Louver undersizing

Undersizing a louver is required to compensate for irregularities in the perimeter of the opening, and to allow for thermal expansion and caulking. In general, Duravent louvers are manufactured 1/4" under measured dimensions, to allow 1/8" gap all around. If a different gap or no undersizing is required, please indicate so at time of order.


Careful handling will ensure smooth operation and maximize the lifespan of louvers and dampers. It is important not to bend or twist frames, blades or linkage. Vents with screens require special attention. Taking care to not press or put weight on screens will ensure that they are flat and they will not interfere with operation.


  1. Unless specified otherwise, all vents are undersized to ensure a smooth fit into rough openings. Nevertheless, care must be taken to ensure vents are installed squarely without twisting or bending. Foreign objects including fastening devices and insulation should not be placed in the side channels of operable louvers and dampers, as they may hinder operation.
  2. To minimize water penetration, caulking should be applied around the perimeter between the opening and vent panel, and along all exposed mullions.
  3. Flange face may be attached using standard fasteners to most opening perimeters. Non-flanged faces should be finished with appropriate flashing material and caulking.
  4. Louver blades protect against water penetration in only one direction and back-draft dampers permit airflow in only one direction. During installation, ensure vents are correctly oriented (refer to shop drawings or mounting diagrams).
  5. Dampers (gravity louvers) are to be installed plumb, unless specifically designed for inclined use (with springs / counterweights).


As with any moving hardware, operable louvers and dampers require regular maintenance including:

  • Obstructions should be removed from between the blades, screen, and airway
  • Dirty linkage should be cleaned and lubricated with a light oil
  • Damaged bladeseals should be replaced
  • Although maintenance frequency depends on conditions such as temperature, humidity, dust and debris level, an annual maintenance check is recommended.