GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A


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.


GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  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

    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:

      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:

      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

    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):

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

    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.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |