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Rigid mineral wool details around windows

Stevesaxenian | Posted in General Questions on


I am planning on installing 1.5 inches of rigid mineral wool over 5/8″ plywood on the exterior of a new building. I am looking for up to date advice and instructions on how to detail rigid mineral wool around windows, including integrating the WRB. I have looked at a lot of articles and discussion threads including Joseph’s Lstiburek article, Windows Can Be A Pain, but nothing seems comprehensive. For instance, in Lstiburek procedure, he doesn’t include using a WRB at all, and on some of the threads there is mention of putting the WRB over the rigid mineral wool, which seems to make sense to me so that it can be taped to the window flange, even though the conventional wisdom seems to be to put it under. So, I am looking for advice/wisdom and instructions on installing this product around windows. Thanks

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  1. Expert Member
    Akos | | #1

    WRB details with rigid mineral wool are exactly the same as a WRB install over sheathing. You can use the standard details from any of the WRB manufacturers.

    Since mineral wool is self draining, you can pretty much treat it as an extra wide air gap.

  2. Expert Member
    Michael Maines | | #2

    Steve, while not fully detailed, I wrote about a few approaches here: I almost always locate the WRB at the sheathing plane, whether using Zip or a separate WRB.

  3. GBA Editor
    Brian Pontolilo | | #3

    Hi Steve.

    There are two common ways to install windows in walls with continuous exterior insulation. You can install them in plane with the sheathing, which means that you can simply integrate flashings with the WRB. This is less complicated, in my opinion, when it comes to water management, though you will need extension jambs for the window trim. This is the method Akos and Mike are suggesting and I agree with them that this is a smart way to go, particularly when using mineral wool.

    The other method is to install the windows in plane with the face of the exterior insulation by using some variation of a window buck (like this: Dudley Boxes for Windows). This alternative is a good option when you are using the exterior insulation as the WRB; so not a great option when that insulation is mineral wool, unless you cover the insulation with housewrap.

    I'd trust Akos and Mike on this one.

  4. Stevesaxenian | | #4

    Thanks all for the help. A follow up question: Akos says that one can treat the mineral wool pretty much as an extra wide air gap. If so, do I need a drainage gap between the rigid mineral wool and siding, such as vertical pine siding, or can I nail the siding directly onto the mineral board?

  5. AJ__ | | #5

    You might find this useful

    You should check your code requirements regarding the rainscreen gap. In my location it is required, plus nailing siding to furring strips seems way easier than trying to hit studs straight through mineral wool and siding

  6. rockies63 | | #6

    There is also Thermalbuck for windows and doors.

    I'd like to see details on adding the metal top flashing after the window is installed, then how to build the trim and sill and then how to attach/flash it to the buck.

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