If there is one frequently asked question about Epilay synthetic underlayments, it’s whether or not our underlayments are considered breathable. The short answer is no, Epilay underlayments are not considered breathable! But as it turns out, breathability isn’t a primary consideration for a roofing underlayment. Let’s take a look at breathability ratings, and then we’ll show you why proper roof ventilation is more important than breathable roofing products.


What are Perm Ratings?

The perm rating, which is short for permeability, is how underlayments are measured for breathability. For synthetics, a perm rating around 0.1 allows for very little water vapor permeation, so these are considered non-breathable underlayments. At the other end of the scale, synthetic underlayments with a perm rating of 9.5 are considered highly breathable, so these would be classified as breathable underlayments.

Among Epilay products, perm ratings are quite low. Epilay Superior features a perm rating of 0.154, Utra has 0.326 and Platinum comes with a perm rating of 0.142, which classifies all of our synthetic underlayments as non-breathable.

Are Breathable Underlayments Necessary?

Breathability is one of those buzzwords that gets a lot of attention, but there are a few misconceptions surrounding it. The primary purpose of an underlayment system is to protect the roof deck from the roofing proper, and to serve as a secondary barrier between the roof deck and the elements in the event that the underlayment become exposed.

Interestingly, even if you do opt to install a “breathable” underlayment, this does not mean that the roof is, itself, breathable. The fact is, the roofing material that is installed over the underlayment will make the roof non-permeable. So even when breathable underlayments are used, moisture won’t be able to evaporate away from the roof deck because the roofing serves as an impermeable barrier.

Designing a properly breathable roof comes down to one thing: ventilation. No matter the perm rating of your underlayment, the primary roofing material will prevent moisture from transferring out of an attic. The only way to remove that excess moisture is with a ventilation system designed to circulate air through the attic, thus removing moist air and allowing the attic to dry properly.

So there you have it! Epilay’s line of underlayments come with low perm ratings, which help protect the roof against excess moisture when the underlayment is exposed. But when it comes to prolonging the life of a roof, it isn’t the underlayment’s permeability that will help it go the distance, but the roof’s ventilation system!