Choosing Foam Board Insulation

hands measuring thick sheet of foam board insulation
  • Intermediate

Foam board insulation comes in myriad shapes and sizes, with different underlying technologies that work best in different conditions. Here's a rundown to help you figure out what kind might be best for your insulation project.

Understanding Foam Board Thermal Performance

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The R-value measures thermal resistance offered by a two-dimensional barrier, such as a layer of insulation, a window, or a complete wall or ceiling. The higher the R-value, the greater the insulating effectiveness. In construction, conduction is the factor measured by the R-value, based on a 25 mm (1-inch) thickness of insulation material tested at a precise temperature of 24° C (75°F).

Permeability Rating

Perm ratings measure a product’s ability to let water vapor pass through. A higher perm rating allows for vapor to escape to the outside, reducing the potential for moisture damage inside the home.

As an example, expanded polystyrene without film facers has a perm rating of up to 5.0.

XPS Foam Boards

Extruded polystyrene foam or XPS insulation is manufactured through an extrusion process. It combines polystyrene crystals with special additives and a gas blowing agent fed into an extruding machine, melting into a viscous fluid. The mixture is then forced through a die, where it is pressed into its desired shape, producing a closed-cell structure to be trimmed to its final dimensions.

Brands of XPS are generally recognizable by their color: blue made by Dow, pink made by Owens Corning, green made by Kingspan, etc.

blue boards of foam insulation


The casting process allows XPS boards to have a high density, resulting in higher compressive strength. This makes XPS ideal insulation for wall panels and especially roofs.

Thermal Performance

When XPS comes off the production line, R-Value is at R-5.0 per 25 mm (1-inch). However, the depletion of the blowing agent slightly reduces long-term thermal resistance (LTTR) to about R-4.7 per 25 mm (1-inch).

XPS foam acting as a vapor block has less moisture permeability in thicker panels—25 mm (1 inch) being at about 1 perm while 50 mm (2 inches) is about .5 perms. When you add tape to the seams, it also acts as an air barrier, increasing the overall insulation effect.


According to the manufacturers, XPS panels don't absorb moisture and are not adversely affected by humidity. Some contractors, though, report potential issues with XPS foam boards retaining moisture. These conflicting results suggest that XPS rigid insulation panels may not be ideal for high-moisture environments like basements.

Your mileage may vary, though. Just pay attention to the details of the specific products you use in damp areas.

Summary of Certified Data on XPS

Closed-cell type / R-value: 4.7 per inch / Perm Rating: up to 1.5

Cost: $$$

XPS prices run higher than EPS or GPS boards.

EPS Foam Boards

hands applying foam board insulation to concrete wall

Expanded polystyrene or EPS is commonly referred to as Styrofoam. It's manufactured by expanding small foam beads and fusing them under heat and pressure inside special molds. It can then also be sent through another process where film facers are added to the foam to provide enhanced rigidity or reflective properties to the boards. Its composition of 98% trapped air and only 2% plastic makes it an excellent lightweight insulator. EPS without film facers has a perm rating of up to 5.0.


This type of foam board is commonly used for structural insulated panels, as well as concrete forms.

Thermal Performance

EPS panels classified as Type-I density offer an R-value of about R-3.9 per 25 mm (1-inch), while Type-II has an R-value between R-4.15 to R-4.2 per 25 mm (1-inch).

Summary of “Certified Data” on EPS

Closed cell type / R-3.6 per inch, 3.8 with film facers / perm rating: up to 5.0

Cost = $

EPS is the cheapest option for foam boards.

XPS and EPS insulation boards of identical densities offer about the same thermal performance or quite close. Comparing them per density, the cost of EPS insulation is lower than XPS. It is usually avoided where a material with less density is needed or if the EPS is not physically applicable.

GPS Foam Boards

Graphite Polystyrene or GPS insulation (Fig. 5) is manufactured just like EPS insulation, with the primary difference being a raw material called Neopor. It is a fairly new concept in the US (more widely used in Europe). It is up to 20% more energy efficient than traditional EPS insulation with its key feature Neopor that is infused in the cell structure of the insulation, providing an R-Value of 4.7 per inch.

GPS insulation is more expensive than EPS but less than XPS insulation.


GPS insulation panels are used in remodeling and new construction applications, such as behind new siding, below grade, and below the slab.

GPS Data Summary

Closed cell type / R-value: 4.7 per inch, 4.9 with film facers / Perm Rating: up to 5.0

Cost: $$

Slightly less than XPS foam boards.

ISO Foam Boards

foam board insulation in roof construction

Polyiso insulation or ISO (polyisocyanurate) is a thermoset plastic product typically produced as a foam and used as a rigid thermal insulation panel. It's most often found with an aluminum foil facing.

ISO is manufactured with materials similar to polyurethane except that the proportion of MDI is higher and different blowing-agent gases are used for the expansion reaction.

It's a thermoset, closed-cell, rigid foam plastic insulation manufactured in board form. Via a continuous lamination process, liquid raw materials that expand and become light (yet strong) are applied between engineered facing materials providing its strength, improved rigidity, and enhanced thermal performance.


Polyiso is used in roof, wall, ceiling, and specialty applications within commercial and residential buildings of all types.

Thermal Performance and ASTM Standards

Polyiso panels are rated with an R-Value of R6.0 - 6.5 per 25 mm (1-inch) when tested according to the standard ASTM methods performed at a temperature of 24° C (75°F) as regulated by Federal Rule.

Polyisocyanurate insulation can also contribute to a more fire-safe home or building.


When Polyisocyanurate is submitted to temperatures below 10° C (50°F), it seems to perform worse than it does at the ASTM standard. According to Chris Schumacher, an engineer and researcher at Building Science Corporation, the reason for this declining performance is that “the trapped blowing-agent gases start to condense at cold temperatures,” decreasing R-value thermal performance to practically nothing at -20° C (-4° F).

In cold weather, the XPS R-5.0 will therefore tend to outperform Polyiso R-6.0, but since the standard measurements favor the Polyiso, many builders might still tend to choose Polyiso based on their lack of knowledge of this drawback. This is another place to be careful when you're picking products yourself.

thick foam board insulation

Observations for Installation

Rigid insulation panels can be attached using screws with large washers for wood frames, with a special adhesive for foam boards or polystyrene construction adhesive for already paneled and solid walls, or by using caulk or spray foam to secure smaller pieces.

If the foil side should be placed in or out depends on whether you want to make the space inside warmer or cooler—if you want it warmer inside, the foil should face inside to reflect the radiant heat inside.

When installing house wrap with rigid foam, the house wrap should be installed under the rigid foam panels and not around to provide an added shield against the wind and the weather.

When taping the seams, use a purpose tape such as Astro-Foil but making sure the surfaces are all clean and dry and using a flat edge to ensure a perfect adhesion and a tight seal.

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