What is Styrofoam?
Styrofoam is a brand name for extruded polystyrene (XPS): rigid insulation boards with a closed-cell structure that resist compression and absorb very little water. The foam was developed at Dow Chemical in the 1940s and the brand now belongs to DuPont. For decades it was recognisable by its light blue colour; after a reformulation, DuPont now makes it in grey in North America.
In the United States “styrofoam” is used loosely for disposable cups, takeaway boxes and packaging foam. Strictly that is wrong: those are made of EPS (expanded polystyrene), a different product.
XPS or EPS
- EPS is made of fused beads; you can see the little balls, with small voids between them.
- XPS is extruded into an even board of closed cells, so it takes up far less water and is stronger in compression.
- The thermal conductivity of XPS is roughly 0.025 to 0.035 W/(m·K), depending on product and thickness. Always use the declared value from the manufacturer.
| EPS (expanded polystyrene) | Made of fused beads; you can see the little balls, with small voids between them |
|---|---|
| XPS (extruded polystyrene) | Even board of closed cells; takes up far less water and is stronger in compression; thermal conductivity roughly 0.025 to 0.035 W/(m·K), depending on product and thickness |
In practice
- Use it where it is wet and loaded. XPS is the standard choice for inverted roofs (insulation above the waterproofing, held down by ballast), perimeter insulation against basement walls and foundations below ground, and under loaded floors.
- Use a solvent-free adhesive or sealant. Solvents in some adhesives, sealants, bitumen products and thinners dissolve polystyrene. Use only products the manufacturer approves for XPS.
- Stagger the joints. This avoids continuous gaps and cold bridges; rebated boards fit more tightly.
- Cut cleanly. A hot-wire cutter gives crisp edges; a fine-toothed saw or knife works on thinner boards.
- Keep it out of the sun. XPS is not UV resistant, so do not leave boards exposed for long.
- Always cover it. Polystyrene is combustible, melts and drips. Never leave it exposed inside a building; cover it as the manufacturer’s system specifies.
Common mistake: fixing XPS with an ordinary solvent-based grab adhesive or bitumen. The board is eaten away on contact and the bond fails.
Related terms
- Polystyrene
- EPS
- Compression-resistant insulation
- Inverted roof
- Perimeter insulation
Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.