What Is Rigid Foam Insulation?
Want to make your building more comfortable, save on energy, and reduce the risk of mould and moisture damage? Rigid foam board insulation may be the perfect solution for you.
Rigid foam board insulation is a type of insulation made out of various polymers, most typically polystyrene. As an insulator, rigid foam board acts as a barrier that prevents air flow and heat conduction. The result is a building that stays warmer in the winter and cooler in the summer.
Let’s dive into everything you need to know about rigid foam insulation: How it works, the scenarios it’s best to use in, how it compares to other insulators, its strengths and weaknesses, some practical considerations to keep in mind when installing it, and more.
What Is Rigid Foam Insulation: Quick Overview
There are, broadly, three types of rigid foam insulation:
- Expanded polystyrene (EPS)
- Extruded polystyrene (XPS)
- Polyisocyanurate
Below, we’ll be focused on the second and arguably most popular type of rigid foam insulation: XPS.
XPS is most popularly known under the brand name Styrofoam—it’s a polystyrene closed-cell foam (in case you ever wondered about the origin of the name). As closed-cell foam insulation, it’s sturdier and stiffer than its open-cell counterparts.
Extruded polystyrene is made up of about 2% polystyrene and 98% air. The air is trapped in the closed cells of polystyrene; trapped air is an incredible insulator, and our XPS products have R-values that range all the way up to R15. R-values are simple to understand—a higher R-value means better insulation.
The closed-cell polystyrene foam makes XPS insulation inherently moisture-resistant; it’s an excellent choice for basements and other areas where water damage and mould growth are a concern. XPS has a water vapour diffusion resistance of 80-250.
Is Rigid Foam Insulation the Right Choice for Your Project?
Rigid foam insulation is an excellent choice if you want an insulator that is:
- Lightweight
- Easy to work with
- Moisture-resistant
Rigid foam insulation materials are easy to cut to size, and can fit in a variety of different spaces. Their inherent moisture resistance makes them an excellent choice for below-grade applications like basement insulation. The average R-value of XPS rigid insulation is about R5 per inch of thickness; we offer materials up to 3” thick, providing an R-value of about R15.
Below-grade applications are the most common use of closed-cell foam rigid insulation, but any place that’s susceptible to moisture is a good candidate for the material. Floors, cellars, foundation walls, and other areas that are prone to moisture are some of the places where XPS insulation is appropriate.
Realistically, XPS insulation can be used throughout your building, but it’s typically best to opt for a less expensive material when moisture isn’t a significant concern.
Comparative Analysis: Rigid Foam Insulation Versus Other Insulation Types
XPS is valued for four qualities: It acts as a moisture barrier, it’s a great insulator, it’s relatively inexpensive, and it’s lightweight and easy to work with. There are other insulation options that meet some of these criteria; let’s compare them:
Rigid Foam Versus Spray Foam
First, there’s polyurethane spray foam; there’s broad agreement throughout the market that this is the best combination of insulation and moisture protection. When comparing the moisture barrier and thermal resistance offered by XPS and spray foam insulation, spray foam comes out on top. Spray foam is also easier to install in some ways; you can spray it continuously, so there’s no fear of any gaps in insulation.
Spray foam is, however, quite a bit more expensive than rigid foam board. As such, many DIY projects—especially those where the goal is to flip a building and turn a profit—will opt for rigid foam instead of spray foam insulation.
Rigid Foam Versus Fibreglass
Fibreglass insulation has almost no moisture resistance and a lower R-value per inch than rigid foam. There is, however, one good reason that developers, renovation experts, and homeowners use it—it’s incredibly inexpensive and costs far less than rigid foam insulation.
Evaluating Rigid Foam’s Performance: Strength, Durability, and Resistance
Structural Integrity: How Strong Is Rigid Foam?
Rigid foam insulation can be quite strong; the XPS products we stock at STAR have a compressive strength of 21-25 psi (145-172 kPa). This is more than enough compressive strength for most insulation applications.
Moisture Management: Do You Need a Vapour Barrier With Rigid Foam?
There’s no need for a vapour barrier with rigid foam insulation—it acts as a vapour barrier on its own. You should, however, take care to ensure there are no gaps between the insulation; fill any gaps you see with spray foam insulation, and you’ll create a surface that is water-resistant and highly insulating.
Water Resistance: Is Rigid Foam Impervious to Water?
Rigid foam is not impervious to water; if it gets soaked, it can become damaged. Fortunately, it is water vapour-resistant, and it’s unlikely to take on moisture unless external factors like basement flooding are at play.
Longevity Assessed: The Lifespan of Rigid Foam Insulation
XPS is long-lasting and can be used for 20-30 years or longer. One problem with XPS, however, is that the R-value can tend to degrade over time; one study found that XPS insulation had its R-value degrade by 52% over 15 years. As such, it can be a good idea to over-insulate with XPS; the extra insulation will also add to the moisture management properties of XPS.
Practical Considerations: Using and Maintaining Rigid Foam Insulation
Rigid foam insulation is easy to cut, but there may be gaps between each board. To eliminate these gaps, couple your rigid foam insulation with spray foam insulation.
Avoid leaving rigid foam exposed, especially to the sun. UV rays can cause rigid foam to degrade over time; within 30-60 days, you may see decreased R-value from your material.
Solving Common Basement Issues: Insights and Solutions
Troubleshooting Sweating Walls: Why Are My Basement Walls Sweating Behind Insulation?
Walls sweat behind insulation when warm air comes in contact with cooler walls; the moisture from the warm air condenses. This can occur when insulation isn’t properly installed. Poor ventilation and water leakage can also contribute to the problem.
By installing ventilation properly, especially with a moisture-resistant insulator like XPS, you can reduce the chances of your basement walls sweating.
Drywall Preparation: Should You Put Plastic Behind Drywall in a Basement?
You should not put plastic behind drywall. Your closed-cell rigid foam insulation will protect your drywall from moisture; plastic can actually trap moisture, causing damage.
Potential Drawbacks: The Disadvantages of Rigid Foam Insulation
Rigid foam insulation isn’t perfect. While XPS comes with a flame-retardant coating, if that coating is bypassed, it can burn rather quickly.
Additionally, XPS tends to lose R-value over time if it’s exposed to too much moisture. In very moist environments, we recommend using a thicker sheet of XPS—this can improve the moisture barrier and add to the overall R-value, so a reduction in R-value is less impactful.
Finally, while rigid foam board is less expensive than spray foam, it can be more expensive than other types of insulation. As such, it’s best used in areas where moisture is a real concern.
At STAR Building Materials, we offer a variety of different types of rigid foam insulation, as well as other types of insulation—we have the perfect material for your next construction or renovation project!