Single Bead vs. Double Bead – What’s Better For Lap Sealant?
Lap Sealant – Single Bead vs. Double Bead
An often overlooked aspect of sealant application is the use of a single bead or double bead of gun grade butyl sealant at concealed flashing laps and material laps. In this blog we will take a look at the reasons why two is better than one.
Why a Double Bead Matters
Most manufacturers recommend the use of a double bead of sealant for a couple of reasons. The first reason is to add a “secondary” sealant row as a failsafe in case the first sealant row fails. The second reason, and most misunderstood aspect for using two beads of sealant is due to the phenomenon of positive and negative pressure created by building envelopes.
Negative pressure inside of a building occurs when the pressure inside is lower than the pressure outside. Warm air in the building tends to rise up, resulting in negative air pressure in the lower areas. Appliances such as roof ventilated fans, range hoods, dryers as well as combustion appliances such as furnaces and fireplaces all draw hot air out of the building envelope, which can create negative air pressure if the exhaust too much air.
The reason a double bead of sealant is so important in certain material laps is a direct result of positive and negative pressure created within building envelopes. The following diagrams illustrates the potential effects of how positive and negative pressure can impact concealed sealant on flashing and material laps.
Utilizing a single bead means that both positive pressure from outside of the building envelope and negative pressure from inside the building envelope are exerting their forces on a single bead of sealant. This can result in sealant failure as the sealant can be drawn out of the lap creating pinholes or other gaps in the sealant bead
Utilizing a double bead of sealant provides what is known as a pressure equalization zone between the beads of sealant. Utilizing a double bead of sealant prevents both positive and negative pressure form hitting a single bead of sealant at the same time. This eliminates the risk of sealant being drawn out of the lap and creating pinholes or other gaps in the sealant bead that can occur using a single bead of sealant.
In conclusion, utilizing a double bead of gun grade butyl sealant at concealed flashing laps and material laps can not only provide you with a double level of protection, but also help reduce the risk of sealant being drawn out of the lap and creating pinholes or other gaps in the sealant bead.
Read our Sealant Type blog for additional installation tips and info.
Jeff Haddock
Technical Services Manager, AEP Span
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