Vapor barriers in post frame construction

Vapor Barrier Applications in Post-Frame Construction

Post-frame buildings or pole barns are popular for commercial, industrial, and agricultural purposes. Built on concrete slabs and often capped with metal roofing, post-frame construction can pose unique vapor barrier challenges. From slab to walls to roof, here are ways to protect your post-frame building from moisture by installing proper vapor barriers.

Vapor Barriers Under the Slab

A vapor barrier under your concrete slab is essential for moisture prevention. A vapor barrier under your slab will also protect you from harmful ground pollutants like radon gas. Post-frame buildings with metal roofs, and especially buildings with minimal insulation, hold warm, moist air. That air condenses on the cooler concrete floor. In larger commercial buildings and warehouses, that moisture can cause the flooring sealer and joints to fail. Often where water pools, the concrete often spalls and cracks. Moreover, where there is moisture, there is a potential for mold growth. Installing the proper vapor barrier beneath the slab will save you money, health, and aggravation.

Americover offers a variety of concrete slab vapor barriers and tapes, which meet ASTM E1745 specifications. This standard ensures the poly sheeting’s resistance to vapor migration and meets industry standards for tensile strength and puncture resistance. The most commonly used vapor barrier under concrete slabs is polyethylene (poly) plastic sheeting with a thickness of 10 mil or 15 mil.

Vapor barriers in walls

It is tempting to forego wall insulation when erecting a post-frame building in a warmer climate zone. Keep in mind the greater the temperature differentiation between the building envelope and the outside, the quicker water vapor in the warm inside will condense onto cooler surfaces. For post-frame buildings in colder climates, it’s best to insulate and protect that insulation and walls by installing a vapor barrier on the inside (the warm side) of the insulation.

Use 6 mil to 10 mil reinforced poly barriers on walls, and depending upon building usage and fire codes, you may need to use a fire-retardant vapor barrier. Install your vapor barrier on the inside of the outer wall between the interior surface (gypsum board) and the wood or metal stud framing. It’s also worth noting that vapor barriers are different than air barriers, such as Tyvek house wrap. House wraps are designed to stop the flow of air, but they allow water vapor to pass through. Even if you have an air barrier on the outside of your building, if you are in a colder climate, you still need to install a vapor barrier on the inside.

If you choose not to insulate your post-frame building, a vapor barrier is still useful in keeping out drafts. If the building has leaks at joints, an interior vapor barrier can help seal the building.

Make sure to use vapor tape to seal seams or any holes, allowing for at least 6 inches of overlap. Don’t underestimate small gaps. In a single heating season, a hole less than one square inch can allow eight gallons of moisture into the building envelope. Americover offers a variety of puncture-resistant wall vapor barriers and tapes. All meet or exceed permeance standards.

Radiant Barriers, Vapor Barriers, and Your Roof

One challenge with post-frame structures is that they often come with metal roofs. Like a car warming in the summer sun, a building with a metal roof traps heat. Two types of barriers are relevant to energy efficiency and moisture control in post-frame attics: radiant barriers and vapor barriers.

Radiant barriers, which are a type of bubble-foil insulation with a shiny aluminumized film surface, reflect radiant heat. Radiant barriers block heat in the summer and reflect that heat back into the building during cold months. Installing a radiant barrier can be helpful if your attic space is poorly insulated, if your roof gets direct sunlight, and/or if you have air-conditioning ducts running through your attic space. In cold weather, that reflective radiant barrier placed below the metal roof will result in an interior surface temperature greater than the outside temperature. As the inside air surface temperature increases, the conditions for condensation become less likely to occur.

A vapor barrier installed on the underside of the metal roofing is an important complement to any insulation system. Traditional fiberglass insulation without a vapor barrier properly installed can become saturated with moisture. Not only does the insulation then become less efficient, but it also turns into an environment for mold and mildew.

The Bottom Line

Control condensation in your post-frame building by installing proper vapor barriers.

What Vapor Barriers Should You Buy?

Polyethylene sheeting is available at most building supply stores. However, not all poly sheeting qualifies as a vapor barrier. Watch out for recycled or “regrind” polyethylene, which may contain impurities and become brittle. Sheeting that tears diminishes the effectiveness of a vapor barrier.

Americover’s virgin polyethylene sheeting is your best bet. Americover offers durable string-reinforced vapor barriers made from 100% virgin polyethylene, which come in 6 mil to 20 mil. If you’re doing a DIY project, don’t forget sealants such as vapor tape and butyl tape. Browse our:

If you have any questions, call our Americover specialists at, 760-388-6294 or email