If you’ve read our blog, you’ll know that we often call Exterra “breathable.” But have you ever stopped to wonder what this “breathability” refers to?
In this post, we’ll explain Exterra’s breathability in simple terms, so you’ll know precisely what you get out of this product in your wall assembly.
How Exterra’s Composition Affects Its Breathability
Exterra is breathable because its Neopor GPS foam core is permeable and is coated with a precision perforated polypropylene film. Thanks to these perforations in the laminate layer, Exterra can maintain a higher level of air and vapor permeance thanks to these perforations in the laminate layer.
This unique ability makes it a capable exterior continuous insulation layer and weather barrier. What’s more, this permanence gives vapour and moisture a path from the wall cavity to the outside.
Here’s a summary of Exterra’s permeance properties (up to a thickness of 1 ⅝ inch):
- Air leakage of 0.2 L/(s-m^2) @ 75 Pa
- Vapour permeance greater than 60 ng
Is Exterra a code-compliant vapor barrier?
No, Exterra is not a vapor barrier or a “vapor permeance material.” According to the National Building Code 2010 (NBCC Section 126.96.36.199(1)), vapor barriers can not have a permeance of over 60ng. Since Exterra exceeds this benchmark, it’s breathable and not a vapor barrier.
And because Exterra is not a “low air and vapor permeance material” per the building code definition, wall assemblies with Exterra require a poly vapor barrier on the warm side of the insulation. This means that you can use Exterra as a code-compliant, continuous insulation AND weather barrier on the exterior face of a wall assembly.
What do we mean when we say that Exterra is “breathable”?
In simple terms, Exterra’s “breathability” relates to its ability to let vapor and moisture escape a wall cavity to the outside.
As we’ve discussed above, the code requires wall assemblies to have the controlling low air and vapor permeance material on their warm side. So, with a poly vapor barrier placed inboard of the Exterra sheet, the vapor, and moisture that accumulate inside the wall have an easier path outside than inside the building.
Keep in mind, however, that the vapor/moisture must also pass through the wood sheathing before flowing through the Exterra. All these weather barriers in the vapor’s path affect the wall assembly’s drying rate.
To this end, be sure you factor these materials in when estimating the rate at which your wall assembly will dry.
Wrapping It Up
Halo’s Exterra is a breathable rigid insulation product — meaning, it allows the wall cavity to “breathe,” or expel vapor and moisture to the outdoors. And it’s all possible because Exterra’s laminate is perforated, giving it sufficient permeance to let air and vapor pass through.
Thanks to its ability to help the wall breathe to the outside, Exterra makes an ideal continuous insulation product to use on a wall’s exterior. Luckily, the perforations in its laminate are not large enough to let water seep into the cavity, so Exterra is also water-resistive.
And, of course, Exterra offers stellar thermal resistance properties, with high, long-lasting R-Values and a durable GPS foam core.
Just remember — to take advantage of Exterra’s breathability, you’ll need that poly vapor barrier on the warm side to block vapor and moisture from traveling inside the building. It’s also a code requirement.