I get asked this question all the time, but the answer is much more complicated. Its fun to go back historically and look at why insulation was introduced into roofs in the first place. It was fairly simple back then, the waterproofing was to keep water out and the insulation was to keep people warm. Today, insulation is so much more and that is why to answer which insulation is the best, we need to look at the most important attributes of the two most popular roofing insulations - Polyisocyanurate and Expanded Polystyrene.
R-value is more than the R-value per inch. Polyisocyanurate (ISO), starts out with a higher r-value per inch, but then the blowing agents escapes and the R-value is reduced. (learn more) Expanded Polystyrene (EPS) has a lower R-value per inch but can be make thicker to match any R-value requirement. EPS’s R-value increase in colder temperature while ISO decreases. Many would give ISO the edge because of R/inch, but taking into consideration overpaying for R-value that does not stay stable in lower temperature, the advantage has to good to EPS. ISO uses a “harmful” blowing agent which escapes into the surrounding area and no one knows for sure what the health effects are on humans. EPS does not use harmful blowing agents and is known as the “safe insulation.” EPS insulation is also 100% recyclable where ISO is not. EPS is by fair the greener product. (learn more)
This gets complicated because EPS has a fire retarder that is in the product and thermal boards, like drywall can be used to make it even more fire resistant. Remember, EPS can go Direct to Deck and has UL approved. ISO by its very chemical makeup is more fire resistant. The question maybe, if ISO does catch on fire what is the chemical by-product that are produced when that product burns and how harmful is that to humans? ISO has the slight advantage on fire but EPS has the advantage on by-product. (learn more)
You can’t have a material discussion without looking at the cost element. This is not even close EPS is a much better value than ISO any way you measure, including cost per R-value inch. (learn more)
Roof design can be looked at in numerous ways such as thickness restrictions and tapered possibilities. ISO is very limited in thickness per board. High R-value systems make ISO more labor intensive as it need so many layer to comply. One EPS board can go up to 200 R. ISO is limited in tapered slope possibilities usually 3. EPS on the other hand is almost unlimited. EPS can use insulation shapes (example rounded) that ISO just can’t match. Design flexibility goes to EPS.
Roof leaks can cause moisture to enter into a roofing system. When exposed to the same test as EPS, ISO absorbs much more moisture and has a difficult time expelling it. The ISO glass facers are even more prone to moisture absorption. EPS can absorb moisture but two anomalies make it so desirable - one it can expel moisture and two moisture has little effect the R-value. EPS has the advantage. (learn more)
Expanded Polystyrene, when compared to polyisocyanurate, certainly has more reason to be use in roofing. It seems over the last few years, many designer have lost sight of the purpose of insulation and the importance of R-value long-term. Some designer only considered combustibility and its superior importance, when in all likely hood this physical property will never ever be used. A feature such as long-term R-value, which performs daily in that system, or even moisture expelling capability, get lower considerations. When all of the major features of insulation are considered, EPS seems to be the clear winner.