Replies

1. GBA Editor
   Martin Holladay | Nov 25, 2014 05:37am | #1

   Colin,
   Here is a link to an article that you may want to read: Installing Roxul Mineral Wool on Exterior Walls.

   In that article, Shannon Cowan and Patrick Walse wrote, "Because Comfortboard is made from fuzzy mineral wool, it’s difficult to attach tape or peel-and-stick to its surface. For this reason, our builder and architect advised us to install “picture-frame” strips of 2-inch-thick XPS foam around each window — just enough to nail the window flanges to, and to provide a surface for sealing the rough openings with peel and stick membrane."

   The top photo below shows part of one of their trimmed windows.

   The lower photo before shows a different approach -- one taken by Mark Yanowitz. (Mark's blog is here: Wrapping an Older House with Rock Wool Insulation.)

   .

   
2. 
   slader99 | Nov 25, 2014 11:08pm | #2

   Thanks for the thoughtful response Martin, I had read the both articles you mentioned. Furring the window out with XPS just seems to shift the problem area in my head (how are you dealing with the mineral wool to XPS joint now?). I guess you'd rely on either caulk or peel and stick to seal the XPS to your window and then the XPS to your moisture barrier.

   After re-reading the second article and in particular Mark's comments I'm beginning to lean towards this approach (tying moisture plane to windows at sheathing wrap) makes the most sense. I guess this makes the window an 'innie' and while the trim and flashing elements proud of this surface shed the bulk of wind driven rain you acknowledge that some water will make it through before being stopped at this plane. With thoughtful application of sloped sill, flashing at head and sill and tight trim joints this should be minimal and with a rainsscreen and rockwools ability to dry probably not a concern. These same areas are going to exist at uncaulked inside and outside corners....

   What are your thoughts on horizontal strapping for vertical siding (like board and batten). I see in Mark's approach he fastened the horizontal strapping directly to the rock wool. is there any reason or benefit to vertical then horizontal strapping application?

   Can anyone else share their experience with vertical siding over MW?

   Cheers,

   Colin

3. 
   Kopper37 | Nov 28, 2014 11:16am | #3

   Hi Colin,

   I haven't been able to write an article about how we install exterior mineral wool, but I'll try to give you some information here.

   The basics: we use sheathing as our air barrier, a single layer of 2" thick mineral wool, standard flanged windows, 5/4 trim boards, and fiber cement siding.

   Basically, we install a "ring stud" or window buck inside the framed rough window opening. We size the rough opening 4" larger than the window---3" for the ring stud, 1" for the installation gap. This ring stud is cut so that it has the same depth as the window frame. We push this ring stud out to a point that it will be flush with the furring strips (it protrudes from the wall sheathing layer 2 5/8"). We flash the ring stud, tie it together with the weather resistant barrier, and install the window.

   Next we install the mineral wool and furring strips. We flush the furring strips to the plane of the window flange using a speed square and level. At this point we install the trim, then the siding. While we're installing the trim we flash the head of the window / trim again --- per the window mfg. and trim mfg. instructions.

   So you flash twice: once at the drainage plane, again at the siding / trim layer.

   Finally, we insulate the ring stud from the inside with 2" EPS foam. This foam reduces the thermal bridging that would otherwise occur.

   We do it this way for a few reasons:

   1) Aesthetics
   2) Ease of flashing the window (believing that it's less sensitive to error than if the window is installed at the midpoint in the wall).
   3) Ease of trimming the exterior.
   4) The window can now be removed and replaced without disrupting the mineral wool, the furring strips, or the siding. You just have to remove the exterior window trim.

   I've attached a few pictures for your reference.

   I don't think the orientation of the furring strips matters at all. We don't ventilate the space between the furring strips. The mineral wool drains and dries very easily---that's one of the main benefits of this type of insulation. However, in order to trim the window you will probably need vertical furring around the window opening.

   
4. 
   jasonn1234 | Apr 17, 2016 12:52am | #4

   sooo, One would have to ask, why not just frame the window with 2x8 instead of 2x6(assuming this is a new build)?

   Just curious, as it seems all of these different types of window bucks are merely cantilevering a new frame over the originally framed window frame

5. 
   user-6970808 | Dec 20, 2017 07:11pm | #5

   Colin, what did you eventually do?
   I am looking at the same issue right now and not really trusting Daniels method. Sheathing is not really an air barrier so his advice is suspect straight out of the gate. I fail to see how he is sealing the window without a real air barrier outside the MW to flash to. The innie method looks best sealed but odd. Any other new ideas here since April 2016?

6. Expert Member
   MALCOLM TAYLOR | Dec 20, 2017 11:48pm | #6

   User-69 etc.

   Sheathing is often used as an air barrier. As long as the joints are sealed it can be a very effective one. Did you mean sheathing isn't a WRB?

7. GBA Editor
   Martin Holladay | Dec 21, 2017 05:31am | #7

   Malcolm,
   I'm guessing that User-6970808 either has board sheathing, or has installed a brand of OSB that he or she fears is air-leaky. (You may remember that I wrote an article about air leakage through OSB).

   But until User-6970808 clarifies his or her concerns, I'm just guessing.

8. 
   qthisup | Dec 21, 2017 11:28am | #8

   Hello
   Recently completed a similar project on the nearby Sunshine Coast so thought I'd share our experience in case of interest.

   I decided on Zip sheathing as the primary air barrier and WRB. Thus all flashing of the openings was against this surface, not the surface of the 2" Roxol we installed. You can follow the same approach with traditional house wrap I believe.

   I ripped 2x material down to 2" (in retrospect would have ripped down to say 1 7/8 to allow for the slight compression of Roxol). Then flashed the opening with Zip flex tape along the bottom, and regular Zip tape and GTape along the sides and top. Along the bottom, you can actually save some money by just using the Zip flex tape in the corners and use regular tape in the middle section of the bottom.

   Along the bottom, a sloped cill or back damn should be installed as well under the tape.

   Windows were installed with sealant behind the flanges (except bottom to allow drainage). Then taped again with the tape adhering not just to the flange, but returned slightly onto the frame to protect the junction between the nailing flange and frame from water intrusion as well. Do this again on top and side, the bottom can have a loose piece of house wrap taped to it but left open along the bottom (as per ProHome 2016 build video in Fine Home Building).

   Feel free to email if you need help, I spent days and days researching and got a lot of help from this community so I'm happy to in turn help others.

   
1. 
   DanShow | Apr 20, 2021 02:00pm | #14

   Quinn,
   Would like to pick your brain on some more specifics of your flashing and assembly as this appears to be basically what we are planning to do this summer on our new build. What email works best to get ahold of you?
   Thanks
   Dan

9. Expert Member
   MALCOLM TAYLOR | Dec 21, 2017 09:45pm | #9

   Martin,

   I was going by this: "not really trusting Daniels method. Sheathing is not really an air barrier so his advice is suspect straight out of the gate." But the misunderstanding is probably mine.

   Quinn,

   Nice job!

10. 
    user-6970808 | Dec 22, 2017 12:58am | #10

    Thanks everyone for chiming in,
    My mistake, I thought Daniel was saying sheathing was both air barrier and WRB. Re-reading his post I see he states later that "We flash the ring stud, tie it together with the weather resistant barrier, and install the window." Prior to that he had not mentioned a WRB so I could not see where the sealing against water was happening.
    I like Quinn's solution minus the ZIP. Seems like if WRB is outside the MW I can just flash to that.
    User69-etc is kind of a cool name but I'd prefer to use something else. In both IE and Chrome I do not see a field next to "Your Name' where I can enter anything. What am I missing?
    Thanks for the offer to help out Quinn, I may take you up on that. Seems like a great community of builder here, lots of good info sharing.
    Mike

11. 
    user-619460 | Jul 22, 2018 09:07pm | #11

    Thank you all for your insights on this forum. I'm trying to finalize my window details. What do you think of the attached?
    My concerns:
    1. How big a deal if the thermal bridge between the 2x3 through the 2x6 into the house. I live in the San Francisco area so it doesn't get that cold, but we do tend to heat much of the year due to the foggy wet summers.
    2. I'm forced to have a 1-hour rated wall due to fire code. That is why the extra layer or 1/2" type X gypsum.
    3. The orange highlight represents the air barrier. I'm leaning toward a fluid applied or maybe Tyvek and tape. I don't know that I have to come all the way in. If I lap the tape over the nail fin to the side of the 2x3 spacer it would probably be fine.

    I'm interested in Quinn's and Daniel's details. If you are able to draw them, that would be really helpful.

    Thanks,
    Jay

    
12. Expert Member
    MALCOLM TAYLOR | Jul 22, 2018 09:36pm | #12

    Jay,

    I'll leave other better able to comment on your questions.

    One small thing: Your detail shows a single stud at the window-jamb and a second one several inches away. Most walls are framed with a king and jack stud to take the loads from the header above. Some Advanced Framing avoids this, but having the two studs together allows the rain-screen furring to be right up against the window flange, which provides a nailing surface for both the siding and any trim.

13. GBA Editor
    Martin Holladay | Jul 23, 2018 06:02am | #13

    Jay,
    Considering the fact that your house is in San Francisco, which is relatively mild, I wouldn't worry at all about the thermal bridge through the 2x3 plus 2x6 framing. (For the record, 8 inches of framing lumber has an R-value of R-10 -- which isn't nothing.)