Extruded polystyrene (XPS) is rigid foam insulation produced by forcing molten polystyrene with a blowing agent through a die, forming continuous boards with a fine closed-cell structure and a smooth skin. The best-known brand, Dow's Styrofoam, was patented in 1944, and the name is often used loosely in North America for all polystyrene foams.
Properties
- Density: about 28-34 kg/m3 (1.7-2.1 lb/ft3).
- Thermal conductivity: about 0.029-0.039 W/(m.K), averaging roughly 0.035.
- Vapour resistance factor: about 80-250, higher than EPS.
- Water resistance: low absorption compared with EPS, which is why XPS is chosen where insulation will be wet, though prolonged immersion or humidity can still raise moisture content.
- Compressive strength: grades are made for foot traffic, paving, parking decks and green roofs.
Use in inverted roofs
XPS is the standard insulation for protected membrane (inverted) roofs, where it sits above the waterproofing, is wetted by rain, and is held down by ballast, paving or soil. Boards often have shiplapped edges, and a water flow reducing layer may be laid on top. Design must allow for rainwater cooling and include ballast to prevent flotation.
Blowing agents
XPS was historically blown with HFCs such as HFC-134a, which have high global-warming potential. In Europe, where HFC-134a was banned in XPS from 2022, manufacturers use carbon dioxide, giving a global-warming potential below 5.
Fire
Like EPS, XPS is combustible and is typically protected by ballast, paving or a non-combustible layer on roofs.
Trademark use
Strictly, Styrofoam refers only to Dow's extruded closed-cell product, which Dow developed from a method first patented in the early 1930s by Carl Munters.
Sources
- PolystyreneWikipedia · en.wikipedia.org
- Inverted roofDesigning Buildings Wiki · designingbuildings.co.uk
- Flat roofWikipedia · en.wikipedia.org