no maintenance required. Of course, it's more expensive, and with the up-charge, a one-coat stucco over the board might be cost-effective.

  2. user-4524083 | | #2

    Mike (and Nate) I assume you mean fiber cement clapboards. You may want to look into a second stud wall with mineral wool batts. The batts are considerably less expensive than the "boards" even with the extra cost of the studs. Basically, you'd be building a double wall with mineral fiber. As you plan it now, the fire potential is pretty minimal, as cement clapboard siding and mineral wool boards are a lot less combustible than foam and wood siding! That said,your current plan, not having a rain screen, given your dry climate and use of mineral wool boards seems very reasonable.

  3. JC72 | | #3

    No. Rockwool isn't flammable and the base of your rainscreen will have an insect screen.

    Paint all sides of the cement siding before it goes on.

  4. mjbroz | | #4

    Thanks Nate and Kevin. To answer your questions:
    - I'm actually in Okanogan County in WA state (yes, it's true, we do have dry areas of the state :-)
    - Yes, when I said "cement board", I was referring to Hardie product (either lap siding or panels for a board-and-batt look...that decision will be up to the wife :-).

    Kevin - I'll look at the prices of Comfortbatt w/double wall vs Comfortboard IS. Thanks for the idea. Note: I have stayed away from double walls primarily because it's not a well known practice in the area while foam over standard 2x6 walls is very common. While I'll be doing most of the time consuming aspects of air/vapor/water sealing myself (and therefore am OK with "less well known techniques"), I'll have a crew doing the majority of the framing and I would like to keep things as "standard" for them as I can.

  5. mjbroz | | #5

    Chris M - I am not sure I understand your response. Does "No" mean you have never heard of anybody doing what I proposed above, or you don't think it's a good idea, or something else?

    I agree painting all sides of the cladding before install is the way to go. Thanks for that.

    What I think I hear you saying is since the Roxul is not flammable (nor is the cement board cladding), you don't see a risk for chimney effects during a fire. Is that correct? I agree the only flammble items are the furring strips and potentially the insect screen (what I was calling "rainscreen vents").

    Thanks for your feedback.

  6. STEPHEN SHEEHY | | #6

    Mike: We did a double wall on our new house. Our contractor had never done one, but found it pretty simple.

  7. Dana1 | | #7

    First, housewrap is not a vapor barrier. Most housewraps are north of 30 perms, and even the most vapor tight versions run about 10 perms, a minimal Class-III vapor retarder. (Traditional #15 felt is about 0.1 perms dry, ~5 perms when damp.)

    Housewraps can be detailed as an AIR barrier, but detailing the OSB sheathing as the primary air barrier is more robust. (OSB is much more vapor-tight than housewrap, less than 1 perm when dry, maybe 5-10 perms at moisture content levels high enough to rot.)

    With fiber cement siding facing one side of the rainscreen cavity and rock wool facing the other, there is nothing to burn but the furring. If it's hot enough ion the exterior to ignite the furring through fiber cement siding the house is already on fire from the heat radiating in through the (long since broken) windows.

    The rainscreen cavity is a capillary break, keeping rain/dew wetted siding moisture from wicking toward the interior, and a convection path to purge any moisture that gets in quickly, keeping the siding & furring much drier. by allowing the siding to dry in both directions, moisture accumulation is minor, reducing the likelihood of paint failure. Painting both sides of the siding helps, but not nearly as much as a convecting rainscreen type assembly.

  8. Expert Member

    Unless you have some compelling reason for wanting more exterior insulation, I would go with your initial plan except for not venting the top of the cavity.

    Whether you include an open cavity or fill it with Roxul, you still should include a perforated base flashing that covers both the gap and exterior insulation to keep out pests and insects and protect it from mechanical damage. The top of the cavity will be blocked by your soffit.

    It's a simple, buildable assembly that will perform well.

    Edit: I've never heard of Hardi-boards or panels having problems with their paint when installed on rain screen furring with just the factory applied primer. I wouldn't bother painting all six sides.

  9. mjbroz | | #9

    Nate - thanks for the hint on silicate mineral paint, I will look into that

    D. Dorsett - thanks for your response. I was too cavalier in my description of the purose of the housewrap; I suppose it's primary function is to protect from bulk water intrusion if something should go catastrophically wrong. As you say, the OSB would be the primary air and I suppose vapor barrier. I do like the value of the capillary break and drying aspects that the rainscreen gap provides. I was just thinking perhaps the porous nature of the Roxul would be a sufficient substitute.

    Malcolm - good point about the base of the Roxul needing flashing with weep holes. That was in the plan but I didn't mention it. I am still working out the soffit details, but I am tending toward no venting at the top despite the fact that it will limit the drying abilities of the rainscreen gap somewhat. I would like R15 on the outside of the OSB for those few years when it gets really cold for longer periods, but it's not technically required. 3" of Roxul only gives me R12. Martin's guidelines from say 11.25 minimum, so R12 would work. Sometimes I just like a little extra insurance.

    This may all be moot if I can't find a place locally to source the Roxul (or equivalent material like Thermafiber Rainbarrier HD) and have to go back to polyiso foam board instead.

    Thanks again to everybody who responded.

  10. LucyF | | #10


    I couldn't find Roxul at first either. Now my local home improvement store will special order it. You do have to wait though, because they "batch" the orders until they have enough to fill a truck. Anyway, that was the case in SC 3 years ago.

    At the time we built a house, I actually ordered Roxul Rockboard 80 from an acoustic supply house. It has the same density as Comfortboard, I assume the same product but just a different name. I had to do that rather than wait for the shipment from the big box store because the guys were ready to install it and the acoustic company got it to me in 3 days. There was, of course, a big shipping charge.

    I vote for Roxul over polyiso. I don't suppose you live in termite land or have carpenter ants? That would be another reason to prefer Roxul or a mineral wool product.

    Also, you mentioned choosing to do exterior insulation over a double stud wall because the framing crew isn't familiar with that building design. If you're not going to be there to make sure that they are doing it right, I don't think it is a good idea. I worked hard educating my crew (which consisted of only 2-3 people who were on board with the stuff I wanted to do) and still had to be there all the time to make sure stuff was done right.

    We elected to do exterior insulation for that reason. Good luck with your house. It sounds like you have some experience with construction. That will serve you well if that is the case.

  11. GBA Editor
    Martin Holladay | | #11

    I think that your plan will work. Since mineral wool drains, most of the benefits of a rainscreen can be achieved with mineral wool, even without an air gap.

  12. jasonn1234 | | #12

    Did this work out?

    On a side note, it looks like Roxul comfortbatt and comfort board 80(aka comfortboard IS) both have the same R value per inch (4)., but the price difference per inch(cubic foot) is not even close. comfortboard IS is 48sqft per bundle and $57 at HD for 1.5" thick this equates to $1.18 per sq ft. Comfortbatt is 37.5 sq ft for 2x6 (5.5)" and $43. This would equate to 137.5 sq ft of 1.5" thick if i cut it up. This then equates to $0.31 per square ft(@1.5" thick), Comfortboard is then approx four times the cost of comfortbatt for the same R value.

    this got me thinking. Perhaps the better solution is to use 3.5" confortbatt on the outside wall.

    To ensure a thermal break, you could rip 2x3s to 2", and rip 1.5" wide comfoartboard IS strips(or foam). Then put the ripped 2x3s on end as strapping onto the ripped comfortboard IS. And then fill in between the strapping with regular comfortbatt 3.5" x22.5" wide insulation.

    The only real downside I see is the intersection of the ripped IS pieces and the comfortbatt. you'd lose continuity I suppose.

    But my goodness, at the price ratio between the products, it would probably save a couple thousand on a house.

    Or is there a flaw in my math?

  13. mjbroz | | #13

    Jason -
    We won't get to the framing until next year, so I don't have an update on how well it went (yet). I did manage to pick up some 2" Roxul Comfortboard off of Craigslist and I found a supplier for 1.25" Comfortboard to go between the 2x4 furring strips, so now the plan is modified a bit. Inside to outside:
    - wallboard
    - 2x6 standard wall w/Roxul Comfortbatt between the studs (on 16" centers)
    - OSB sheathing
    - WRB (details still in flux...considering a liquid applied WRB at this point)
    - 2" Roxul Comfortboard
    - 2x4 furring strips
    - 1.25" Roxul Comforboard between the 2x4 furring strips

    This leaves me with a 1/4" rainscreen gap that may help a little with bulk water intrusion if I have any in addition to the draining ability of the Roxul. Of course my R-factor has dropped (somewhere around 12.3 taking the 2x4s into account I think), but the price was right for the 2" stuff.

    All that said, I will have to let others comment on your idea to use Comfortbatt on the exterior. Others have proposed similar ideas to yours with respect to attaching foam to the backside of the furring strips to create a thermal break and avoid the compressibility issues of Roxul. I am guessing using insulation on the outside of a home that is designed for inside will be an interesting conversation though :-).

  14. user-2310254 | | #14

    @Jason. Is there a reason you aren't using advanced framing techniques such as 24-inch on-center studs? You would save quite a bit of lumber.

  15. Expert Member
    Dana Dorsett | | #15

    A 1/4" rainscreen gap is plenty. In fact, 1/8" would be plenty if it could be applied consistently. It's primary function is as a capillary break between rain/dew wetted siding and the rest of the assembly, not as a bulk-water drain for a fire-hose. Code min on the western slopes of the mountains in B.C. is for 10mm (3/8") but there's not much data to indicate that 10mm is dramatically better than 1/4" (6mm), even in that far wetter climate.

    Rock wool in direct contact with wet siding will in fact wick moisture toward the sheathing, but in your climate the high air permeability of rock wool at that thickness might be sufficient protection even without the capillary break, but keeping rainscreen would still fall under "best practices".

  16. jasonn1234 | | #16

    Mike, I thought about the foam to the backside of the furring strips as well, and now that I cut the 2x6s to 2" and basically wasted an 1.5" strip of wood, I think buying two inch thick foamboard would have been better as then I could have used 2x2 everywhere. I was hoping to find 2: comfortboard IS here, but it is not readily available. Ah well. But you didnt say if you have the same price differential on the 1.5" comfortboard vs comfortbatt that we have. Do you?


    I have tall walls, engineer required 16 o.c as well as blocking every 48". I plan on trying my little plan above tommorow and spacing 24" o.c. for the outside, and hitting the blocking with the strapping screws. This should provide enough support(I'll use GRK fasteners maybe) for the eventual horizontal lap siding. It also eliminates my WRB conundrum, as I will now have the WRB fully on the outside of everything. This will allow tie in to the window drip flashing and sill flashing to be easier. After reading multiple comments on this site I could never realy figure out how exactly to properly flash around a flanged window if there was a rainscreen. Looking at Roxul's website and videos, they tell you to flash out the window from the OSB. Too much of a pain in the a$$ in my opinion as you then have to batten the inside of the window as well. That and the rainscreen requires insect screening at the bottom and top. I could never find a video on how that is affixed (stapled and then brought over the lap siding and stapled on the front?). Maybe I'll post a couple pics tommorow as I'm going to try this on my little wall first

    oh and in case anyone is wondering why I am not simply building another wall on the interior, I am not doing that for a couple reasons:
    1) This is a second storey addition. Square footage is precious. a second interior wall framing would lose 3.5" where there is floor. And where there is "open to above", this would not work because the main floor is 2x4 wall (I'd have to bring in the main floor walls too (and adjust all the heat vents, electrical etc)
    2) I may cheat in places and not put strapping exactly every 24" on the outside. If I was to do this on the inside it needs to be proper in order for the drywall to be affixed. As well I do not have to put in a top plate for the horizontal siding. At the end of the day I just dont have to be as fussy on the outside
    3) whan I go to reside my main floor I can employ the same stategy, so eventually the hole house will have the same outside dimensions, and same R value. I do not have to take 3.5" off my interior dimensions (which simply would not be feasible for say, the bathroom)

    This should be good, My upper floor will be R22+R14 nominal, and eventually my main floor will be R12+R14 nominal. The new roof is a vaulted split truss system with room for 13.5" inches of insulation. This should given me near R50 if I use Roxul. The only problem will be the windows, I think they are only going to be R7 :(

  17. jasonn1234 | | #17

    so i tried out the above.. I glued(PL'd) the strips of 1.5" wide by 1.5" thick confortborad to my 2" ripped 1.5" wide spruce lumber.(12' long as I have mini 6' walls on this side of the house) The PL works to keep the insulation attached for the most part if you are carefull. I then cut to length. Because I felt guilty about not following roxul instructions, I decided to put the WRB on first (but I will do it again later). I then attached the cut pieces with 6" screws, and then backed out the screws to ensure the total distance from the wall is 3.5". I then put the normal comfortbatt between the pieces. Below are pictures. I do think I will need to put a nail with the plastic washer in the middle if nothing else ot make it look good. There is a weakness and that is the left to right movement of the wood/confortboard pieces. I think for the remianing large walls I'll attach styrofoam to give it more rigidity

  18. jasonn1234 | | #18


  19. charlie_sullivan | | #19

    Jason, that's a very interesting idea and experiment. Thanks for posting the pictures as well.

    The side-to-side floppiness will be helped some by the siding. Another strategy would be to angle the screws that hold the 2" thick strapping very slightly left and right, alternating. I also have recommended angling some of them very slightly up to help the vertical rigidity. You could probably achieve the same effect just by not being care to get the screws in straight.

    Am I right that you are not planning any rainscreen gap between the siding and the batt? (Sorry if you said otherwise and I lost track among all the different ideas flying around on this thread.) A way to do that might be to use a random filament drainage mat between the whole assembly you have now and the siding. (Keene Driwall or Benjamin Obdyke Slicker) If you are planning a second layer of WRB, you could opt for a combined product, either dimpled WRB, or a two layer combination of WRB and drainage mat.

  20. jasonn1234 | | #20

    pics below.

    As well, I used small nail mending plates to secure the batt insulation instead of plastic or metal washers as i could not find the former and the metal washers were the same price as the simpson mending plates, but the mending plates(truss connectors)have metal grabbers which worked well to hold against the insulation. Because my sheating is OSB, and screwing into 3/8" OSB wasnt exactly strong, I nailed 2x4" of plywood from the inside of the house the support the screw. I also then came up with the idea of firing 3.6" nails from the inside to the outside(At a slight upward angle) for additional support of the insulation. In hindsight, doing this probably eliminates any need to support the roxul from the outside

  21. jasonn1234 | | #21

    the end result below. The only real odd thing is I have two layers of WRB at the end of the day, which I dont think has any impact other than cost (and it is cheap as people readily sell partial rolls).

    Because this window is for 2x6 wall, I will need to supplement the casing with 1x3 primed MDF. For my other windows I will be getting the Window manufacturer to build extended casings to match the new wall width (3.5+3/8 OSB"+5.5+1/2" drywall) for a total of 9-7/8"

  22. jasonn1234 | | #22

    There is no real raisnscreen, . However, it is a little "cushiony" between the vertical members, which will likely mean there is an actual "variable" rainscreen, because I am using comfortatt instead of comfortboard. Although this "dimpled WRB is intriguing me. Do you have a brand name? What do you suppose would be the advantage over two WRBs? better breathing?

    With regard to the side to side, for the two side walls of my house, these vertical pieces lined up with the trusses. I considered attaching a 2x3(or a simpson H4 connector) at the top from the truss to the vertical member for stability, but in the end, I figured for the height of the wall (6') it would be okay.I'll try the screw angle strategy on my other 6' wall (but remember it is only 1.5" wide by 2" deep, when looking at the front).

    For the front of my house, There is no real advantage(other than better insulation) to keeping the 1.5" wide strips as they will not line up with a truss(it is a gable wall). And I was readiing the LP smart side installation instructions and they actually indicate to use 2x4 strapping. And because the peak of my wall is 16', maybe it is just simpler to use what mike was proposing (2x4 on flat). But I think I will use foam as the backing unless I can find 2" comfoartboard.

    One other thing, I decided to use a full piece of 2x4 on edge for below my window and used GRK screws for this piece only as I was concerned with supporting the window. I also noticed because this window has a built in brickmould and a nailing flange, the holes for the nailing flange missed the vertical and horizantal wood pieces. I ended up screwing in screws at an angle to catch the wood. This was a "returned window" I got cheap. I need to make sure that before I do all my other windows I get the window drawings first (perhaps another good reason to do the 2x4 on flat. I have some larger windows where I intend on making a window buck due to size and weight of the window

  23. jasonn1234 | | #23

    well I found a problem with the above. The problem is my intended horiz lab siding will have nothing to attache to beside the window. To avoid this problem on my front wall, and due to the fact that I will be putting 3.5" battons around the windows at the front (which also need something to nail to) I revamped the front wall to have 2x6 on flat for the vertical sides next to the windows. The rest I will do with either 2x4 on flat or use the rest of the 1.5x2" I have left. I was also given some free 2" foam, so I tried than as backing for the 2x6. Much easier to work with. 2" roxul needs to be special ordered here, so it the foam is same or cheaper and readily available I will use that. But so far the regular comfortbatt in between seems to work just fine. I also ran a 2x6 at the top of the gable(with foam backing) to ensure I had something to nail the soffit and siding to. I did not put in a top header at the top of the window on the outside of the wall like I did with the sill as I'm not sure what the point of it would be. Pictures below

  24. KeithH | | #24


    How did your experiment turn out? I think you need a full article here on your experiment.

  25. jasonn1234 | | #25

    yes it worked out

  26. jasonn1234 | | #26

    I was trying to decide on what type of horizontal siding, LP or hardieboard. I bought some spare hardieboard someone was selling and did a test wall. I was concerned about the weight, but it turned out fine. See the pics. In hindsight there are a couple items I would change. 1. I bought two 2ft by 2ft windows for the top of my gable. Because the thickness of the wall is 10", and the window glass is on the outside, it created a 9" inside frame. That in itself is not the problem, but when you look at the window from the inside, at an angle, you lose seeing the full 2ft by 2ft' window because of the wide frame. I did originally get prices on a recessed window (which would minimize the effect) but the price premium was too high. In hindsight I should have specd out 2'6 by2'6 windows at a minimum. Also, in the end I ended up boxing out the entire window (up top I only say the sides and bottom) to aid in the window attachment. There is a bit of a flaw in this method, in that if water gets behind the roxul, there is nowhere really for it to go, except where it hits the top of a window or a roof line, and there is no way really to get it "back to the front" so to speak (as I spray foamed around the window from the inside). so hopefully the WRB I put on top of the outside roxul keeps the water out.I'm not overly concerned as we do not get a lot of rain here, and really because it is horizontal lap sidiing, I dont expect any to get behind it, excpt perhaps at the windows(over time)

  27. jasonn1234 | | #27


  28. jasonn1234 | | #28

    attached again

  29. jasonn1234 | | #29

    during this whole experiment I changed a couple things along the way.

    Once I went to styrofoam backed 2x4 (or ripped 2x6) on flat to hold the roxul, life was easier as the styrofoam is more rigid.

    The window above has no battens, but for the front an rear of my house I planned for 3.5" battens, which meant I had to put 2x6 on flat on the left and right of windows and doors so I had something to nail the batten to, as well as something to affix the end of the hardieboard to

    The weakpoint as noted in the prior post is the top of the window. The picture above show the tyvek on the shaethigng wrapping down and into the house. I realize if water came down the tyvel it would then sit on the window and/or the spray foam surronding the window. So in later versions of the window, I put a 2x4 on flat backed by tyrofoam at the top of the window to hold the batten. But I also change the tyvek on the sheating to be over top of the 2x4 on flat so that if water ever came down that way it would at least either sit on the tyvek (as it is 90deg there, or shed to the outside of the window. I came to the conclusion this problem is why on the roxul site, the installation of the roxul IS is always to the outside of the window. There is a video on their site where then it shows a guy painfully flashing out the widow to meet the siding.

    One could argue the tyvek against the sheathing is not even needed as I tyveked the outside of the roxul. so perhaps it is a moot point. And I suppose with the Roxul comforat board that is not an option because there is nothing to staple the tyvek to after you apply the comfoardboard IS, where I had the 2x4 on flat

    1. kirk_ellis | | #30

      Jason, I am very intrigued by your use of ComfortBatt rather than ComfortBoard on the exterior of your structural OSB and WRB. Technically, because you have a second layer of WRB outside the ComfortBatt, does this even qualify as "exterior insulation" or is it more of a quasi-double-wall where the outer wall is non-structural but just supports the siding and insulation ? Did an inspector ever look at this arrangement and give it his blessing ? I am planning a whole new house build and going through the same quasi-double-wall w/ batts (or even cellulose) vs. ComfortBoard plus furring strips debate with myself.

      I am surprised you had any wiggle in your ripped 2x2s that were attached with 6" structural screws. Aren't those screws claiming to be substitutes for 3/8" lag screws ? I'd think those screws would keep things rock solid even if there was nothing between the 2x2 and the wall.

      I am considering 2x4 blocks only at the attachment points between 2x4 furring strips and the studs behind the OSB rather than continuous strips behind ripped furring strips. While it would not create a continuous thermal break, the screws prevent anything from being "continuous" anyway. I am thinking it would make for a solid connection while leaving only a 2"x2" patch every 4sf that was wood bridging the otherwise continuous insulation. What would you think of that, given your experience of squishy insulation between furring and wall ?

  30. mjbroz | | #31

    I thought I would post what I finally ended up doing with my wall insulation.

    Basically, I used a combination of Roxul board on the exterior and blown in fiberglass in the wall cavity. See attached image of a "view from the top".
    Specifically, from inside to outside:
    - 1/2" drywall
    - Certainteed MemBrain "smart" vapor barrier
    - standard 2x6 framing on 16" centers with blown in fiberglass (packed pretty dense)
    - 1/2" plywood
    - Blueskin house wrap (the sticky stuff)
    - 2" of Roxul "board" material (more details below), using 1" blueboard and 2x4s on a flat as a nailer
    - 1/2 rainscreen air gap
    - cement board clapboard siding

    This gives me somewhere close to an R30 wall (depending on what R-factor you give blown-in fiberglass...range seems to be about 3.7 to 4.3, so I assumed 3.9).

    - I could have used 1x4s with 1.75" of blueboard instead of 2x4s with 1" of blueboard. The 2x gives a better purchase for the siding and helpful for attaching trim around the window, but probably not necessary.
    - The use of Blueboard behind the nailers worked really well. Since it is not compressible, we could use some heavy duty 4" deck screws and screw them down tight. I did use some 4.5" fancy screws (the kind with the ridges at the top so you can back out the screw to pull the board back into alignment) a few times where the framing or sheeting was funky.
    - There were a few occasions where the nailers warped/cupped a bit and it took a little power-planing hackery to get things coplaner again for siding. If I were to do this again, I would make sure to put the nailers down so if they cup they cup toward the building.
    - The Roxul was awesome to install. I cut it with a bandsaw to the exact width needed between each nailer plus about 1/8th to 1/4 inch (using an old fine tooth blade that I didn't care about). Very little dust created and nice straight cuts. This let me wedge the roxul in tight between nailers so I didn't have to use screws to hold it in place and a very tight install to keep thermal leaks to a minimum.
    - Note that I bought all the Roxul on Craigslist. It took me a couple years to find it (this was back in 2015/2017) but it came from different folks that had excess from commercial applications. Therefore I ended up with 3 different densities (I think equivalent with the Roxul 60, 80, and 110 products). The densest stuff was so hard you could walk on it with boots and not leave impressions, where the softest stuff was very compressible. The majority was the middle density.
    - The BlueSkin is awesome stuff and I highly recommend it. That said, be careful with the edges of the rolls...if they get scraped up, etc. the backing likes to tear such that it is a pain to remove the backing. We hung it vertically (after talking to the manufacturer) which I think was much easier to keep straight and avoid it sticking to itself (especially for tall walls).
    - The MemBrain held up well when used for the blown in fiberglass. It is strong and doesn't break or stretch much under pressure.
    - The blown fiberglass worked really well. It filled the spaces nicely and was packed in there pretty tight. They might have put up a mesh and then the MemBrain over top of it...I can't remember. See attached image.
    Finally, my goal was to build a "pretty good" house insulation-wise and I think we managed to do that. While I do believe a big part of our success was adding the full basement (different insulation approach obviously) that helps mitigate extreme outside temperatures, overall the home stays very comfortable without spending too much on HVAC. The best example I can give is we were evacuated due to wildfires this past summer for 3 weeks. During this time I turned off the HVAC system and temps outside went from low 70's at night to upper 90s during the day. The house was 76 degrees inside when we moved back in. I was quite surprised.

    1. Jon_R | | #32

      Mineral wool inset between thermally broken furring - a nice wall design that allows lots of drying in both directions and has warmer sheathing.

      Even low density batts would have worked?

      I'm curious why you didn't use 24" centers.

      Would there be any advantage to strips of Zip-R-12 or nail-base as furring?

    2. Expert Member
      MALCOLM TAYLOR | | #34


      Thanks for the update and insights. It's great to hear back once a project is complete.

  31. mjbroz | | #33

    @Jon - From what I can tell, low density batts probably would have worked, but since I got the material of of CL for a good price, I didn't go that route. Being able to pressure fit the boards between nailers and not have to worry about fasteners for the Roxul was an added bonus...although I got some choice swear words from my helpers during install when I cut them a bit too times we would have to lay a 2x4 on the Roxul and pound it in with a framing hammer :-). The walls were engineered with studs on 16" centers and I wanted to screw the nailers into the studs to support the weight of the cementboard siding. I've never worked with the Zip panels, but it seems to me the cut edges of the OSB would be susceptible to water that gets behind the siding. As we all know, OSB really likes to swell and lose structural integrity when it gets wet.

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