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Roxul Comfortboard vs. rigid foam vs. Tyvek Drainwrap vs. Thermalbuck

Scott Wilson | Posted in General Questions on

Oh, the humanity! For months I’d settled on using Roxul Comfortboard for the exterior insulation over Stoguard Gold Coat but then I read an article by Matt Risinger about the backyard tests he did on various brands of liquid applied membranes and Stoguard failed miserably.

Then I read another article on the thickness needed for exterior insulation in climate zone 6A and it is 2 1/2″, not the 1 1/2″ I’d planned for. I’m planning on using Thermalbucks for the door and window rough openings and there is some question on how to seal the liquid membrane to the edge of the Thermalbuck (the staff at Thermalbuck have been very helpful in this regard) but my main concern is the thickness needed for the mineral wool boards.

After reading a Joe Lstiburek article on drainage membranes I’m now thinking of using Tyveks Drainwrap over the sheathing, then 2 1/2″ of rigid board, a furring strip then the cladding.

However, with the flanged window installed onto a Thermalbuck that extends 2 1/2″ out from the sheathing does that count as an “outie” window? Should the Drainwrap be omitted between the sheathing and the rigid board and a different kind of membrane be installed OVER the rigid insulation (and then attached to the Thermalbuck)? I do like the advantage of not having to worry about mineral wool  insulation boards compressing when I attach the furring strips and cladding through it but which rigid insulation board to use?

Oh, why do I read so many articles?

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  1. GBA Editor
    Martin Holladay | | #1

    Provide us a drawing so we can figure out where you intend to install the windows. Where is the plane of the nailing fins with respect to all of the other layers you are contemplating?

    That information will allow us to determine whether these are innie windows, outie windows, or in-betweenies.

    Once we know that, we can provide advice on where your WRB belongs.

    For more information on these issues, see these two articles:

    "Where Does the Housewrap Go?"

    "Installing Windows In a Foam-Sheathed Wall"

  2. Scott Wilson | | #2

    I don't have a drawing at the moment. Basically the Thermalbuck will be installed around all the door and window openings. It will take the place of a wooden "picture frame" window buck (basically a 2x3 on edge). So the Thermalbuck will stick out 2 1/2" from the sheathing and be about 2" wide. The exterior insulation will butt up against the Thermalbuck.

    The flanged windows will be attached to the front surface of the Thermalbuck. If I use rigid insulation should I use a product like Drainwrap behind it or not? Should I use a WRB on top of the rigid insulation and extend it over the front of the Thermalbuck before I install the window?

    I was reading in a Joe Lstiburek article about a Net Zero house he'd helped with where he advised wrapping the entire house in a self-adhered WRB. This would suggest that the Drainwrap product wouldn't be sufficient since it is not self adhered.

    If the self adhered WRB is a better solution, which brand to use, along with what type of rigid insulation?

    1. GBA Editor
      Martin Holladay | | #3

      It sounds like you are installing outie windows.

      The method that Joe Lstiburek is talking about -- the method that involves installing peel-and-stick over the entire exterior sheathing -- is called PERSIST. You can read more about PERSIST here: "Getting Insulation Out of Your Walls and Ceilings."

      I doubt if you are building a PERSIST house, but you can if you want. Note two points: (1) You had better be absolutely sure that your exterior rigid foam is thick enough if you go this route, because you can get wet sheathing if your rigid foam is too thin, and (2) Peel-and-stick is expensive. There are cheaper ways to obtain an air barrier.

  3. Jon R | | #4

    If you stick with external Roxul Comfortboard, you can use any thickness you want.

  4. Scott Wilson | | #5

    So basically the question is, do I stick with 2 1/2" of Roxul Comfortboard over a liquid applied WRB from Tremco (which one though?), Comfortboard over Drainwrap, rigid over liquid applied WRB, rigid over Drainwrap, or WRB over rigid?

    I do like the Roxul Comfortboard 80 for its fire resistance, but I've read that the Comfortboard 110 is stiffer.

    1. GBA Editor
      Martin Holladay | | #6

      Any of the approaches you mention can work if well detailed, so your choice comes down to your preferences and your budget. Note that a liquid-applied WRB can double as an air barrier, whereas Drainwrap isn't a particularly good air barrier.

  5. GBA Editor
    Peter Yost | | #7

    Hi Scott -

    The current Thermal Buck installation instructions for flanged windows does not include an independent sill pan flashing with backdam and/or sloped sill.

    I think this is an important consideration when installing outie windows.

    I hope to address this issue in an upcoming GBA building science blog, including ThermalBuck's perspective.


    1. qofmiwok | | #8

      Did you ever do a blog post on this ThermalBuck issue?

      1. Expert Member

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