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Musings of an Energy Nerd

Choosing Rigid Foam

Knowing when to use polyisocyanurate, expanded polystyrene, and extruded polystyrene

The three most common types of rigid foam are expanded polystyrene (EPS), which is white; extruded polystyrene (XPS), which is usually pink or blue; and polyisocyanurate (polyiso), a yellow foam which comes with a variety of possible facers.

UPDATED on January 25, 2018 with information on phenolic foam.

Maybe you’ve decided that your floor, wall, or roof assembly needs one or more layers of rigid foam. Which type of foam should you choose: polyisocyanurate, expanded polystyrene (EPS), or extruded polystyrene (XPS)?

The answer depends on several factors, including your R-value target, your local climate, whether the insulation will be in contact with soil, and your level of environmental concern.

R-value per inch

Manufacturers of insulation products are required to provide consumers with R-value information. If you’ve purchased rigid foam insulation that isn’t clearly labeled, contact the manufacturer to learn the product’s R-value.

Over a period of decades, the R-value of polyisocyanurate and XPS gradually declines. For more information on this phenomenon, called “thermal drift,” see Thermal Drift of Polyiso and XPS.

Cold weather performance of polyiso

Rigid foam manufacturers are required to perform R-value tests using an ASTM method specifying that the test be performed at a mean temperature of 75°F. At lower mean temperatures, EPS and XPS perform better than their R-value label indicates. In other words, as the temperature drops, the ability of EPS and XPS to resist heat flow improves.

Polyiso behaves differently: as the mean temperature drops, it does a worse job of resisting heat flow. For that reason, some cold-climate builders assign a lower R-value for polyiso — perhaps R-4.5 or R-5 per inch — than the R-value on the product label.

For more information on this issue, see Cold-Weather Performance of Polyisocyanurate.

Soil contact limitations

XPS and most types of EPS are rated for ground contact. These products can be buried without worrying that the materials will lose their insulating value or absorb significant quantities of moisture. If you are in doubt about the suitability…

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  1. User avater GBA Editor
    Allison A. Bailes III, PhD | | #1

    Global warming impact
    As I pointed out shortly after Alex Wilson wrote that article on the global warming impact of insulation, he used a whole lot of assumptions to come up with his conclusions. I don't consider that article to have much validity. Here was my response back in 2010:

    Don't Forget the Science in Building Science

  2. User avater
    John Semmelhack | | #2

    The ABC's of GWP for XPS
    From the link that Charlie posted from the XPSA: "All of the blowing agents currently used by the
    industry have a life cycle positive benefit, as measured by greenhouse gas emissions." So if I use it to make a super-insulated birdhouse, it'll have life cycle positive benefit in terms of green house gases? Or if I use 10 inches of it...below Houston, it'll have life cycle positive benefit in terms of green house gases? To refuse to acknowledge that there's a threshold for each application where the application is no longer life-cycle positive is disingenuous.

    Charlie - do you have a link to more information on the phase-out of HFC blowing agents in North American XPS? It certainly could be good news...

  3. Charlie Sullivan | | #3

    no question
    The fact that XPS and ccSPF use high GWP blowing agents in the US is well known and admitted by the industry. For example, here is a statement from the industry association that is quite clear and direct about it.

    The good news is that HFC blowing agents are scheduled to be phased out in XPS [edit: by Jan 2021--I said before "in 2017 IIRC." but IDRC--see below.] Unfortunately, there is no corresponding agreement on ccSPF, but, good news again, Lapolla has a low GWP "4G" product already available, using HFO blowing agent.

  4. Charlie Sullivan | | #4

    Phase-out of HFCs
    The EPA SNAP program "Rule 20" has the schedule. Here's the fact sheet:

    and the full rule:

    You can see in these documents that polystyrene extruded sheet has a deadline of Jan 1, 2017; Polystyrene extruded boardstock and billet has a deadline of Jan 1, 2021. The trick is figuring out whether the stuff we use if buildings is sheet or boardstock. It's boardstock, so the data is 2021. From the full rule:
    "• Polystyrene (extruded sheet) includes foam for packaging and buoyancy or flotation.
    • Polystyrene (extruded boardstock and billet) includes insulation for roofing, walls, floors, and pipes."

    Perhaps some insulation manufacturers will start differentiating their XPS by transitioning sooner. It's already the case that Owens-Corning pink foam uses a lower GWP mix than Dow blue foam, but only by a factor of 2, not by the factor of 100 to 1000 that we will get by 2021.

    There's also work on an international agreement, with a rule under the Montreal Protocol to be finalized in 2016.!OpenDocument

  5. Mark Hays | | #5

    Beware of polyiso sheathing - a sponge around your house
    Thanks Martin for another great article! As you noted, polyiso foam board absorbs water -- unlike XPS and EPS. If you wrap a house with sheets of foam board, there will be many edges and joints, just waiting for a trickle of water. With polyiso, each large sheet can turn into a large sponge. With perfect flashing, sealing, housewrap, etc, no problem, of course. One mistake by a sub could equal a big sponge, however -- with a reservoir of moisture to encourage rot.


  6. Bruce Lepper | | #6

    What about rodent resistance? Do these three all have the same RR value? I seem to remember mice having a whale of a time in XPS insulation.

  7. User avater GBA Editor
    Martin Holladay | | #7

    Response to Mark Hays: "A sponge around your house"?
    The hypothetical situation you worry about -- "With polyiso, each large sheet can turn into a large sponge" -- is highly unlikely, and in fact is one that hasn't been reported on GBA. Nor is this situation one I have ever seen. Nor is this situation one that I have heard about at any building science conference. It sounds like a scenario invented by EPS distributors or XPS distributors.

    GBA recommends that builders who install polyiso on the exterior side of wall sheathing also install vertical furring strips to create a ventilated rainscreen gap (unless the polyiso measures 1.5 inch in thickness or less and the siding is vinyl siding, which is inherently well ventilated). If these recommendations are followed, the rainscreen gap will be dry -- in fact, this will be one of the driest parts of your house. The siding keeps off almost all of the rain, and the ventilation air encourages evaporation. Sunlight contributes to the extraordinary dryness of a rainscreen gap.

    A builder would have to be extremely sloppy with his or her flashing details -- criminally sloppy -- to end up with the type of scenario you describe.

  8. User avater GBA Editor
    Martin Holladay | | #8

    Response to Bruce Lepper: "Mice?"
    In my experience, using rigid foam does not result in more problems with mice. Mice are far more likely, in my experience, to live in fiberglass-insulated cavities than in rigid foam.

    The best way to reduce rodent infestations is to pay attention to airtightness. The lower your blower door number, the less likely that your house will have mice. Mice like holes -- that's how they enter your house. If you seal your holes, the mice will (generally) stay outside where they belong.

    GBA readers who have encountered mouse nests in rigid foam are invited to post comments here. I have lots of tales about mice in fiberglass-insulated walls... but most GBA readers are familiar with that problem.

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Mark Hays
    Your wrote, "I have seen a number of cases where water traveled under and into a wall assembly, particularly in colder climates, e.g. via backed up gutters with rotten fascia or water dams on the edge of the roof."

    So have I. Have you ever seen damage like that in a house with a ventilated rainscreen gap?

  10. Mark Hays | | #10

    The polyiso sponge
    Dear Martin:

    A rainscreen will certainly help, but I have seen a number of cases where water traveled under and into a wall assembly, particularly in colder climates, e.g. via backed up gutters with rotten fascia, water dams on the edge of the roof, etc, etc. I am sure other builders have as well. If these leaks reach polyiso, it will absorb the moisture literally like a sponge -- and remain damp for a long time. With virtually identical R ratings and pricing, I would not increase the risk for a customer or myself with this product in exterior applications. Polyiso is great, however, for interior insulation.

    I have no connection to any building supply manufacturer, by the way -- except as a customer.


  11. Sean Wiens | | #11

    Rigid foams absorb water - full stop
    I am disappointed that GBA is still purporting the myth that rigid foams do not absorb water in a below grade or below slab application or even on a roof applications. This is far from accurate. If water is present, these boards wet up. EPS absorbs moisture very quickly but may dry faster between wetting cycles if spaced far enough apart and installed over gravel so that it can dry. XPS wets up much slower than EPS but still does wet up over time, but also dries at a much slower rate. My own testing showed a 250% mass increase in EPS due to wet up and a 31% for XPS over a 8 month test. ( And ask any building envelope remediation firm and they will confirm that they have pulled saturated samples of both types out of roof assemblies and plaza decks.

  12. User avater GBA Editor
    Martin Holladay | | #12

    Response to Sean Wiens
    GBA has consistently advised readers to include drainage systems for every foundation. All of our details show drainage systems.

    Moreover, when rigid foam is installed on walls, the wall assembly should usually include a ventilated rainscreen gap between the rigid foam and the siding.

    Finally, when rigid foam is installed above roof sheathing, it needs to be covered by roofing. When roofing fails and begins leaking, you have a problem that needs to be addressed immediately. Many types of insulation will be degraded when faced with a roof leak -- and roof sheathing and framing lumber are also at risk when roofs leak. So by all means, fix roof leaks promptly.

    So if your point is that rigid foam absorbs water when it is allowed to sit in a puddle or a pond, I agree. So don't install rigid foam in a puddle or a pond.

    Drainage systems for foundations include a layer of crushed stone (drained with perforated piping) installed under horizontal rigid foam, and free-draining materials (or a dimple mat) adjacent to foundation walls. These systems will help maintain the R-value of your rigid foam.

  13. Sean Wiens | | #13

    Below Grade still wets up
    Martin - thanks for response.

    The point is that foam does absorb moisture and your posting "XPS and most types of EPS are rated for ground contact. These products can be buried without worrying that the materials will lose their insulating value or absorb significant quantities of moisture." did not have the caveat that you have now included in your comment above.

    Of course if you remove the water source it probably cannot absorb moisture (I will be doing long term testing below slab in my upcoming build - but there is a suspicion that even in high RH environments, the foams will wet up).

    Re foundation walls, how often do you see the backfill right up against the foam without dimple sheet or any form of drainage plane? All the time?

    regarding roofs : plaza decks and inverted roof assemblages often use rigid foam. These systems by their nature have the foam exposed to moisture. These systems do wet up.

    So again the point is that these foams do absorb moisture and when discussing their use, a strong recommendation to include drainage plane should always be included in the same sentence.

  14. User avater GBA Editor
    Martin Holladay | | #14

    Response to Sean Wiens
    Over the years, I think that my advice on this topic has been consistent. See, for example, these two articles:

    All About Basements

    Fixing a Wet Basement

    My advice in those two articles is almost identical (yes, I tend to repeat myself). I wrote:

    "If you are building a new home on a basement foundation, you should specify ... a layer of dimple-mat drainage board installed on the exterior side of the foundation walls; failing that, the foundation should be backfilled with coarse, free-draining material like crushed stone, topped with an 8-inch layer of dirt (ideally, dirt with a high clay content)."

  15. Sean Wiens | | #15

    Thanks Martin
    Thanks Martin - I was only reacting to your article above which stated "XPS and most types of EPS are rated for ground contact. These products can be buried without worrying that the materials will lose their insulating value or absorb significant quantities of moisture." and did not include the caveats that you have now mentioned in the comments. Every article that discusses rigid foam below grade needs to include the warnings that the foam does wet up if water is present.

  16. Kenneth Gartner | | #16

    How can one reliably identify 'type II' EPS on the used market?
    Martin, I would like to follow the GBA advice to seek reclaimed ~4 inch EPS here in MA for my underslab project. However, I am finding that many people offering such foam rarely indicate attributes such as PSI rating which makes me think that manufacturer markings are not typically stamped onto the full sheets. How can I reliably determine from picture or in-person inspection whether it is type II? I understand that many walkable roofs will use Type II, so that might help if the seller knew where the material came from. Can I run a simple strength test using a vise clamp or such? Thanks for all the pertinent advice on this site.

  17. User avater
    Dana Dorsett | | #17

    Under a 4" slab it doesn't really matter (@Kenneth Gardner)
    Almost all used EPS from reclaimers is Type-II or denser. It typically comes without facers (although some has a heavy facer on one side to withstand the heat of torch-down roofing a bit better). It's fairly stiff stuff at 2", and isn't easy to poke a finger into (but fingernails, yes). Lower density Type-I often has lightweight facers, and with a facer peeled back it doesn't take a martial arts black belt to poke finger in 1/4" or more, but it's not particularly squishy the way some ultra-low density packing foam can be. Type-I foam without facers is easy to scar up, and corners are often dented or broken in handling.

    The most reliable method is to weigh it: Type-II EPS has a minimum density of 1.3lbs per cubic foot (3.5lbs per inch of thickness for a 4x8 sheet) and a nominal density of 1.5lbs per cubic foot (4lbs per inch of thickness for a 4x8 sheet.) If it's been sitting out in the rain for months/years it can theoretically have some amount of additional water weight stored in the interstitial spaces between the beads, but that would be rare. If it's been exposed to too much sun it's surface will be chalky/dusty from UV degradation of the polymer, but most reclaimers keep it under tarps or in sheds.

    You can't tell anything about it's density from a picture.

    But unless the foam is under a footing, even Type-I foam doesn't present a problem for a 4" slab.

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