POLYURETHANE vs POLYSTYRENE

The question always arises “What’s the difference between Polyurethane and Polystyrene”.

The review below is a general comparison and does not necessarily represent a particular manufacturer’s product.

There are 5 main differences that need to be evaluated.

1. R-VALUE .

Urethane has an R-value of 7.0 to 7.5 per inch, whereas Styrene has an R-Value of 3.5 to 4.0 per inch. The R-Value for both materials may vary by the manufacturer and density of the material.

2. CHEMICAL RESISTANCE.

Urethane is resistant to damage caused by other chemicals, whereas Styrene is subject to attack by petroleum based solvents such as : gasoline, many types of insect sprays and common glues such as “liquid nails”. These problems would normally not affect a building unless there is an insect infestation that must be treated.

3. FIRE DAMAGE.

Urethane is a thermal set plastic, so it will not melt. It is not damaged until the temperature reaches 700-800 degrees and then it “chars”. SIP’s use Urethane that has been treated with a flame retardant and has been tested as a Class 1 foam. A Class 1 foam means that when burned, an external heat source is required to sustain combustion. If left on its own the foam will self extinguish? Styrene on the other hand will soften at 190-220 degrees and melt at temperatures of 240-300 degrees. Many Styrene products are also classified as a Class 1 foam . They will not sustain combustion, but will melt at a very low temperature. There is much debate whether the Class 1 fire test should apply to styrene.

4. DIMENSIONAL TOLERANCES.

Due to the manufacturing process, Styrene panels have the opportunity to have tighter tolerances for thickness than Urethane panels. Styrene cores are cut with a hot wire and then laminated to the skins. This cutting process should be very controlled. Urethane cores are injected molded into the skins. The expansion of Urethane is very dependent upon temperature, humidity and other factors. This expansion process is more difficult to control resulting in some thickness variations.

5. FOAM CUTTING TOOLS.

There is a difference in the two materials when routing out the foam for modifications. Styrene can be cut out using hot wire, due to the low melting point. Urethane cannot be melted and must be mechanically removed. This process requires a special router.