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Originally published in Interiors & Sources

10/30/2012

Packaged Terminal Air Conditioner Guide

Avoid mistakes with packaged terminal air conditioners that undermine wall systems

By Thomas A. Schwartz, Mark A. Brown, and Octavian Vlagea

 
  • /Portals/1/images/Magazines/2012/1112/B_1112_PTAC1.jpg

    FIGURE 1 is an example of a manufacturer’s schematic figure that guides installation of the PTAC. Manufacturer instructions fail to show fundamental waterproofing components. The manufacturer’s recommendation consists solely of sealant at the perimeter of the PTAC sleeve penetration.

    FIGURE 2 is an example of a solution for this problem. Although some design drawings include flashing, they commonly lack necessary aspects. Make sure your metal sill flashing includes a panned-up interior leg and end dams to collect water that penetrates the back-up wall construction, as illustrated below.

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  • /Portals/1/images/Magazines/2012/1112/B_1112_PTAC7.jpg

    The PTAC louver is typically set flush with the adjacent wall surfaces. The vulnerable joint between the louver and the wall sleeve is inside the wall. The copper apparatus in the photo is a water spray rack that investigators use to diagnose leakage problems.

  • /Portals/1/images/Magazines/2012/1112/B_1112_PTAC4.jpg

    The bird screen between the wall sleeve and grille is vulnerable to infiltration, allowing water to flow through the joint and onto the floor.

  • /Portals/1/images/Magazines/2012/1112/B_1112_PTAC5.jpg

    The bird screen between the wall sleeve and grille is vulnerable to infiltration, allowing water to flow through the joint and onto the floor.

Deficient Design Drawings
Construction drawings often fail to provide proper direction for integrating the wall sleeve with exterior wall waterproofing. Ensure your designs include essential elements that avoid failures.

Even when designs show a fundamental waterproofing component such as a flashing beneath the PTAC sleeve, the flashing generally lacks a panned-up leg at the interior and end dams to collect water that penetrates the back-up wall construction.

The drawings generally fail to provide sufficient detail, such as blown-up section details or isometric views of flashing terminations and other complex transitions, to guide the installation of a water-resistant assembly.

Three Typical Water Leakage Problems
Water leakage paths associated with PTAC installations include: through the joint between the inboard surface of the louver and the front face of the PTAC sleeve, through unflashed perimeter joints between the louver or sleeve and the adjacent wall cladding, and through unsealed corners of metal sleeves (plastic sleeves are molded and do not have vulnerable corners).

Be mindful of these leak sources and how to combat them:

1) Leakage through Interface of Louver and PTAC Sleeve
Louvers shed water toward the outside but do not prevent the entry of wind-blown rain or rain drawn into the assembly by the PTAC fan. Some of the water bypassing the louver flows against the joint between the louver and the sleeve, and some flows into the sleeve.

The joint between the inboard face of the louver and the sleeve is inherently vulnerable to leakage. Manufacturers recognize this vulnerability and typically stipulate that this joint be placed outboard of the wall system by at least one-quarter inch to allow leakage to fall harmlessly outside the face of the wall, as explained above. This method, however, ignores the typical architectural requirement that louvers be set flush with, or slightly recessed from, adjacent wall surfaces.

When the louver is set flush or recessed, the vulnerable joint ends up inside the wall. Sometimes there is an attempt to seal this joint though the use of double-stick tape, foam, or glazing tape, but the surfaces that mate are irregular and often have sleeve fasteners that obstruct or penetrate these seals, rendering them ineffective.

Best Practices for Waterproofing at Wall Penetrations

One of the most common building performance problems faced by designers, contractors, and building owners is water leakage resulting from improper detailing and construction at wall system penetrations. Waterproofing practices that can eliminate leakage include the following:

  • Use sheet membrane water resistive barriers (WRBs) with long-term, proven track records, such as unperforated asphalt felt and self-adhering rubberized asphalt membranes. The membrane’s durability should match the expected service life of the wall cladding.

  • Provide drainage space for water to flow down the wall outboard of the WRB.

  • Provide flashings at all horizontal interruptions in the WRB.

  • Combine water and air barriers into one sheet wherever possible to simplify installation.

  • Create water- and air-tight seals between the wall membrane(s) and the penetrations.

  • Slope surfaces that convey water for prompt drainage.

  • Avoid wall systems that eliminate the sheet membrane in the wall and instead rely on taping of the joints in the sheathing boards. The upper edges of the tape buck the flow of water and can lose adhesion over time, permitting leakage.

  • Extend flashings slightly beyond the face of the wall and turn down the edge to form a drip to ensure that water does not flow around the edge and run back in.

 

Many sleeves are designed to drain condensate through weep slots in this outboard edge of the sleeve, directly through this seal between the louver and sleeve. Some installations include birdscreens inboard of the louver, and in some cases the birdscreens are sandwiched between the louver and the PTAC sleeve, creating an open pathway for leakage.

Since leakage through the louver-to-sleeve joint results from both rain penetration and condensation, this path of leakage can occur even in the absence of rain.

Secure protection of the building from this path of leakage requires a full pan flashing unless the louver-to-sleeve joint is placed outboard of the exterior wall face (see Figure 2).

2) Leakage at Perimeter of PTAC Sleeves
PTAC sleeves often lack perimeter flashing around the wall opening. In the absence of a perimeter flashing, the installation relies solely on continuous adhesion of sealant between the wall cladding and the wall sleeve to prevent water entry.

Gaps or loss of bond in the sealant – which result from improper sealant installation, incompatible materials, joints in substrates, sealant degradation due to exposure to the elements, and other causes – are inevitable and result in water penetration to the interior. This path is often a source of profuse leakage.

Even if the perimeter sealant is completely effective for a period of time, in the absence of head flashing, water draining down within the wall assembly above the unit can be deflected to the interior by the sleeve, which interrupts the downward flow of water.

3) PTAC Sleeve Leakage
Drainage of water from the sill of the sleeve is either internal or through weep slots as noted above. If internally drained, the sleeves are generally installed level. Since the sleeves don’t pitch to the internal drains, there is usually some residual water that does not drain out. The water can become stagnant, fostering organic growth.

The problem of ponded water is exacerbated if the sleeve deflects due to insufficient support for the sleeve and chassis. The problem of sleeve deformation is particularly acute with plastic sleeves that creep over time.

With both forms of sleeve drainage, some ponding is common; thus the watertight integrity of the sleeve sill corners is critical to avoiding leakage to the interior floor.

Plastic sleeves have molded corners which are reliably watertight unless damaged. Metal wall sleeves typically have a return flange at the interior that is cut and lapped at the corners. The lap joint is covered with sealant to prevent ponding water in the sleeve from leaking to the interior. The sealant-to-metal bond tends to degrade over time when exposed to prolonged ponding conditions.

Lack of proper pitch together with reliance on sealants results in leakage at the corners of the wall sleeves, causing deterioration of interior finishes adjacent to and below these sleeve corners.


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