Tyvek Low Perm vs. High Perm - Building Science Bulletin - Vapor Permeability Explained

Oct 13, 2026

Upload by Rowan

Building Science Bulletin explains the practical meaning of vapor permeability by comparing high-perm and low-perm housewrap behavior within a wall weather resistive barrier (WRB) system. It details four core WRB functions—air resistance, water resistance, moderate-to-high vapor permeability, and jobsite durability—then focuses on why permeability drives wall drying performance. It clarifies vapor vs bulk water, summarizes typical code targets (about 5 perms), discusses modern energy needs, and presents DuPont Tyvek’s engineered nonwoven structure for moisture management. The text also introduces permeability measurement approaches including laboratory ASTM E96 and real-world evaluation concepts.

LanguageEnglish

CategoryGeneral

FormatPDF

Pages6

Reading Time2 min

AccessFree to download

Understanding High Perm vs. Low Perm

WRB functions and why permeability matters

What is vapor permeability?

Vapor permeability vs bulk water

Building code targets and modern construction needs

What is DuPont™ Tyvek®

Measuring permeability: ASTM E96 and field evaluation

Building Science Bulletin Understanding High Perm vs. Low Perm High perm(eability) vs. low perm(eability) within the housewrap category…what does it mean and how does it affect your building practices? The intent of this paper is to explain this property thoroughly so that a better understanding of this building science principle can be gained. The first step in understanding high perm vs. low perm housewraps is to break down the physical attributes of what a high quality housewrap can offer. A good weather resistive barrier (WRB) housewrap has 4 equally important functions: 1. It MUST have a high level of air resistance – to help prevent drafts, reduce energy bills and resist the flow of moisture laden air though wall cavities. 2. It MUST have a high level of water resistance – to help protect the wall cavity from water that gets behind the cladding. 3. It MUST have moderate to high vapor permeability – to promote drying in wall systems. 4. It MUST be durable-to withstand the rigors of the construction site and continue to perform once construction is completed. While all 4 properties are important, it is permeability that is probably the most ignored and the least understood function, yet it can have the greatest impact on how a wall system performs. So let’s take a look at this critical property. What is Vapor Permeability? Vapor permeability (commonly referred to as breathability) is a material’s ability to allow water “vapor” to pass. This is often confused with the concept of holding out bulk water. So what’sthe difference? Bulk water is moisture in liquid form; water vapor is a gas. Also, the amount of moisture that moves and the method of movement are significantly different. A single drop of water consists of thousands of molecules. In general this can be compared to a crowd of people who do everything together. This is the way they like to travel. Now just imagine this crowd of people trying to go through a door all at the same time. They won’t fit so no one gets through. But just suppose everyone in the crowd spreads out, and in an orderly fashion and one-byone, they attempted to exit. Some of them would get out. Resists air penetration Resists water penetration Allows moisture vapor to pass through Figure A In just the same way, when thousands of molecules of water get behind cladding, they can’t fit through the small pores in the housewrap. When you have moisture vapor in your wall, one molecule can break away from the pack and is able to escape by diffusion. When a material allows these individual molecules to pass we call it vapor permeable. Building codes require that WRB’s be vapor permeable and establish the minimum allowable level to be around 5 perms. This value is based on traditional building practices and does not necessarily address the requirements of modern energy efficient construction, new cladding systems and materials. DuPont Building Scientists believe that 5 perms are not enough to ensure the kind of consistent performance you need from a WRB. DuPont suggests that a WRB have a moderate to high vapor permeability. So why is vapor permeability important? Because wall cavities do get wet, roofs leak, condensation occurs, plumbing leaks, construction materials are installed wet and internal moisture loads can be very high. However it happens, walls get wet and require a way to dry out. When a wall can’t dry out, it becomes vulnerable to moisture-induced damage including mold and rot. DuPont Building Scientists believe in designing and building forgiving wall systems. All WRBs produced by DuPont, are engineered to allow maximum drying while still maintaining the other critical properties. DuPont™ Tyvek® is very different from every housewrap on the market! DuPont™ Tyvek® weather resistive barriers help provide the right type of protection you need and require for your wall systems. Figure B – DuPont™ Tyvek® fibers (magnified 75x) form a strong web that main

FAQs

High-perm and low-perm housewrap are compared based on which key property?

They are compared primarily by vapor permeability (breathability), which describes how water vapor passes through the material while resisting bulk air and water penetration.

Why do building codes require WRBs to be vapor permeable?

Because wall cavities and assemblies can get wet and must dry out. If a wall cannot dry, it becomes vulnerable to moisture-induced damage such as mold and rot.

How is permeability measured in standard laboratory testing?

Permeability is measured using a moisture vapor transmission test referenced in building codes, using ASTM E96, which assesses how much moisture vapor passes over a 24-hour period.

Oct 13, 2026

Upload by Rowan