Q: I have a project where I will be installing an air barrier on an exterior wall. Are there any weather-related issues that I should consider?
A: All building materials are affected by swings in temperature and humidity. Panel products are affected by temperature and humidity changes, and drying times for water-based materials increase with rise in humidity. For example, a 100-foot gypsum panel will grow approximately half an inch when the temperature increases 50 degrees Fahrenheit. Further, that same panel will grow, again approximately half an inch, when there is a swing of relative humidity from 13 percent to 90 percent. Related to drying, the amount of time that it takes for drywall finishing joint compounds and plaster to dry is drastically altered by changes in relative humidity. For drying type joint compounds, the range can be from 18 hours at 60 degrees and 30 percent relative humidity, to 18 days at 60 degrees and 98 percent relative humidity. Although drywall finishing joint compounds should never be used on the external side of an exterior wall, that change in drying time is fairly common knowledge in our industry. The same environmental changes affect air barriers.
The following information is provided by Scott Wolff, building science manager for the Henry Company. A common fluid-applied air barrier that is used today is either a water-based acrylic or a silicone membrane. Manufacturers of water-based air barriers list an application temperature of 40 degrees Fahrenheit or above. Some manufacturers recommend avoiding application when ambient temperatures fall below 32 degrees for a period of 48 hours before or after application. If this cannot be avoided, tenting of the structure is advised. Some manufacturers produce lower temperature fluid-applied membranes with application temperature capabilities down to 20 degrees. Silicone membranes, while only available in vapor-permeable varieties, boast, in some cases, no minimum application temperature limitation.
During the spring and fall of each year, ambient temperatures often rise above a membrane’s minimum application temperature but may fall below this at night. When temperatures fall to the dew point (or below), surfaces recently sprayed or rolled with a water-based air barrier may experience varying levels of re-constitution of the membrane back to a fluid state. The result can end up being a “slumping” of the material off the wall. This occurs more readily with concrete masonry unit backup walls. CMU may already contain moisture that, when compounded with exterior-side dew point moisture collection, can re-constitute not yet fully cured liquid air barriers. It is important to understand how fluctuating temperature and humidity impacts the installation of an air barrier.
Like concrete and plaster, full liquid air barrier membranes cure. Also like concrete and plaster, the time the air barrier takes to cure is a function of temperature and humidity. Typical manufacturer’s data sheets suggest a two-hours dry time, meaning dry to the touch. A membrane considered “firm” dry can be reported as much at 24 hours after application. These data are not uncommon to the myriad available liquid air barriers. Another and equally important point is that dry times listed on manufacturer’s data sheets is often measured at 70 degrees and 50 percent relative humidity. Rarely is this experienced in the field. The farther the temperature drops below 70 degrees, the longer the cure time. Lower humidity, too, will hamper the curing process. As mentioned above with the drying time for joint compound, the difference in drying time with increased humidity can be measured in days, not hours. This is not uncommon in the air barrier world as well.
The process of spray applying fluid-applied membranes is temperature sensitive as well. Production in cooler weather can be an issue, so it’s recommended to keep liquid (typically water-based) material at or near room temperature. This is especially important when spraying fluid-applied membranes. Room temperature material simply flows more easily through hoses and helps alleviate nozzle clogs. Simply put, fluid-applied membranes should be kept at room temperature until just before use.
Storage of water-based materials, in many cases, should be kept above freezing. Failure to do so may render the material unusable. There are a small number of products that can be used after freezing. However, their temperature must be slowly brought up to the point of room temperature, that which is advised for optimum spraying performance.
A final thought is for the contractor to work closely with their preferred air barrier manufacturer. A lot of information can be found in the manufacturer’s published literature, specifically, in this case under what is called “Limitations.” Also, it is a prudent idea to request handling and training from a manufacturer’s field service technician. This is best done long in advance of production.
Robert Grupe is AWCI’s acting director of technical services. He can be reached at (703) 538.1611 or [email protected].
