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EPS Foam on over exterior OSB Sheathing

Allamon13 | Posted in Energy Efficiency and Durability on

Hello,

I am new here and I apologize if this has been asked. I am currently an Owner Builder (subbing out as I go for items such as framing, etc) and am thinking for the future of energy efficiency. My question is the following and I am not necessary looking for someone to give me the answer but links and other material that would help would be great! I am an electrical engineer so I convince myself at least that I can understand technical explanations but take it easy on me!

I am located near Pittsburgh, PA which is zone 5 on the climate chart. I currently am in the beginning process and framing has begun. My exterior walls are 2×6 with 7/16 osb sheathing followed by Tyvek House Wrap. https://www.lowes.com/pd/Tyvek-9-ft-x-150-ft-House-Wrap/3227658. The house will be sided with vertical vinyl. 

I was debating whether to use closed cell spray foam on the interior but the cost is astronomical and the more and more I read was that if I add foam sheathing over my osb and house wrap I can create a thermal break which would effectively increase my homes energy in a multitude of ways for much less than spray foam just on the interior. 

Is there a guideline available for this method? Do I need to use a different house wrap? I know I will need to shim out my windows and doors with 2x pressure treated material which is not an issue. I am concerned if whether or not 1.5″ of https://www.homedepot.com/p/R-Tech-1-1-2-in-x-4-ft-x-8-ft-R-5-78-Rigid-Foam-Insulation-320817/202532855#overlay is a large enough thermal break or could I incur moisture issues? The reason the 1.5 is tempting is that there is not a lot of cutting involved for the window and door trims. 

Any guidance would be great or even links to others with this scenario. Most of the other links I found were from the mid 2000s and nothing was very recent. 

Thank you,

Greg

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Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Greg,

    The two main considerations are:

    - What your code requires. Building codes typically lay out the minimum thickness of exterior foam for each climate and wall type to keep them safe. https://www.greenbuildingadvisor.com/article/calculating-the-minimum-thickness-of-rigid-foam-sheathing

    - Going beyond a certain thickness means you can't just fasten cladding directly through the foam, and requires furring strips.

    1. Allamon13 | | #2

      Thank you for the link! I am not entirely sure thought what differences between a 2x4 wall vs a 2x6 wall would have to do with the exterior foam based on the chart provided:

      Zone 5 R-5 for 2×4 walls; R-7.5 for 2×6 walls

      1. Expert Member
        MALCOLM TAYLOR | | #3

        Greg,

        The proportion of cavity insulation to the amount of exterior foam is important. More interior insulation means the inside face of the foam and the sheathing will be colder and at risk of having moisture condense on it. To counteract that you need thicker foam so that the inside face stays warmer.

        1. Allamon13 | | #5

          Thank you for the reply! It makes sense now.

      2. Expert Member
        BILL WICHERS | | #4

        You need a certain minimum ratio of exterior (rigid foam) to interior (batts) R values to avoid moisture issues with the sheathing. The issue is that any kind of rigid foam reduces the drying ability of the sheathing, so you need that minimum R value ratio to keep the average temperature of the sheathing high enough to limit the moisture risk. You can put as much more rigid foam on the exterior as you want though, with more being better, and safer.

        The reason for the prescribed amounts for 2x4 and 2x6 wall assemblies is that with regular batts, R15 is as much as you can fit in a 2x4 wall, so the code gives you about 25% of the total R value of the assembly on the outside. For a 2x6 wall, the most you can fit with batts is about R23, and the code puts you at just about 25% of your R value on the exterior again with that R7.5 requirement for exterior insulation. It's the ratio that matters.

        R7.5 would need a bit more than 1.75" EPS. I'd use 2" (about R8.4) as a minimum here. You could use GPS instead, then you'd be OK with 1.5" since GPS is about R7.5 for 1.5" thickness.

        You'll get a much better overall performance improvement with any kind of rigid foam over what you'd see using spray foam in the studwall. The reason for this is that the rigid foam is CONTINUOUS insulation, without thermal bridges from the studs. The thermal bridging from the studs really reduces the overall R value of a wall assembly, especially when going with very high R per inch materials like spray foam between the studs.

        BTW, I'm an EE and I figured this stuff out, so I have confidence you can do it too :-)

        Bill

        1. Allamon13 | | #6

          Thank you Bill! I will look into this GPS foam but will probably end up with the r-tech 2” for the ability to return the unused portions. I did however inquire about the GPS (I work for a foam machinery manufacture and we actually sell machines to Carlisle Construction Materials). I have a few contacts I may inquire and see if they will even sell direct since I am not a contractor.

          I guess the next step is to rip some boards.

          Is there any information regarding the house wrap before it’s all in tyvek?

  2. artisanfarms | | #7

    I am just a little north of you near Lake Ontario in NY and went a little different route on my renovation project. I am happy with the results so far. My framing was 2x6 with 7/16 OSB like you have. On the exterior, I taped all of my OSB seams and followed with 2 1/2" of recycled polyiso which I also taped. This was topped with furring strips to create a rain screen which my siding was installed on. I used mineralwool insulation in the stud cavities and finished the interior with drywall. The house is tight and seems to be very efficient.

    1. Allamon13 | | #8

      Thank you for the feedback! Did you do new windows and doors for the new depth of the exterior? I have a concern over adding extensions for the doors (opening them I am not sure if the two extra inches will hinder this or not). My windows are planned to be drywall returns but lucky for me my wife picked a house with just over 30 windows….

      1. artisanfarms | | #12

        Almost all of the windows on the house were Facebook Marketplace finds, either NOS, or relatively new takeoffs when someone remodeled. The house has more than 20 windows, so I was very happy to find suitable windows on Marketplace. I am making my own jamb extensions on the inside for the windows.

        I purchased new doors with extended exterior jambs so as not to interfere with how the door opens. The extended exterior jambs should end up working well with my screen/stormdoors and don't really look very different than a regular door jamb from a distance.

  3. maine_tyler | | #9

    As much as possible, it is good to work out as many insulation (and other) details before starting work on the ground. It will make everything easier along the way.

    Definitely consider air tightness of the building. Consider continuity of both said air barrier and the insulation. This is when planning can be most helpful-- to map out the route at important areas like doors, windows, wall to roof and wall to floor transitions, etc.

    Are you taping sheathing joints for air tightness?
    Here's another article: https://www.greenbuildingadvisor.com/article/combining-exterior-rigid-foam-with-fluffy-insulation

    1. Allamon13 | | #10

      Thank you for the reply. When you say taping does an overlap of tyvek house wrap that will be taped at the seams suffice? Or apply a similar tape before the house wrap? I will click the link and read the article!

      1. maine_tyler | | #11

        I think the general consensus is that Tyvek—even when taped—makes a poor air barrier. Or doesn't as readily lend itself to being installed as a high quality air barrier, I should say. I have taped a high-end membrane from 475 as a secondary air barrier and felt good about it. Taping the sheathing directly with the right tape will probably be easier to get better results.

        What's most important is that the air barrier, whatever it is, be continuous and not 'jump' from one plane to another with no connection. This includes transitions at corners, to roofs, etc.

        I think some people tape the foam itself for an air barrier; I have no experience with that. You need the right tape to stick and it depends on the foam. Foam can shrink over time too and just doesn't seem as robust to me personally.

  4. Allamon13 | | #13

    Just a quick bump here... would unfaced EPS provide the same as the faced foam boards with the metallic facing? For example:

    https://www.menards.com/main/building-materials/insulation/foam-board-insulation/expanded-polystyrene-foam-board-insulation-4-x-8/1632118/p-1444435971902-c-5779.htm

    1. Expert Member
      BILL WICHERS | | #14

      I'm guessing you mean "would unfaced EPS provide the same R value as faced EPS", in which case the answer is "yes". The facer is there mainly to provide some structural integrity to the EPS panel, to make it easier to install and handle with less risk of breaking. Sometimes the facing is there to act as a vapor barrier too (which can be a problem in some wall assemblies).

      A foil facer also gets you a radiant barrier, but only if it's installed facing an air gap. If you have a rain screen going over the foam, face the foil facer towards that rain gap to gain the "free" radiant barrier provided by the foil. The radiant barrier only gains you about R1 or so though, so it's not a big deal if you can't make it work in your assembly.

      Bill

      1. Allamon13 | | #18

        Bill,

        I should clarify. If I used a "faced" foam board will my interior insulation be unfaced and vice versa. If its unfaced foam will I need a faced insulation on the interior wall. Reading about how messy the unfaced is I will probably stick to the R-tech series at 2" but I need to prepare my interior solution properly!

  5. fromPok | | #15

    This is an interesting. I am going through something similar with my house build and debating choices. I have read quite a bit about wall assembly and trying to optimize dollar cost for best insulation for exterior walls. I saw Jake Burton recommendation and Micheal Chandler sill / wall assembly.
    We are thinking intello wrap, cdx, 2×6 wall with thermal fiber, 1/2″ sheet rock.
    Question for the experts.
    1. I considered zip-R vs cdx ply and the cost adder is significant. $100 for zip-R vs 25$ for CDX. Is it cost effective to go to 2×8 and use closed cell foam insulation + thermal fiber to get better air sealing or does intello give good enough air sealing? If it does then I could use R30 rock wool for insulation.
    2. Would the above assembly require an interior vapor barrier?
    3. For the exterior of the house, we are thinking of using Hardie backer? Does the wall assembly need any kind of rain screen or can the Hardie go on top of intello?
    Thank you experts

  6. Expert Member
    BILL WICHERS | | #16

    1- Comparing CDX (plywood) to Zip-R (OSB with integrated exterior rigid foam) is not an apples to apples comparison. You'd need to add at least an equivalent R value worth of exterior rigid foam to your "CDX" number to get to a number you could use for cost comparison purposes here. Keep in mind that "Zip-R" is a particular model line, basically, with various R value options within the line, so you need to know how much R value you're getting with the Zip-R product you are using for comparison purposes.

    I wouldn't waste money on spray foam in a wall assembly on a new build. You could air seal using the CDX layer alone if you were careful with your detailing (tape the sames and put a bead of sealant around the framing perimeter prior to hanging the plywood panel). I would stay with the 2x6 wall, use mineral wool batts, and then add at least the code minimum amount of exterior rigid foam. If you want to have a better than code performing wall, additional exterior rigid foam is the best way to do that from a performance perspective in this case.

    2- It depends how much exterior rigid foam you use. You would generally want a vapor RETARDER, not a vapor BARRIER here. You don't necassarily NEED either, but a smart vapor retarted will almost always add some additional extra insurance against moisture problems, which makes the wall assembly more robust. As an example, in my own home I've been adding way beyond code minimum levels of exterior rigid foam, but I still put an interior side smart vapor retarder even though it's not really needed. The vapor retarder isn't terribly expensive, and it adds extra insurance against moisture issues, so it's a plus.

    3- Hardie backer is usually used behind tile. Are you thinking of one of their fiber cement products made for siding applications? A rain screen will really help make for a more robust exterior, and is highly recommended. If you go with thick exterir rigid foam, you pretty much have to put a rain screen, since Hardie wants you to hang the siding from furring strips anyway, which essentially gives you a rain screen "for free". Hardie likes 1x4s, but many (including me) prefer to use strips of 3/4" plywood instead (same thickness, different material).

    Bill

  7. Expert Member
    KOHTA UENO | | #17

    "...if I add foam sheathing over my osb and house wrap I can create a thermal break which would effectively increase my homes energy in a multitude of ways for much less than spray foam just on the interior. Is there a guideline available for this method?"

    BSC has the following guidebooks on exterior insulation/continuous insulating sheathing over wood frame walls. BA-1406 is a DOE guidebook we wrote a while back. The DER Guide is focused specifically on retrofits of existing buildings.

    BA-1406: Final Measure Guideline: Incorporating Thick Layers of Exterior Rigid Insulation on Walls
    https://www.buildingscience.com/documents/bareports/ba-1406-final-measure-guideline-incorporating-thick-layers-exterior-rigid-insulation/view

    GM-1302: Mass Save Deep Energy Retrofit Builder Guide
    https://www.buildingscience.com/documents/guides-and-manuals/gm-mass-save-der-builder-guide/view

  8. fromPok | | #19

    Bill & Kohta - Thank you. My builder has lot of horror stories with external rigid insulation layers. Not sure I can convince him otherwise. If I can't, what would be my next best choice based on my above proposal?

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