Vapor retarders, air barriers, and roofing systems: What architects need to know

GAF partner content - Vapor retarders and air barriers

Manufacturers such as GAF, whose roofing products are shown here, can advise on designing roofing systems that manage air and moisture efficiently and effectively.

Which class of vapor retarder is best for not only blocking air but allowing moisture to escape? AIA partner GAF dives into the science.

The design, manufacturing, and construction industries are quite good at keeping bulk water and capillary water out of buildings. They have also brought tighter focus on the importance of keeping air out of buildings. Most notably with the 2012 International Energy Conservation Code (IECC), which first required all new buildings to include an air barrier.

The main purpose of air barriers is to keep conditioned air from escaping and exterior air from infiltrating, but air-leakage prevention also keeps the moisture in the air from passing into and out of buildings. Understanding how air and moisture move is crucial to designing roofing systems that not only perform well, but remain dry. If a system blocks air movement and vapor diffusion, any moisture that gets inside will be trapped, potentially leading to mold issues and degradation of the system over time.

Understanding heat, air, and moisture movement

The 2nd law of thermodynamics defines how heat, air, and moisture move. What it means in terms of building and roofing science is this:

  • Hot moves to cold
  • Moist moves to dry
  • High pressure moves to low pressure

This helps explain why warm, moist interior air (e.g., 75º F, 50 percent relative humidity) infiltrates up into a roof system when there is no air barrier. There can be other reasons why this happens, like the stack effect, membrane billowing, and internal pressurization from mechanical systems.

Choosing the right vapor retarder

With the new focus on air barriers, designers may be quick to add them to roofing systems. Recognize that all vapor retarders are air barriers, but not all air barriers are vapor retarders.  Some air barriers are vapor open and allow moisture to escape.

There are three classes of vapor retarders. The lower the perm rating, the less diffusion (i.e., less drying) occurs through a material.

• Class I, Impermeable (vapor-proof), ≤ 0.1 perm:

Built-up roofing membrane, perm rating 0.00-0.02

Single-ply membrane, perm rating 0.03-0.06

Polyethylene film, perm rating 0.06-0.08

• Class II, Semi-Impermeable, > 0.1 ≤ 1.0 perm:

Asphalt felt, perm rating 0.3-0.8

Polyiso roof insulation, perm rating 1.0

Extruded polystyrene, perm rating 1.0

• Class III, Semi-Permeable, > 1.0 ≤ 10.0 perm:

Expanded polystyrene, perm rating 1.2

Wood fiber, perm rating 3.0-5.0

Most roofing membranes, as well as a single-layer, self-adhered, bituminous sheets, are Class I vapor retarders. However, if a Class I vapor retarder is used, there is concern that any moisture (e.g., construction moisture due to installation methods, bulk water from weather, etc.) that enters a roof system won’t be able to dry out. In many cases, a better option might be a Class II or Class III vapor barrier, which will allow for some amount of drying from diffusion, similar to the way walls are now being designed. Exceptions to this idea include roofs over indoor swimming pools or other high-humidity-producing activities, and over new concrete roof decks to prevent the moisture in the concrete from drying into the roof system.

When we use a vapor retarder in a roof system it will also act as an air barrier as long as it is sealed at all perimeters and penetrations and is tied to the wall air barrier. This eliminates the need for a separate air barrier that could potentially prevent the moisture from escaping.

A roof design that includes an adhered roof membrane with multiple layers of insulation (with board joints offset and staggered) over a vapor retarder/air barrier helps lower the risk that air—and the moisture it carries—will infiltrate the roof system. That reduction of air and moisture infiltration can then help improve roof longevity.

Another option for the roofing system is to recognize a hardboard or sheathing layer can be a vapor open air barrier. A gypsum-fiber board has a perm rating of approximately 24 to 30 perms, depending on thickness, and if this board is fastened to a steel deck and the joints and transitions are taped, it could be an effective air barrier that allows some drying.

Above all, during the design and specification process, it’s important to work closely with product manufacturers, like GAF, who understand the building science behind roofs and ensure the products and systems work together as a system to manage air and moisture efficiently and effectively.

For the latest trends and challenges in building science, check out GAF’s ProBlog. James Kirby, AIA, can be reached at james.kirby@gaf.com.

AIA does not sponsor or endorse any enterprise, whether public or private, operated for profit. Further, no AIA officer, director, committee member, or employee, or any of its component organizations in his or her official capacity, is permitted to approve, sponsor, endorse, or do anything that may be deemed or construed to be an approval, sponsorship, or endorsement of any material of construction or any method or manner of handling, using, distributing, or dealing in any material or product.

Image credits

GAF partner content - Vapor retarders and air barriers

Slyworks Photography

Recommended on this topic

Topic

Another minor downtick in architectural firm billings

Lorem ipsum dolor sit amet, consectetur adipiscing elt. Cura bitur amet et commodo turpis ...

Topic

Another minor downtick in architectural firm billings

Topic

Another minor downtick in architectural firm billings

Lorem ipsum dolor sit amet, consectetur adipiscing elt. Cura bitur amet et commodo turpis ...

Recommended products

From our store

Earth Water Air Fire Book

From our store

Earth Water Air Fire Book

From our store

Earth Water Air Fire Book