Understanding Vapor Barriers: Key Factors for Determining Whether to Use Them with Insulation

A sustainable, energy-efficient building envelope requires various control layers within the building envelope, including the facade, a water penetration barrier, an air and water-resistive barrier, a thermal barrier with continuous insulation (CI), and a vapor barrier. Properly integrating the control layers ensures the structure's exterior will repel moisture, wind, ultraviolet rays, temperature changes, etc., to ensure a durable, efficient, and safe building.

For decades, stricter building codes and customer demands have prompted architects and builders to apply continuous insulation (CI) over the entire building envelope. At the same time, today's market also wants structures with long-term integrity and healthy air quality, which requires effectively controlling moisture within the building envelope, often by installing a vapor barrier.

A vapor barrier prevents water vapor from infiltrating the wall system by diffusion, which can lead to structurally damaging and unhealthy mold. However, because building codes don’t always mandate vapor barriers, many builders question when to use a vapor barrier with insulation.

NOTE: The long-term integrity of a wall system depends on the correct placement of the right type of vapor barrier. Polyguard strongly suggests that you contact a waterproofing professional, like the experts at Polyguard, on when and how to place a vapor barrier with insulation most effectively.

What is a Vapor Barrier?

Vapor barriers prevent vapor diffusion, which happens when moisture flows from a space of higher moisture concentration toward a lower concentration. It can also occur when water moves from a hot to cooler area within a building product, like insulation.

The ASTM E96 determines a product's capacity to restrict moisture from moving through it, and designates a Class based on its permeability. Vapor barriers (Class I) stop diffusion, and vapor retarders (Class II and III) slow vapor diffusion.

  • Class I vapor barrier: 0.1 perm or less

  • Class II vapor retarder: 0.1 < perm <1.0 perm

  • Class III vapor retarder: 1.0 < perm <10 perm

When to Use a Vapor Barrier with Insulation

Climate Consideration When Using a Vapor Barrier with Insulation

To keep water vapor out of the wall cavity, builders should place the vapor barrier on the side of the insulation that will face higher moisture and humidity levels.

  • Cold Climates: In cooler climates, where the humidity inside a structure is greater than the wall cavity, install a vapor impermeable vapor barrier before the drywall on the inside face of the insulation.

  • Mixed-Humid Climates:
    • If the structure requires more cooling than heating, locate a semi-permeable vapor barrier on the exterior of the insulation.

    • If the structure requires more heating than cooling, locate a semi-permeable vapor barrier on the interior of the insulation.

  • Mixed-Dry Climates: Place a semi-permeable barrier on the inside face of the insulation.

  • Hot or Humid Climates: In hot or humid climates, where the moisture level outdoors is higher than inside a building, install the semi-permeable vapor barrier on the outside face of the insulation.

Cladding Materials

Cladding materials that absorb moisture, like brick, stucco, wood, fiber cement, and stone, can lead to moisture problems within the wall cavity. Therefore, when using an absorbent cladding type, builders should apply a vapor barrier on the exterior face of the wall insulation to protect the cavity from moisture accumulation.

Polyguard UV2-40 for Climate Zones 1, 2, and 3A

UV2-40 is an impermeable weather-resistant vapor barrier that prevents vapor and bulk water infiltration into the wall cavity, to virtually eliminate the chance of condensation, rot, and mold - a superior vapor barrier solution for hot and humid climate zones 1, 2, and 3 A.

Case Study: Over ten years, building scientists conducted an extensive WUFI analysis on ten structures that utilized Polyguard UV2-40 barrier in hot and humid Austin, TX, Houston, TX, Miami, FL, and Birmingham, AL. The study produced the following results:

  • The study found no evidence of long-term moisture accumulation with individual building components.

  • The study found a predicted Mold Index below 3 (lowest visible) based on the latest VVT (Viitanen) model.

  • They found no evidence of long-term moisture accumulation in the wall assembly, which indicates that the barrier stopped air and moisture from inflating the wall system, and helped to create a longer-lasting, healthy, and energy-efficient building.


Straightforward and fast to apply, sunlight- and chemical-resistant UV2-40 barriers successfully adhere when applied according to the manufacturer's installation and performance requirements. UV2-40 is a 40-mil rubberized asphalt waterproofing membrane laminated to two strong polyethylene films with a top protective aluminum layer, which makes it a high-strength waterproofing product.

Its elastomeric nature allows it to stretch and not tear if the building moves slightly. The aluminum surface of durable UV2-40 also works efficiently with higher exposure buildings because it reflects heat, which reduces energy use.

The Importance of Knowing When To Use a Vapor Barrier with Insulation

Creating an energy-efficient, healthy, and long-lasting building requires knowing when and how to use a vapor barrier with insulation. Codes, climate, and cladding will determine the need for a vapor barrier.

The temperature and humidity levels inside and outside of a building will determine the vapor barrier's permeability and placement in relationship to the wall insulation. Properly installing a high-quality vapor barrier, according to the climate and cladding type, will help ensure a building's long-term integrity and health.

Contact our Polyguard professionals today for more on when to use a vapor barrier with insulation.