A good vapor retarder should not be underestimated; without facing, fiberglass insulation is highly susceptible to moisture infiltration, which can eventually wreak havoc on a building’s envelope. Vapor retarders are intended to help control the diffusion of water vapor through an assembly such as a wall or roof. To be considered a vapor retarder, the material should have a permeance or “perm” rating less than 1.0 as tested in accordance with ASTM E-96. The lower the perm rating, the better the ability of the material to control the diffusion of water vapor.
For metal building systems specifically, the challenges of using the proper vapor retarder presents their own unique set of design considerations. In many cases, the vapor retarder facing is laminated to the fiberglass insulation and this composite is installed either between the roof and wall sheets and the secondary framing, or it is installed between the roof purlins and wall girts. It can also be installed in the cavity of the purlins and girts. For the majority of these types of applications, the vapor retarder also functions as the final interior finish. Because of this, the following should be considered when choosing the right vapor retarder:
- Perm Rating — The rate of water vapor passage. A vapor retarder should have a perm rating no greater than 1.0.
- Air Permeance — In many applications, the vapor retarder can be an integral part of an air barrier system in a metal building. To be considered an air barrier it should be tested in accordance with ASTM E 2178 (Standard Test Method for Air Permeance of Building Materials) and have an air permeance not exceeding 0.004 cfm/ft2 which is the target of the latest energy code requirements for materials. Controlling air infiltration and exfiltration is becoming recognized more and more as an area that offers substantial energy savings opportunities. All seams and joints need to be sealed and special attention to construction details involving the vapor retarder need to be addressed.
- Fire/Smoke Rating — For exposed application and use in return air plenums*, an ASTM E84/UL-723 Flame Spread and Smoke Developed rating of 25/50 is required on the faced insulation composite, which includes the fiberglass, vapor retarder facing and laminating adhesive. This is a critical fire rating for safety and building code compliance. Users should always make sure that they are receiving material that is tested in accordance with ASTM E84/UL 723.
- Temperature Resistance — Most vapor retarders for metal building systems have very good ratings for both low and high temperature resistance. They should be rated to at least 0 F for low and 140 F for high. Installation in colder climates can cause damage to vapor retarders that do not have a proper low temperature resistance.
- Strength and Toughness — A vapor retarder can carry the proper ratings for permeance and fire, but it may vary drastically when it comes down to actual performance. An important consideration when choosing the strength of a vapor retarder is installation, as the process can be stressful on the facing. Additionally, the use of the building plays an important role as well. A gymnasium or tennis facility, for example, will require an exposed vapor retarder that has very high puncture resistance and tensile strength. It is far more prudent to pay a few cents more per square foot for a heavier, more resilient product than to have to go back and repair or replace an inferior one.
- Appearance and Light Reflectivity — Choosing a bright white product with high light reflectivity not only looks appealing, but it can also reduce lighting loads. An 80 to 85 percent light reflectivity rating is available with most vapor retarders.
- UV Resistance — Most currently available vapor retarders have UV inhibitors to reduce the chance of damage from ultra violet rays, which are produced from sunlight as well as select lighting fixtures. Special attention should be paid when “up” lighting is utilized and where the vapor retarder is exposed to direct and reflected sunlight. WMP-UV has been designed for use in interior applications where high light exposure is anticipated.
Installation
Vapor retarders should be placed on the warm side of the building assembly, which is typically the inside of the building. An exception might be an air-conditioned building in a high humidity/high temperature climate zone where keeping moisture from entering the building becomes an important design consideration. In this situation, the vapor retarder might be placed on the exterior of the assembly. This same logic would apply to ice arenas or cold storage in warmer climate zones.
Important Considerations
As important as the vapor retarder design is for any building, it will only function as well as the HVAC design allows. The mechanical system must be properly designed, and just as importantly, the building owner must be trained to the proper use of the system.
Specialty Vapor Retarders
Not all vapor retarders are created equal. Before selecting a vapor retarder, purchasers should consult with an insulation specialist regarding the needs of the project, as well as application specifications. There are many different types of vapor retarders, and buyers should be aware of which facings are best for their job.
Low Perm Facings
Typically, a low perm facing has a rating of 0.02. The lower the perm rating, the better the water vapor permeance. Most builders choose low perm facings because they avoid moisture problems more effectively than high perm facings. Additionally, there is typically very little cost difference in low vs. high perm vapor retarders, so low perm facings are the better value.
Bridget Mahovlic is the marketing manager at Therm-All, North Olmsted, Ohio. For more information about Therm-All’s selection of vapor retarders, visit www.therm-all.com. For technical information, visit www.lamtec.com.
*Always consult building officials for applicable codes when installing materials in return air plenums.
