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Community and Q&A

GPS vs EPS vs XPS

Judy5 | Posted in Energy Efficiency and Durability on

I would appreciate any comments about Graphite Polystyrene (GPS) as an insulation product.  It appears to have an R-value of 5 (per inch) and a permeance of 4 for a one-inch product depth.  In addition, the graphite in the polystyrene product causes the R factor to trend higher (!) with colder temperatures (for example, 5.0 at 75 degrees F; 5.2 at 40F; 5.4 at 25F for Type I). 

Is there anything not to like about this product?  I am considering it for continuous insulation under vinyl siding.  I do not have room to have a thick layer of insulation (to overcome the dew point concerns), and need to have a product that allows for drying to the outside.

For background information, I am in climate zone 5 (New England), using 2×6 studs and planning to use R-23 rock wool insulation in the cavities.  The sheathing is 1/2″ plywood which will be taped for air sealing, then covered with a WRB (HydroGap?) before some insulation and vinyl siding.

Judy

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Replies

  1. Expert Member
    Dana Dorsett | | #1

    The vapor permeability of graphite loaded changes with density the same way other EPS. The graphite doesn't affect the permeability by very much. The specified permeance @ 1" is 4 US perms, only for Type-I (1.0lb per cubic foot nominal density) density EPS.

    Graphite loaded EPS is preferable to XPS, since unlike XPS doesn't use HFC blowing agents, and it's performance doesn't decline over time the way XPS does. XPS gets it's performacne advantage from those blowing agents, but they don't stay put over time- it slowly releases the HFCs and takes a performance hit in the meantime. Most R5/inch XPS is only warranteed to R4.5/inch at 20 years, and may not meet that performance level at age 50.

    In your stackup and climate zone it's legitimate from a dew point control design point of view to use the specified 40F mean temp temp through the foam, since the sheathing usually has to be 40F or colder to take on wintertime moisture (unless the house is being actively humidified, or every tight and under-ventilated.) But from a code compliance point of view the 75F mean temp used for labeling purposes is what counts.

    In zone 5 the IRC prescribes R7.5 minimum for dew point control on the exterior of 2x6 framing, at a presumptive R20 cavity insulation. You have 15% higher cavity-R, so you'd also need 15% more exterior R, or R8.6 minimum. That takes at least 1.75" of the the product you're looking at to have any margin, 2" (R10.4 @ 40F mean temp through the foam) would be even better. At 2" it would test at about 2 perms, which is still a decent drying path for the sheathing.

    But is that going to be thin enough for you?

    If you dropped back to R20 fiberglass or cellulose in the cavities you could go with 1.5" with the graphite loaded foam, which would run about R7.8 at outdoor temperatures that mattered, and ~2.7 perms.

  2. GBA Editor
    Martin Holladay | | #2

    Judy,
    Dana is referring to the issues described in this article: "Calculating the Minimum Thickness of Rigid Foam Sheathing."

    Q. "Is there anything not to like about this product?"

    A. Graphite-enhanced EPS is more expensive than other similar products. You might want to consider polyisocyanurate, which has a labeled R-value of about R-6.5 per inch, and which many designers use with a de-rated R-value of R-5 per inch to account for the cold-weather performance of polyiso.

    For more information, see "Choosing Rigid Foam."

  3. STEPHEN SHEEHY | | #3

    Judy: Whatever foam you use, use reclaimed foam, which is widely available in MA.i
    Martin- the new site is fine, but every time I open it, I need to log in, whereas on the old site, I was usually recognised without needing to enter my log in info.

  4. Judy5 | | #4

    Alas, I may not have 1.5 inches of exterior foam at R-7.5 with R-19 insulation in the wall cavities. I will talk to the builder. He has been very gracious about my raising these issues even after the building has started, and in trying to come up with solutions for us.

    Unfortunately, I may have only one inch available without modifications to all the windows. The drawings and specifications were done about two years ago, long before I had discovered GBA and started paying attention to construction details and preferences (and not just general layout and design issues). I have read many GBA articles about using rigid foam externally.

    I am now well aware that thicker foam sheathing is better than thin foam sheathing, and that Table R702.7.1 should guide one’s decisions. Unfortunately, my builder was working from plans that specified closed cell foam to be used on the interior of the sheathing, with fiberglass batts to finish filling the 2x6 space. (No external continuous insulation is required locally.) When we started building this year, I decided I want to minimize use of closed cell foam -- although some closed cell must be used in the basement and the unvented conditioned attic. I also wanted to ensure thermal bridging was addressed.

    At this stage, it is not likely I can use an R-10 external foam product in order to get to the required 27% to 30% exterior insulation for 2x6 walls with R-23 batts in CZ 5. And it may not be possible to have R-7.5 exterior foam with R-19 batts, although I will try.

    So, if there is only one inch for exterior wiggle room, do I add what foam insulation I can in order to deal with the thermal bridging issue, and rely on ensuring good exterior permeance so that the sheathing can dry out every March or April? Or revert to the builder’s plans with interior closed cell foam and deal with thermal bridging internally at the drywall or externally with insulated vinyl siding? Or something else?

    I do so appreciate the comments you provide to me and other people who post questions. You are generous with your help to each of us. Both the blogs and Q&A sections are my favorite reading at GBA. Thank you for sharing your experience and wisdom.

    1. Expert Member
      Dana Dorsett | | #7

      R19s when compressed to 5.5" in a 2x6 cavity perform at R18:

      https://www.greenbuildingadvisor.com/sites/default/files/Compressing%20fiberglass_2.JPG

      The IRC prescriptive R7.5 presumes R20 cavity fill for a ratio of R7.5/R27.5= 27.3%.

      With 1" of graphite loaded EPS on the exterior R5 nominal, R5.2 at temperatures that matter, you're looking at a ratio of R5.2/R23.2= 22.4%.

      That's not even close.

      Unlike most polyisocyanurate products, Dow Thermax does not derate with falling temperatures. At 1" it's labeled at R6.5 @ 75F mean temp through the foam, but at temperatures that matter it's closer to R6.7. See Figure 1 in this bit o' marketing fluff:

      http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_098a/0901b8038098a7e2.pdf?filepath=styrofoam/pdfs/noreg/179-00379.pdf&fromPage=GetDoc

      So with 1" of Thermax you'd be looking at a ratio of R6.7/R24.7= 27.1%

      That's VERY close the IRC prescriptive ratio and should be good enough, as long as you don't go higher-R in the cavities.

  5. GBA Editor
    Martin Holladay | | #5

    Judy,
    You are a rule-breaker. How worried should rule-breakers like you be? Here are links to two articles that attempt to answer that question:

    "The Exterior Rigid Foam is Too Thin!"

    "Rethinking the Rules on Minimum Foam Thickness"

  6. Expert Member
    Dana Dorsett | | #6

    If only one inch, use ONLY Dow's Thermax branded polyisocynaurate foam, which is rated ~R6.5/inch at 75F mean temp through the foam but (unlike most polyisocycanurate products) monotonically increases rather than loses performance as the temperatures drop. See Figure 1 in this bit o' marketing fluff, and note the R-value with mean temp curves for Thermax (the top curve) relative to generic 2lb roofing polyiso (the weirdly shaped curve):

    http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_098a/0901b8038098a7e2.pdf?filepath=styrofoam/pdfs/noreg/179-00379.pdf&fromPage=GetDoc

    When compressed to 5.5" in a 2x6 cavity an R19 batt's performance drops to R18:

    https://www.energyvanguard.com/sites/default/files/styles/panopoly_image_original/public/owens-corning-compressed-fiberglass-insulation-r-value-chart.png?itok=jq3gsGh8

    The ratio of exterior R to total R is what determines the average temperature at the sheathing, which is what determines how much moisture accumulation to exect.

    The IRC prescriptive R7.5 presumes R20 cavity fill which is a ratio of R7.5/R27.5= 27%.

    With R6.5 on the exterior and R18 in the cavities you have R6.5/R24.5= 26.5 % of the total R outside the sheathing, which is pretty close to the prescriptive.

    At outdoor temperatures that matter from a dew point control perspective you'd be closer still, due to the up-rating of Thermax when colder.

    If you're the type who would worry about it, installing a "smart" vapor retarder such as 2 mil nylon (=Certainteed MemBrain) on the interior side under the wallboard would be cheap insurance, giving it a bit of margin. In winter the entrained air in the cavity will be dry enough to keep the vapor permeance of the sheet nylon low, reducing the amount of moisture that gets in. But in spring (or when there is a bulk water leak) the humidity in the cavity goes up, making the nylon more vapor open than standard latex paint on wallboard. This characteristic allows the assembly to dry more quickly than it accumulates moisture via diffusion.

    1. DanS355 | | #8

      Any issues with foil face insulation under vinyl siding. Due to excessive heat?

      Thank

      1. Expert Member
        BILL WICHERS | | #9

        Unfortunatly, Dana hasn't been seen around these parts for some time now, but I'll see if I can answer your question here.

        The usual concern with polyiso, which is what I assume you mean by "foil faced insulation" here, is that the R value drops with reducing temperature, which is the opposite of most other materials. This property is usually overstated and based on older materials, but it's common to derated polyiso to about R5 per inch in cold temperatures.

        All materials drop in performance as temperatures increase, polyiso is no exception here. We're usually more concerned with cold weather performance though, since most on this site are building in climates where heating needs are the dominate concern. I wouldn't worry about polyiso's performance behind vinyl siding, which is a very common application for the material. Worst case, derate to R5 per inch and don't worry about it, just as is commonly done in colder climates.

        Bill

        1. DanS355 | | #10

          Hello Bill, I have a similar situation trying to ahieve an R10 CI on a remodel and use vinyl siding.
          Trying to avoid using furring strips. It appears there's nothing on the market other than 2 inch foam to achieve that.
          Thanks

          1. Expert Member
            BILL WICHERS | | #11

            You're pretty much stuck with at least 2" thick foam to hit R10. XPS is R10 at 2", polyiso can be found in a 1.75-1.8" thick R10 variant, but it's unusual. EPS would be a bit over R8 at 2" thick.

            Your best option is probably 2" polyiso here, for R13. Polyiso is greener than XPS, which is why XPS is less desireable here. EPS won't hit your R value target in the space available. Furring strips are usually put over the foam to support the siding. I prefer ripped strips of 3/4" plywood for this. I suppose you might be able to hang vinyl siding directly against the foam since it's relatively lightweight, fastened through the foam to the structural sheathing behind, but I'd stick with furring strips myself.

            Bill

  7. DanS355 | | #12

    I assume your talking the foil face Polyiso? This would be in climates on 67. How about th GPS foam it an r5 per inch it does derate I guess.
    I appreciate your advice I hate to say most builders and building inspectors are not very familiar about CI in this area...
    Side question do you know of any website or calculation that can tell the cost-benefit payoff time of adding exterior insulation?
    Thanks

    1. Expert Member
      BILL WICHERS | | #13

      Correct, the most common polyiso used on residential projects is the foil faced type.

      GPS is usually rated at R5 per inch, just like XPS, but it's more stable over time than XPS is. EPS is about R4.2 per inch. None of those (GPS, EPS, XPS) need to be derated for cold weather. I really don't think polyiso needs to be derated in most cases, either.

      Building inspectors are just going to look at the labeled R value of the product. If polyiso says "R13", then they'll "count" it as R13. They won't think about any fancy stuff like derating in cold weather.

      There are various ways to do energy modeling to work out how much energy, and thus money, exterior insulation will save you. I don't really work with that, but there are others on GBA who do.

      Bill

    2. Expert Member
      Michael Maines | | #16

      Foam R-values come in three flavors:

      1. The rated/marketed value, which is measured after careful aging for 6 months.
      2. The LTTR (long-term thermal resistance) for polyiso only, which involves accelerated aging for 5 years to simulate 15 years in service.
      3. The actual long-long-term value for any foam, which mainly matters for situations where it's being used to control the dewpoint (i.e., moisture issues).

      When the main concern is meeting building code thermal requirements, I use #1--foam is a good insulator but bad-to-terrible for the environment so I only use as much as necessary.

      When the main concern is moisture control, I use the most conservative values--#3 on the list. I want the buildings I design to last at least 50 years without mold or rot.

      All insulation except polyiso performs better the colder it gets. Polyiso performs worse as the temperature drops, but that mostly affects the outer portion of the foam.

      The values I use for #3:
      EPS: R-3.8 to R-4
      GPS: R-4.8
      XPS/GPS: R-4.3
      Polyiso: R-5.5, with the outer third of exterior insulation at R-5.0.

      Those values are based on tons of research over many years, but new products and new information are always becoming available.

  8. conwaynh85 | | #14

    Does anyone know where GPS can be sourced In New Hampshire ?

    1. begreener | | #15

      You might wanna try these guys:

      https://www.atlasfoam.com/

  9. DanS355 | | #17

    How critical is it really?
    I'm doing a remodel in climate zone 6/7.
    The minimum prescribed continuous exterior installation is R10 (using foam) with a 2x4 wall having R13 in the wall cavity. What about using r15 mineral wool or fiberglass?
    The prescribed is 46% outside r vaule.
    Is using r15 in the wall cavity would be 40%.
    Any issues?
    Thanks

    1. Expert Member
      BILL WICHERS | | #18

      If you increase the interior side insulation, you would need to increase the exterior foam R value too. It is the RATIO of interior to exterior insulation that matters for moisture control. The material used to arrive at that R value doesn't matter in this case, at least as you described...

      Note that there ARE other ways to deal with the problem. Using vapor open exterior insulation like rigid mineral wool allows for exterior drying and makes the ratios less of an issue. Using an interior side smart vapor retarder cuts down on how much moisture can get into the wall, but still allows for inward drying, which also allows for a safe assembly with slightly less exterior insulation for the same amount of interior insulation.

      More exterior R value is always safer. Less interior R value is safer, but less efficient. I would try to stick with the prescribed ratios if at all possible. If you can't quite get to the needed exterior R value to hit the right ratio for your climate zone, but you can get close, using an interior smart vapor retarder behind the drywall is usually enough to make up the difference. Personally, I try to exceed the ratio (more exterior R value than needed for my CZ), AND use an interior side smart vapor retarder (CertainTeed's MemBrain, usually, because it's relatively inexpensive), as a belt and suspenders approach.

      Bill

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