Fundamental Waterproofing Concepts
Unfortunately, fundamentally sound, time-proven waterproofing concepts are generally absent from manufacturers’ standard details and most architectural designs. PTAC wall sleeves require perimeter flashings for reliable and durable performance, whether installed in curtain walls, cavity walls, or barrier walls.
The following design guidelines are recommended to avoid PTAC leakage.
PTAC designs, like the designs of other exterior wall penetrations, need to recognize the inherent lack of reliability and durability of perimeter sealant joints. The design should provide flashings to collect water that bypasses the perimeter seals and drain it to the exterior.
- Sill: The sill should have a metal flashing that extends the full depth of the wall sleeve, with an upturned leg inboard of the wall sleeve, end dams at the sides of the wall sleeve, and a down-turned drip edge that projects beyond the outer face of the exterior wall. The metal flashing should be properly integrated with adjacent wall materials beyond the PTAC opening.
- Head: Because the PTAC sleeve cannot provide proper support to the brick above the sleeve penetration, the brick is supported by a loose steel lintel. A metal flashing with drip edge projects outward beyond the top of the louver and engages a continuous metal hook strip fastened to the wall sleeve and embedded in sealant. The metal flashing covers the horizontal leg of the lintel, turns up at the vertical leg of the lintel, and extends upward onto the exterior wall sheathing. The water barrier laps over the vertical leg of the metal flashing. The metal flashing should have end dams that turn up into a masonry head joint beyond the PTAC opening.
- Jamb: The water barrier should extend into the wall opening the full depth of the wall, and lap over the end dam of the sill flashing below.
Flashings at Curtain Walls and Window Walls
Flashing concepts for PTACs installed within curtain walls or window walls are similar to those outlined for cavity wall systems. The flashings must drain either directly to the exterior face of the wall, or into a glazing pocket that is wept to the exterior. For flashing installations where drainage into the glazing pocket might overwhelm the glazing pocket weep holes, the sill pan should project beyond the outer face of the wall to drain water outboard of the glazing pocket.
Slope to Drain
One of the most fundamental concepts of good waterproofing practice is to provide adequate slope to promote prompt drainage from waterproofing surfaces and avoid ponding water conditions. A slope of one-quarter inch per foot, a minimum requirement for roof slopes, should be provided for PTAC wall sleeves. It may be necessary to provide leveling shims for the chassis to meet the manufacturer’s requirements for the levelness of the chassis. Adequate slope to drain will reduce the exposure of inboard wall sleeve corners to ponding water and subsequent leakage.
Ensure Proper Design and Installation
PTACs are a common source of water infiltration through exterior walls. Installation instructions typically provided by the manufacturers and the details on most design drawings ignore fundamental principles of wall waterproofing. PTACs installed in accordance with these instructions and designs without due consideration of fundamental wall waterproofing principles are prone to leakage.
However, PTAC leakage can be minimized through proper design and construction, including the following:
- Recognize the inherent lack of reliability and durability of perimeter sealant joints. Provide perimeter flashings at the sill, head, and jambs of the PTAC opening that integrate with the back-up water resistive barrier (cavity walls) or other secondary waterproofing and drainage system (glazing pockets of curtain walls or window walls).
- Install PTAC wall sleeves with slope to the exterior to promptly drain water to the exterior. Shim the PTAC chassis to meet the manufacturer’s requirements for levelness.
Make sure your PTACs are designed and installed to withstand a PTAC attack.
Thomas A. Schwartz is the president and a senior principal, Mark A. Brown is a senior project manager, and Octavian Vlagea is a Senior Staff II at Simpson Gumpertz & Heger Inc., an engineering firm that designs, investigates, and rehabilitates structures and building enclosures.