Polyisocyanurate (PIR), Extruded Polystyrene (XPS) or Expanded Polystyrene (EPS)?
When building or renovating a high-performance building envelope there are really three main kinds of rigid foam panels you are going to have to choose from - Polyisocyanurate known as Polyiso (PIR), Extruded Polystyrene (XPS), Expanded Polystyrene (EPS).
Before choosing, you should know exactly what you expect these foam insulation panels to do, to make sure you buy and benefit from the right one. These three products we're comparing here are all petroleum-based, but their characteristics, performance and ecological impacts vary significantly. Alternatively, depending on application and budget, you could always choose natural Green Insulation Products, Like Hemp Insulation panels or Batt Insulation or mineral wool - demonstrated here in roof insulation.
Polyisocyanurate Foam Panels:
PIR, polyiso, or ISO, is a thermoset plastic product typically produced as a foam and used as rigid thermal insulation panel - most often with aluminum foil facing. Thermal performance is rated at R6-6.5 per inch, but don't count on that if your winters are cold for the reasons we're about to explain. Most insulation products actually perform a bit better the colder it gets but polyisocyanurate breaks that rule. As of about 15°C its performance starts to deteriorate, and badly. By the time you get down to the -20s Celsius it's nowhere near that. It can be a great product to use as long as you keep it warm, which is a really odd thing to say about an insulation product.
The news of Polyiso's R value petering out when you need it most still hasn't permeated entirely through the building industry, so you still see it being installed occasionally on the exterior of walls in cold climates. It won't offer nearly the thermal protection you think it will in the dead of winter, and it may cause moisture damage due to its lack of permeability.
Polyiso insulation boards are the most widely used low slope, above-deck commercial roofing insulation. As a versatile choice for commercial roofing applications, polyiso is designed to be part of any modified bitumen, built-up, or single-ply roofing system. Polyiso products feature a facer for high strength and excellent absorption for both hot mopping and adhesive attachment methods. The product also is designed to perform well with mechanical fasteners, possibly under Green Roof membranes.
Polyiso Foam Panel Insulation Conclusion: In real terms, using polyiso foam insulation panels is probably a poor choice if your winter temperatures dip below 50°F or 10°C. To put that statement into perspective, a wall or roof assembly in Chicago was tested for the whole of December then averaged out. The first assembly using 2" Polyisocyanurate foam panels was compared to the same assembly using 2" of EPS foam and was found to be losing 30% more heat in this study (see below)!
XPS - Extruded Polystyrene Foam Panels - The blue, pink or green ones (eg. Styrofoam):
XPS is Rated at R5 per inch, but it will off-gas and lose some insulation performance over time - especially below-grade and when tested in real-world applications. Above grade XPS foam acts as a vapour retarder (and becomes even less moisture permeable the thicker it is - 1 inch is about 1 perm, 2 inches about .5 perms); when taped it can act as an air barrier; the manufacturers and standardized testing state that it does not absorb moisture, nor is it affected adversely by it. However, with some of the EcoHome team having real world experience to the contrary, we "dug-deeper" and found that many contractors have also noticed potential issues with XPS foam boards retaining moisture, backed up by reports like this citing a large source of correlated testing and which would lead us to conclude that XPS rigid insulation panels should be avoided for below-grade applications like basement insulation - which is the opposite of much of the information out there.
Note: 1 perm and 60 ng are U.S. and Canadian equivalent rates of permeability, below that rate of permeability classifies a material as a type II vapour retarder, suitable for residential construction.
Also, rather regrettably for traditionally produced XPS, the hydro fluorocarbons (HFCs) most commonly used as blowing agents are far more damaging to the climate than those used with other rigid foam insulation boards. Some manufacturers speak of a transition to more eco-friendly foam insulation blowing agents; that will be great news when it happens across the board! And credit where it's due, the "real" Styrofoam, as in the DOW Chemicals blue product, is now manufactured with HCFC blowing agents which have 94% less ozone depletion potential. As HFCs have a global warming potential (GWP) that is 1430 times worse than carbon, this really does demonstrate how important it is to choose rigid foam insulation boards very carefuly to reduce their environmental impact. All XPS panels are not equal!
EPS - Expanded Polystyrene Foam Insulation Boards:
Rated at R4 per inch; EPS foam insulation boards are more permeable to air and moisture than XPS, but it doesn't retain moisture to the same extent because of it's more closed cell structure and it's breathability which lets it dry out. Two inches of EPS foam board has a moisture permeability rate of between 60 and 75 ng (1 to1.25 perms), which is on the cusp of qualifying it as a type II vapour retarder, but on the more 'breathable' side of the scale which we would probably consider a good thing in most applications.
For reference sake, the traditional 6mil polyethylene vapour barrier has a permeability rating of 3.4 ng, making it about 18 times more vapour resistant than building codes allow.
The permeability of EPS can be handy at times if you want to add insulation to an existing wall assembly but are worried about trapping moisture, like retrofitting the exterior of buildings with additional insulation. Though to be absolutely sure you may be better with a mineral wool board which lets moisture pass right through - it all depends what moisture barriers are already in place or ar about to be installed.
The lower stated R value of EPS compared to XPS is in a way compensated for by having a higher R value per dollar, as it is somewhat cheaper. If you're not worried about losing an inch of space here or there, you'll get a higher R value with EPS for the same amount of money, albeit with a thicker wall, and this R-Value also stays pretty consistent over time - unlike XPS which tails off as it ages.
The performance of EPS may drop slightly when it's wet (reports I've seen indicate somewhere in the area of 10-15 %, so nothing too catastrophic), it will also dry out just as quickly as it got wet and return to its original performance. But there is nothing wrong with putting a little effort into keeping it dry if you can. The GWP of expanded polystyrene blowing agents is about 7 times worse than carbon, but that's a lot less than being 1430 times worse like standard XPS is. There is also the potential for a miniscule amount of off-gasing of some chemicals including potentially troublesome brominated fire-retardants from EPS foam products used in construction, but speculation as to whether or not this poses a significant risk to health doesn't seem to be based on any hard facts or testing. We'd be really interested to find any verifiable sources for EPS chemical off-gassing testing - if you find any please post them to the comments section below.
Additional considerations with Polyisocyanurate:
Polyisocyanurate foam insulation panels come with a layer of foil on each side to keep the gases in, so there is the potential to solve a bit of a growing problem in wall assembly durability. Foil is a vapour barrier and a very good one at that, it fact it stops even more moisture than the normal 6 mil polyethylene normally used. So if you use it on the interior of a high performance stud wall design, you won't need to add an additional vapour barrier.
Here is the fun part - Since there is foil on either side of the panel, you end up with a harmless second vapour barrier, which is usually heresy in building design. But this can help in summer months when there is a risk of the vapour drive reversing due to air conditioning during hot humid weather. Any inward-bound moisture would be stopped at that inner layer of foil, which will be warmer than the foil on the other side, so you reduce your risk of summertime condensation.
That foil is the reason it can be problematic on the exterior, as you would be adding an exterior vapour barrier where you likely don't want one.
On the good news side, the GWP of blowing agents in Polyiso is similar to those in EPS, and in the right circumstances its R value is significantly higher, which deservedly or not helped earn it the reputation of being the 'greenest' foam. It can be a great choice when kept above freezing and away from moisture - so above grade for sure, and it makes a great interior thermal break when it's kept a bit warmer by batt insulation in stud cavities.
Being petroleum based should not result in foam being condemned by green builders on principle alone; it should be looked at in perspective. There are other great types of insulated sheathing (mineral wool, wood fiber and fiberglass to name three) and each will have their own benefits, drawbacks, carbon footprint and embodied energy through manufacturing, so even the greenest of the green will have some measurable impact. It takes energy to save energy, and manufacturing insulation is arguably one of the more noble things we currently do with fossil fuels. If you really want to go into depth about the tested performance values of Polyiso Foam panels in hot and cold clinates compared to EPS and XPS foam Insulation panels - see the article from Building Science Corporation here
An Alternative to Foam: Mineral wool / Rock wool rigid insulation panels:
The only rigid board insulation that is not a petroleum-based foam product shown in the main photo above is the brown one, which is mineral wool. It is a recycled product made with the stone dust from industrial blast furnaces.
Like any particulate, it is best not to inhale the fibers while installing rock wool insulation board, so like any batt insulation we recommend using eye protection and a mask, not to mention gloves to prevent skin irritation. As an insulation it is non-toxic, and unlike foam products there are no blowing agents that will off-gas into your home. That means it can lead to cleaner indoor air quality, and the R value will never change, whereas foam products will deteriorate over time as the gases escape.
Mineral wool is rated at about R 4, it is unharmed by moisture, it is not a vapour barrier at any thickness, nor is it an air barrier. It also provides better soundproofing, and being made with stone, it is fireproof.
Not being affected by moisture makes it is a great choice for retro-fitting insulation when finishing basement, either interior or exterior. It is also something of a fool proof exterior board insulation when renovating, as it will not trap moisture from the interior inside the wall.
In batt form is it a common alternative to fiberglass for its higher R value, and easier installation (at least for doing a good job) It does however cost more than fiberglass per batt.
In conclusion - Which rigid insulation panel is best?:
Our personal overall preference due to its recycled content and versatile applications is probably mineral wool, but as for the petroleum plastic rigid foam insulation products - polyisocyanurate gets top marks for being 'eco' if you are in a warmer climate and can handle its moody disposition. EPS foam is versatile, great for below-grade applications and in the middle ground for performance, financial and ecological cost, and whilst XPS foam is a top performer on paper it comes with some unfortunate environmental baggage for which we'd probably rate it in last place. As soon as XPS completes its transition to less harmful blowing agents, I'm sure it will be welcomed into the green building community for above-grade applications.
For Additional Information on Insulation & Building High-Performance Walls:
Be sure to read the EcoHome Guide to High Performance wall construction here