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If there is one frequently asked question about Epilay synthetic underlayments, it’s whether or not our underlayments are considered breathable. It’s an interesting question—but one thing to keep in mind is that for most underlayments, breathability isn’t a primary concern. We’ll show you why—and we’ll examine breathability ratings, plus what it takes to create a properly ventilated roof.

Underlayments and Perm Ratings

When addressing breathability, the stat you want to look at is the perm rating, which is short for permeability. This is the measurement used to factor breathability—and it is set forth by the ASTM E96 standard. Here’s a quick breakdown of ratings:

  • Less than 0.1: Class I impermeable vapor barrier (highly impermeable)
  • 0.1 to 1: Class II semi-permeable vapor barrier (relatively impermeable to semi-permeable)
  • 1 to 10: Class III permeable vapor retarder (Semi-permeable to permeable)
  • Greater than 10: Considered highly permeable, not a vapor barrier at all

For underlayment in general, a perm rating of around 0.1 allows for very little water vapor permeation, which would make the underlayment a non-breathable one. At the other end of the scale, underlayments with a perm rating of 9.5 are considered highly breathable, so these would be classified as breathable underlayments.

To put that in real-world terms, most synthetic underlayments have a perm rating that comes in somewhere under 1, making them impermeable. Asphalt felt, on the other hand, is typically considered semi-permeable, with a rating somewhere between 5 and 7.

Are Epilay Underlayments Breathable?

The short answer is no, they’re not! Among Epilay products, perm ratings are quite low. Epilay Superior features a perm rating of 0.154, Ultra has 0.326 and Platinum comes with a perm rating of 0.142, which classifies all of our synthetic underlayments as non-breathable.

If you want something super impermeable, then look to Plystik Plus. Its ASTM E96 perm rating is 0.03.

The Big Question: Are Breathable Underlayments Necessary?

Here’s another short answer: No, breathable underlayment isn’t necessary. Actually, it’s better to focus on creating a solid roofing ventilation system.

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 becomes 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 the 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.

In fact, it’s better to use an impermeable roofing underlayment because the focus should be to prevent water from getting in rather than letting water vapor back out. Underlayment is your second line of defense should the rooftop’s primary covering develop leaks, sustain storm damage, or fail in some other way.

So there you have it! Epilay’s line of underlayment features low perm ratings, which help protect the roof against excess moisture when the underlayment is exposed or when the primary roofing material develops a leak. 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. Letting water vapor out means creating a well-designed ventilation system for the roof.

If you’d like to learn more about how to choose the right Epilay underlayment for your roofing project or to find a distributor near you, please contact us.