Extruded polystyrene boards are probably your best bet in terms of home insulation material. Such insulation boasts exceptional characteristics at an affordable price point.

Characteristics of Extruded Polystyrene (Rigid Styrofoam)

Lightweight, Rigid, and Insulating Properties

To better delve into the details tailored to the characteristics of rigid Styrofoam, let’s start with the following:

Insulating Properties

The table above highlights the fact that extruded polystyrene (XPS) is 8 inches thick, with a thermal resistance (R-value) of 6.5. As for thermal effusivity, a decent insulation value starts below 200, whereas thermal lag starts at 8 hours. Given those figures, XPS is rather worthwhile. 

However, in terms of thermal diffusivity, extruded polystyrene isn’t as efficient given that the borderline value for materials to be considered effective is 0.6. Nonetheless, compared to other insulation materials commonly found, extruded polystyrene is the best insulator on the market, followed by next-generation polyurethane. 

XPS is all the more worthwhile than next-generation polyurethane since it’s commonly found in all hardware stores.

Rigidity

In terms of rigidity, it’s measured based on its Lamé parameters. The latter determines the shear strength of a material, which is measured in kPa. The test method is better known as an in-plane shear (IPS) test, standardized as such: ASTM C273/C273M-11. 

Here are the entire sets of values regarding extruded polystyrene rigidity:  

• Compression force: 250 kPa 

• Tensile strength: 450 kPa 

• Shear force: 250 kPa 

Its Lightweight Factor

Still missing from its list of characteristics, is its lightweight factor. Extruded polystyrene has one of the lowest densities of any insulation material retailed in hardware stores—33 kg/m³.

Its density is remarkably low, so much so that in Japan, extruded polystyrene boards are favoured during construction, as such material mitigates seismic shocks passing through structures.

Type 4 Extruded Polystyrene Foam and its Specificities

Take the following table into consideration to really appreciate the technical specificities of Type 4 foam. 

Type 4 foam is more efficient compared to other types of insulation boards. It’s mainly used in areas requiring superior density and heavy load resistance:

• Slab foundations 

• Parking lots

• Protected membrane roofing systems

• Flat roofs  

Type 4 foam is marketed by some of the largest manufacturers, such as Soprema, which retails Sopra-XPS 40, standard CAN/ULC S701.1 compliant foams. 

What is the difference between expanded and extruded polystyrene?  

Similarities are few and far between. Here’s a chart detailing the insulation efficiency differences between extruded polystyrene (XPS) and expanded polystyrene (EPS): 

To crown the best insulator, extruded polystyrene solely takes a backseat to expanded polystyrene on account of its effusivity. In all other areas, it surpasses the latter. 

What about in the long run? Note that both materials are as efficient as mentioned in the table when new, but over time, any given material will lose some of its efficiency. 

Note that XPS wins it in terms of long-term durability too. As such, researchers from the University of Alaska Fairbanks studied the R-value of EPS and XPS during a wear period of 1 and 31 years. Here's a rundown of the results: 

• EPS must be 1.5 to 2 times thicker than XPS to achieve the same R-value.

• Standards ASTM C272, D2842, and C578 don’t account for the decreasing R-value long-term in real-time use.

• New-generation EPS isn’t more efficient than its former version.

• Moisture absorption further negatively affects EPS compared to XPS.  

In other words, they’re both excellent insulation materials, but extruded polystyrene is far better than expanded polystyrene. 

Extruded Polystyrene and Floor Insulation

How to Secure Extruded Polystyrene Boards

The key to efficient thermal insulation is a sturdy fit and optimal adherence. Let’s take a moment to explore the fundamental steps of a proper installation.

Step 1: Level your floor

Make sure the floor on which the extruded polystyrene boards are installed is perfectly flat. If not, level the floor using a self-levelling cement. 

A flat floor doesn't have any bumps or recesses exceeding 0.19 inches (5 mm) along a 6.6 foot (2 m) long surface, or 0.07 in (2 mm) along a 7.87 in (20 cm) distance. 

Step 2: Install XPS boards

To insulate a floor, simply lay the boards down and trim them accordingly to ensure the entire floor space is properly covered. We’ll delve into how to properly cut your polystyrene boards down below. Don’t be too surprised if the boards are identically shaped. Depending on how they were stored by retailers and then in your home prior to being used, the edges can be more or less straight. To ensure proper weatherproofing, you can fill in any gaps between two boards with spray foam. 

Step 3: Trim excess foam and clean floor

If you’ve filled in the gaps with foam, use a hand saw to trim any excess foam for a smooth surface. 

Once the boards are installed, vacuum the surface to ensure it’s clean prior to installing your new flooring atop the insulation.

What glue should you use with extruded polystyrene?

Polyvinyl acetate (PVAc) is the best type of glue paired with polystyrene. It’s a water-based glue with a neutral pH that’s easy to apply (paintbrush or roller) and clean, but also compatible with: 

• OSB panels

• Wood 

• Drywall panels

• Fabric

• Cardboard

• MDF

How to Cut Extruded Polystyrene Boards Down to the Right Thickness

Polystyrene boards are often cut using a hand saw meant for either wood or plastic. Such a tool will give you a straight and accurate cut, but depending on your skill level, the incision might:

• coil;

• generate a decent amount of dust; 

• create static electricity; and

• produce a lot of debris.

Therefore, saws aren’t the best tool, especially since they tend to create spongey or granular-like edges. 

Instead, use a $6, 9 mm utility knife for a straight, dustless, and accident-free cut. Avoid using a bigger utility knife, the result won’t be as clean.

For the perfect cut, make the first incision where you intend on cutting, then make another, deeper cut with the blade, separating the material into two parts. 

Extruded Polystyrene Coating and Paint: What to Know

You can paint extruded polystyrene boards, just not with whichever type of paint. Solvent-based paints attack and dissolve polystyrene foam. 

To paint a polystyrene board, turn to water-based coatings, meaning:

• Vinyl paints 

• Acrylic paints 

If you have to apply a solvent-based paint, use a water-based paint as a primer base to protect the material. 

Rigid and Extruded Insulation Board: Products & Costs

There are numerous brands of insulation boards. However, in retail stores, you’ll mainly find Sopra-XPS by Soprema. The price point will obviously depend on material characteristics: 

Extruded Polystyrene Insulation: Wrap Your Home in Tomorrow’s Energy

Extruded polystyrene insulation is a major contributor to modern construction's energy revolution. Its thermal efficiency, durability, lightweight and low maintenance factors are all advantageous characteristics that make this material the go-to choice. Whether used for residential, commercial, or industrial construction, extruded polystyrene has reliable efficiency and contributes to reduced energy use and fewer greenhouse gas emissions.