Polyisocyanurate, often called polyiso, is a rigid foam insulation board offering the highest R-value per inch among common foam board products, typically faced with foil or another reflective material on both sides, making it a popular choice where maximizing thermal performance within limited thickness matters significantly.
This superior R-value per inch allows polyiso to achieve comparable thermal performance to thicker alternatives using less overall material thickness, a genuine advantage in applications where space constraints limit how much insulation thickness can be accommodated.
One notable characteristic of polyiso is that its actual R-value per inch decreases somewhat in very cold temperatures, a performance consideration worth understanding when comparing published R-values across different climate conditions.
What Does Polyisocyanurate Involve?
- Rigid foam board with the highest common R-value per inch
- Foil or reflective facing on both sides
- Some R-value reduction in very cold temperatures
- Common use in roof and exterior wall applications
Why Is Polyisocyanurate Important?
The superior R-value per inch polyiso provides makes it particularly valuable in applications where achieving high thermal performance within limited available thickness matters, such as roof retrofit projects or exterior wall continuous insulation where adding excessive thickness isn’t practical. This space efficiency has made polyiso a preferred choice for many high-performance building envelope applications.
Because polyiso’s effective R-value decreases somewhat in colder temperatures, this characteristic is worth factoring into insulation planning for very cold climates, where a different material or additional thickness might be needed to achieve the same effective performance polyiso’s published rating would suggest at moderate temperatures. This nuanced understanding helps ensure realistic performance expectations across varying seasonal conditions.
Polyisocyanurate vs. Extruded Polystyrene
Polyisocyanurate offers the highest R-value per inch among common foam boards but with some cold-temperature performance reduction. Extruded polystyrene provides more consistent R-value across temperature ranges and better moisture resistance, particularly valuable for below-grade applications.