Insulation is a critical component to consider when designing a functional, cost-effective, and energy-efficient building. Two of the most popular polystyrene insulation solutions you’ll come across when looking into rigid foam insulation are expanded and extruded polystyrene (EPS and XPS). Just for reference, XPS stands for eXtruded Poly Styrene, Expanded Poly Styrene (or EPS) is the typical white polystyrene used to pack TVs and hardware that crumbles into little white balls. In this case, we’ll be talking about it in the context of insulation. Both materials have a closed-cell rigid structure and are made from the same base polystyrene resin. However, they both differ in crucial ways.
In order to fully understand the differences between their properties and performance, we must, first and foremost, take into consideration their manufacturing differences. Let’s take a look at each process below:
Extruded polystyrene insulation (XPS) is manufactured using a process of extrusion. This process results in a closed-cell structure that boasts smooth, dense skin on both faces. The closed-cell structure of extruded polystyrene (XPS) prevents water penetration, thus providing long-term strength and durability.
Expanded Polystyrene insulation (EPS) is manufactured using beads of foam within a mould. Heat or steam is then applied directly to the beads which cause them to expand and fuse together. This process produces a closed-cell structure but not a closed-cell insulation board. This is due to the fact that voids can occur between the beads.
Perhaps one of the most notable differences between these materials is their sustainability. Unlike XPS, EPS can be constructed with recycled content, deeming it the more environmentally friendly option of the two. However, this sustainability does not come without its disadvantages. The voids between the polystyrene beads of EPS also make it more permeable and susceptible to water absorption. And with the absorption of water, comes the dreaded decrease of R-Value (dulling down its already quite low thermal properties). Resisting water is critical for high-performance insulation due to the fact that water is an excellent conductor of energy. Water ingress will ultimately hurt the overall thermal effectiveness of the board since water allows energy to short circuit or bypass its way through the board’s structure; reducing the overall effectiveness of thermal performance.
It should be noted then, that EPS’ potential to absorb water is 10 times greater than XPS, which does not absorb water at all, due to its closed-cell structure. This enables it to maintain insulating power in the presence of water.
In general, when comparing EPS and XPS board densities, the compressive strength of XPS is greater than that of EPS. For EPS to achieve the same compressive strength as XPS, the density of the foam would need to be increased, often resulting in a greater thickness of the board being required.
Below, we will further note the advantages and disadvantages of each board.
EPS (Expanded Polystyrene):
This is the most basic insulation board as it offers the lowest R-value (meaning it has the least effective thermal performance).
- Cheapest insulation board
- Most environmentally friendly (does not use uses HCFC in its production)
- Not a closed-cell insulation board
- Allows water vapour/moisture to penetrate over time
- To achieve similar U-Values to PIR or Phenolic boards you’ll need a much thicker block of insulation
XPS (Extruded Polystyrene):
- Able to resist water and moisture due to its closed-cell structure
- Better suited to wet and moist environments where moisture is a challenge; such as cavity walls
- Better thermal insulation
- More tough and durable than EPS
- Greater compressive strength than EPS
- Cannot be recycled (less environmentally friendly than EPS)