Choosing Rigid Foam

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Choosing Rigid Foam

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

Posted on Feb 5 2016 by Martin Holladay

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 (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.), or extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.)?

The answer depends on several factors, including your R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. 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.

  • The R-value of EPS ranges from about R-3.6 to R-4.2 per inch. In most cases, the higher the density of the EPS, the higher the R-value per inch.
  • The R-value of XPS is R-5 per inch.
  • The R-value of polyisocyanurate is R-5.7 to R-6 per inch (although cold-climate builders sometimes use a lower R-value because polyisocyanurate performs poorly at cold temperatures).

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 ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. 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 of an EPS product for burial, contact the EPS manufacturer for more information. If you are specifying EPS for use below grade, remember that denser types of EPS tend to perform better in this application than products with a lower density.

Polyiso is not rated for ground contact or burial, so polyiso should never be used to insulate the exterior side of a basement wall; nor should polyiso ever be installed under a concrete slab on grade. It’s perfectly OK to use polyiso to insulate the interior of a basement wall.

Compressive resistance

When specifying rigid foam for use under a slab or a footing, it’s useful to know the compressive strength of the EPS or XPS you’ll be using. Compressive strength may also matter when rigid foam is installed directly under membrane roofing.

XPS and EPS manufacturers produce rigid foam products with a variety of densities and compressive strengths. Type IX EPS and common XPS products like Dow Styrofoam or Owens Corning Foamular have a compressive strength of 25 psi. That’s more than many soils that are routinely used to support a footing and a house.

If your project engineer recommends a higher compressive strength, you can purchase XPS with a compressive strength of 40, 60, or 100 psi.

According to Michael Maines, a residential designer and GBA blogger who lives in Palermo, Maine, “For most situations, Type II EPS is fine for sub-slab use. Its compressive strength is similar to that of sandy soil. A 4-inch slab contributes about 50 psf of load; add a 40-pound live load and you're still below 1 pound per square inch. The compression curve for foam plastic is usually pretty straight, so insulation rated at 15 psi would compress somewhere in the neighborhood of 0.015 of its thickness. For 3-inch foam, that's less than 1/16 inch — not enough to worry about. Type IX EPS is better for areas that support roof upper floor loads, such as below footings.”

For more information on this issue, see:

Environmental concerns

Specifiers of rigid foam insulation have two environmental concerns to worry about: brominated flame retardants and blowing agents with a high global warming potential.

All brands of EPS and XPS sold in the U.S. include a brominated flame retardant — hexabromocyclododecane (HBCD) — that many environmentalists find worrisome. (That said, EPS or XSP isn’t usually exposed to interior living space, so it’s unclear whether these worries are justified.) For more information on this issue, see Making Healthier, Greener Foam Insulation.

Most green builders avoid using XPS because it is manufactured with a blowing agent with a very high global warming potential. For more information on this issue, see Calculating the Global Warming Impact of Insulation.

That leaves polyiso, which enjoys a reputation as the most environmentally friendly type of rigid foam insulation. Because polyiso doesn’t perform very well at cold temperatures, however, green builders in cold climates tend to prefer EPS, while green builders in hot climates tend to prefer polyiso.

What about vapor permeance?

In most cases, designers who are planning to install rigid foam on the exterior side of wall sheathing or roof sheathing will want to specify a thick enough layer of rigid foam to keep the sheathing above the dew point during the winter. For more information on this topic, see Calculating the Minimum Thickness of Rigid Foam Sheathing.

In some cases, however, a designer may want to install a thin layer of semi-permeable rigid foam that allows wall sheathing to dry to the exterior. A full discussion of the circumstances under which this approach might make sense is beyond the scope of this article; suffice it to say that there are circumstances when it's important to know the vapor permeance of a layer of rigid foam.

  • One inch of unfaced EPS is relatively permeable; it has a permeance of 2.0 to 5.8 perms. If you are looking for vapor-permeable EPS, though, be careful: if the EPS is faced with polyethylene or foil, it will have a very low vapor permeance.
  • One inch of XPS has a permeance of 1.1, while a 2-inch-thick sample of XPS has a permeance of 0.55. XPS is classified as a semi-impermeable material.
  • One inch of foil-faced polyiso has a very low permeance of 0.03 perm. Polyiso with a glass-mat facing or a felted-paper facing has a permeance ranging from 1 to 10 perms, depending on the type of facing used.

Phenolic foam

Important information has come to light since this article was first published: An Irish manufacturer of rigid foam, Kingspan, has begun U.S. distribution of a previously unavailable kind of rigid foam insulation called phenolic foam. For more information on this development, see Kingspan Kooltherm Phenolic Foam Rigid Insulation.

It's cheaper to buy used foam

Every week, roofers and demolition contractors remove usable rigid foam from buildings that are being re-roofed or demolished. Much of this used foam is later offered for sale by marketers of "reclaimed" or "recycled" insulation.

If you are a green builder, it makes sense to consider the use of reclaimed rigid foam. Users of reclaimed rigid foam not only help reduce the amount of foam that is disposed of in landfills — they save money on their construction project.

What if you hate rigid foam?

For a variety of reasons, some green builders hate foam insulation. If you are a foam-hater, you may want to consider using semi-rigid mineral wool panels instead of rigid foam. For more information on this option, see the following articles:

Martin Holladay’s previous blog: “Smart Vapor Retarders for Walls and Roofs.”

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Feb 5, 2016 9:33 AM ET

Global warming impact
by Allison A. Bailes III, PhD

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:

Feb 5, 2016 1:35 PM ET

Edited Feb 6, 2016 5:14 AM ET.

no question
by Charlie Sullivan

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.

Feb 5, 2016 9:47 PM ET

The ABC's of GWP for XPS
by John Semmelhack

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...

Feb 6, 2016 5:24 AM ET

Phase-out of HFCs
by Charlie Sullivan

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

Feb 10, 2016 7:17 PM ET

Beware of polyiso sheathing - a sponge around your house
by Mark Hays

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.


Feb 11, 2016 5:15 AM ET

by Bruce Lepper

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.

Feb 11, 2016 8:26 AM ET

Response to Mark Hays: "A sponge around your house"?
by Martin Holladay

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.

Feb 11, 2016 8:31 AM ET

Response to Bruce Lepper: "Mice?"
by Martin Holladay

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.

Feb 11, 2016 2:56 PM ET

Edited Feb 11, 2016 3:41 PM ET.

The polyiso sponge
by Mark Hays

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.


Feb 11, 2016 3:10 PM ET

Response to Mark Hays
by Martin Holladay

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?

Apr 6, 2016 6:43 PM ET

Edited Apr 6, 2016 6:50 PM ET.

Rigid foams absorb water - full stop
by Sean Wiens

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.

Apr 7, 2016 5:53 AM ET

Response to Sean Wiens
by Martin Holladay

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.

Apr 7, 2016 11:44 AM ET

Below Grade still wets up
by Sean Wiens

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.

Apr 7, 2016 11:58 AM ET

Response to Sean Wiens
by Martin Holladay

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)."

Apr 28, 2016 1:21 PM ET

Edited Apr 28, 2016 7:44 PM ET.

Thanks Martin
by Sean Wiens

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.

Apr 24, 2017 6:23 AM ET

How can one reliably identify 'type II' EPS on the used market?
by Kenneth Gartner

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.

Apr 24, 2017 7:06 AM ET

Under a 4" slab it doesn't really matter (@Kenneth Gardner)
by Dana Dorsett

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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