Drainable blades capture water before it can infiltrate the louver. Drainable features give that water an exit. Learn more about these features on the MCDLG Newsstand. 

Your building will have ventilation openings to allow fresh air into your HVAC system. You can protect those openings with louvers. Louvers can be equipped with many types of blades, which can have features that help reject unwanted elements. The top concern is preventing water infiltration. Rainfall can cause serious water damage to ductwork and other HVAC equipment, if left unchecked. Adding blade features will increase resistance to airflow.

Louver blades are typically installed at an inclined angle to deflect incoming rain. The exact angle will differ depending on the louver. Blade angle will typically be between thirty and forty-five degrees. Air easily passes over the blade and through the louver. Heavier elements, like rain and dust, will have a tougher time moving across the blade. During light rainfall, droplets will naturally slide off the slope of the blades. A greater blade angle may affect air performance since the air will need more speed to pass over the blades. However, this angle will also make it more difficult for water to infiltrate the louver. This creates a natural barrier against light rainfall, but this protection can fail in heavy rain. Water can collect on the louver's blades if the rate of rainfall increases. With nowhere to go, this excess water can infiltrate into the duct. The problem with a standard louver blade is that the collected water has nowhere to go.

Louver blades are set at an incline to reject rain as it hits the front of the louver. This design works for light rain, but it may need more to reject heavy rain. 

Most blade types can have drainable features, like catches, face troughs, and channels. Any blade with these features is called a drainable blade, and louvers equipped with these blades are drainable louver. The primary purpose of a drainable louver is to prevent rainwater from infiltrating the ventilation opening. Drainable louvers provide a pathway for captured water to drain out of the louver. As the water collects on the blade, the drain troughs and blade channels will direct the water towards channels inside the louver’s jambs. The jamb channels drain water down towards the louver’s sloped sill. Then, the water slides down the sill and out of the louver’s face. Drain catches can also capture water as it passes over the blade, directing it to the troughs on the face of the blade.

Straight blades, baffle blades, and chevron blades can be drainable blades. Adjustable blades can have drain troughs along the face side of the blade. Adjustable drain catches will align with channels in the jambs when the blades are set to the fully open position, channeling water out of the louver. 

The drain trough captures water along the face of the louver. From here, these troughs can direct rainwater to channels in the jambs.

Accessories provide additional tools for water rejection. Drain pans sit under the sill of the louver and capture excess water that drains out of the louver. They have an open face for water to spill out and a raised backplate to prevent water from spilling back through the louver’s sill. Drain pans are optional for drainable louvers and severe weather louvers, but they come with hurricane louvers as a standard feature. This design works well for louvers installed flush with the opening. For recessed louvers, rejected water will collect along the space between the louver and the edge of the ventilation opening. Sill extensions provide a pathway for this water by extending the sill to the edge of the opening. It is sloped, just like the louver’s sill, so that water can easily slide down the ramp and out of the opening.

Choosing the best louver is a balancing act between protecting the opening and allowing airflow in. A smooth blade profile will provide the best air performance, but it won’t protect against rainfall like a blade with catches. Drainable blades are not designed for air performance. When you add features to a blade profile, these features decrease the amount of free area between the blades. A smaller free area will increase pressure drop. You may find that the benefits of water rejection outweigh this loss in air performance. All louvers will have a requirement for airflow. Your louvers should accept the necessary amount of air while rejecting rainwater.

Severe weather louvers are designed to protect ventilation opening during serious weather, where heavy rainfall will be driven by high winds. These louvers will have the same drainable features as drainable louvers, but severe weather louvers are designed to reject heavy rainfall during severe weather events. And more importantly, they are tested and rated for severe weather scenarios. These louvers will have an AMCA rating for Water Penetration, Wind Driven Rain, or both. Severe weather louvers provide greater protection, second only to hurricane louvers. Hurricane louvers are heavily reinforced and should only be chosen for applications that call for hurricane-rated protection.

On the left: a drainable louver with drainable blades. On right: a severe weather louver with drainable chevron blades.

It is important to know what you need. Drainable louvers can provide the necessary protection and keep your ductwork dry during a storm. If you are expecting rain, consider adding drainable louvers to your project.

Read more about storm-ready louvers with the Newsstand:

• AMCA Testing - Wind Driven Rain
• Combination Stationary - Two-in-One Louver Solutions
• AMCA 540 and 550: Tougher Tests for Hurricane Louvers

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