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.50″ exterior rigid foam, and new construction windows

Larkin86 | Posted in General Questions on

Hello, 

Edit: I have updated my Question. I have decided on foam and purchased .50″ rigid foam by Foamular. I got a price I couldn’t pass up on. I have also heard from the Window company and found out the windows have 1.5″ of depth to the trim. So, my question now is. Can I just install the house wrap, then install and flash the windows to the house wrap. After that, I can put the rigid foam on and then the fiber cement siding. The Foam + Siding will be less than 1.5″ in thickness so it should be about flush with the windows? 

I really appreciate this site and community. I have been reading it like crazy over the past couple months. A lot of the things discussed in this site are not commonly used or talked about in my building area. We live in Zone 4 in rural KY. 

We are remodeling a home build in the late 80s to move into. We are removing all the vinyl siding, and the windows completely, and starting over. There is no WRB or insulation on the outside of the home. I plan on adding a tyvek WRB to the home first — Then installing .50″ Rigid Foam Insulation with a R3 value. 

Then on top of that will be fiber cement siding. 

Most people in my area don’t tyvek wrap houses or worry about any foam insulation. But I think it is important. 

I am concerned about installing the windows though. I have read extensively about “innie” and “outie” windows, and how there are several methods to tackle. Most the things I read only speak about larger foam insulation though. 

 

I haven’t read anything specifically for .50″ sizes. 

My question is… is that because at those sizes you do not need to do anything special? Is it an easier install? Or do we still need to “buck” around the windows first? My preference is for an “OUTIE” window. 

We made a last minute siding change, and I am nervous it will send a ripple through our cost. I ordered all my new construction windows already, and they have the jam extension already fitted to them for standard 2×4 walls. Im afraid if I put the windows “on top” of the rigid foam, those window jams will be useless. Maybe even add labor cost to remove them and put new ones on? 

Should I use “innie” windows to be on the safe side with that? 

Sorry to be so long winded, this is my first post, and this is the issue that is most driving me crazy! 

 

Thank you, 

Andrew L. 

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Replies

  1. Expert Member
    Armando Cobo | | #1

    If you are using outsulation to avoid thermal bridging and possible condensation in CZ4, you need 1” R5 min. taped rigid foam. I prefer to used 1x4 window bucks and outie window installation since I use 1x4 rain screens, which is the best practice to install fiber cement siding. On the window jam, no need to worry, usually folks return the open frame of the jam with drywall or wood trim.

    1. Larkin86 | | #2

      Respectfully,

      There is a calculator on this site that shows the R-Value foam you need for each climate zone. Z4 says 2.5R value, and Z5 needs R5.

      This that incorrect or am I misunderstanding what it means ?

      1. Expert Member
        Armando Cobo | | #6

        See 2015 IRC Table N1102.1.2 (R402.1.2) - CZ4 except Marine - 2x4 Wood frame wall R-value is R20 or R13+5

      2. Expert Member
        Dana Dorsett | | #15

        Armando has it right- the IRC's thermal performance has to be the equivalent of 2x4/R13 + R5 c.i.. Even with R15 fiber insulation and R3 c.i. on the exterior it's not quite getting there.

        In zone 4A Kentucky there is no requirement for exterior foam for dew point control using Class III vapor retarders. The R2.5 prescriptive only applies to Marine zone 4 (=4C), per TABLE R702.7.1 CLASS III VAPOR RETARDERS.

        So, you don't really need R5 for dew point control, but it IS necessary for getting to a code-minimum R value.

        Also, half inch Foamular is labeled R3 due to rounding up, and is only warranteed to R2.7 (90% of labeled R) It will eventually drop to R2.1 when fully depleted of it's HFC blowing agents.

        Most 3/4" foil faced polyisocyanurate would be labeled R5 and should deliver that in a zone 4A climate. It's manufactured using comparatively low impact hydrocarbons, and is fairly easy to reliably air seal using aluminum HVAC tape.

  2. GBA Editor
    Brian Pontolilo | | #3

    Hey Andrew,

    You have probably seen these two articles, but I wanted to make sure, because they will be helpful if you haven't:

    https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing
    https://www.greenbuildingadvisor.com/article/innie-windows-or-outie-windows

    In your area, you can install any thickness of exterior rigid foam without causing problems, but that doesn't mean you should. I wonder if instead of installing 1/4 inch foam you should upgrade your WRB choice to something that will do a better job of air sealing the walls, like Henry Blueskin or another self-adhering WRB. Combine that with some other air sealing work around the house and you may improve performance more than you will with the R-1 insulation.

    It sounds like you'll need to either modify the interior extension jambs, if you go with an outie installation, or add exterior extension jambs if you go with an innie window install. If your not furring out the siding, I'd install the windows to the sheathing and do a little extra carpentry work when installing the exterior window trim. However, like Armando mentioned, it would be a good idea to install your siding with a rainscreen gap, which would make an outie install the easier option.

    1. Larkin86 | | #8

      I have some new information that makes things a little easier. The windows I ordered have an inch and a half protrusion from the nailing flange to the edge of the casing. (It’s 1.5” thick if that’s clearer said)

      With the half inch of foam(r3), the Tyvek wrap, and the fiber cement board all added up together I am still under a inch and a half in thickness.

      So shouldn’t I be able to install the windows against the house seal them to the Tyvek wrap and then put the foam up against the window frame and then the siding and I should be good to go? Without needing to buck them out any?

  3. Deleted | | #4

    Deleted

  4. Jon R | | #5

    You don't strictly need any foam at all. And even with it, you will probably still get some condensation and you will certainly still get some thermal bridging. But exterior foam is a good idea and thicker is better.

    If you can't install a gap, consider using StuccoWrap for better drainage.

    1. Larkin86 | | #7

      This confused me. It will cause problems and I don’t need it but it is a good idea?

      1. Jon R | | #9

        In terms of condensation and thermal bridging, it will reduce the problems (not cause them but also not completely eliminate them).

        1. Larkin86 | | #10

          Thank you. I understand now.

          It almost gets to the point I get so confused I wonder if I should even install the foam at all. Or do like most do around here and housewrap it then put the cladding on.

          My logic for wanting to use the R3 foam, is because I know with it being an older home my in the walls insulation isn’t great. I was hoping the foam would help make my walls closer to what a new home would bit like without the exterior foam

          1. Expert Member
            Armando Cobo | | #11

            Perhaps you need to ask your building official, and for the record, there is no personal logic in code enforcement. Most municipalities require you to update to the latest code if your remodel and/or addition is over 50%. If so, the Kentucky Building Code, which is essentially the 2015 International Residential Code published by the International Code Council, Inc., with the specific Kentucky amendments, requires you to install R13+5 minimum on 2x4 walls. See comment #6.

  5. Larkin86 | | #12

    Armando,

    I really respect what you are saying. But in the county I live in in Kentucky there is little to no code. I've talked to the city. We are even allowed to do electric and plumbing here without a license. It's sort of crazy.

    They have no requirements for foam sheathing or tyvek wrap.

    Every contractor I've talked to that has made bids on my house have been licensed contractors with a good reputation. All have told me they don't see foam being put on hardly any homes in the area except for by people like me that tend to care about that stuff!

    I also drive an electric car which is crazy for the area.

    I can't do 1" foam. I will either have to do none, or the 1/2".

    If the 1/2" is worse for me than nothing, I would rather know that an use nothing.

    1. John Clark | | #13

      If cost is an issue have you considered reclaimed foam?

    2. Expert Member
      Malcolm Taylor | | #14

      Larkin,

      If the only issue is the depth of y0our windows, why not install 1" foam, and picture-frame your windows with 2"x4" or 2"x6" trim? They will look a lot better than having the Hardi-siding butted straight up to the window frame.

      1. Expert Member
        Dana Dorsett | | #16

        #1 on the 2x6 window framing!

        Don't use 1" XPS, use 1" or 1.5" foil faced polyiso. With R9/1.5" polyiso and the foil facing a small air gap it roughly doubles the performance of a 2x4/R13 batt wall.

        Or for the cheap stuff go with 1.5" reclaimed roofing polyiso (R8.5-ish) or 2" (R11-ish).

        Try running this search on your more local sites to find foam reclaimers:

        https://lexington.craigslist.org/search/sss?query=rigid+insulation

        https://louisville.craigslist.org/search/sss?query=rigid+insulation

        With 2" of exterior polyiso and an 2x4/R13-R15 type wall it will come in at about R20 whole wall, which is still in the "financially rational" range per Table 2, p10 of BA-1005

        https://www.buildingscience.com/sites/default/files/migrate/pdf/BA-1005_High%20R-Value_Walls_Case_Study.pdf

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