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Indoor airPLUS

Technical Guidance to the Indoor airPLUS Construction Specifications

1. Moisture Control

Sections 1.5 - 1.6: Water-Managed Wall Assemblies

1.6 Flash All Window and Door Openings

Fully flash all window and door openings, including pan flashing at sills, side flashing that extends over pan flashing and top flashing that extends over side flashing.

Detailed Illustrations - Window Flashing

Click on the images for a full-page version

Steps 1 and 2 - Window Flashing - House wrap drainage plane

Window Flashing - house wrap drainage plane

Step 3 - Create back dam or slope

Step 3 - Create back dam or slope

Step 4 - Install pan flashing (Option 1)

Step 4 - Install pan flashing

Step 4 - Install pan flashing (Option 2)

Step 4 - Install pan flashing

Step 5 - Finishing window installation

Finishing window installation


Exterior walls that have properly installed cladding and are backed by the drainage plane described in Moisture Control Section 1.5 will keep liquid water out of the wall system. However, window and door openings create many potential new leakage points. Water leaks in a completed home may be quite obvious as wet spots on interior walls or floors, or they may be undetectable inside the wall for years. Either way, the results are rotting of the wall sheathing and framing, wetting of insulation rendering it ineffective and providing a perfect environment for extensive mold growth.

Windows and doors can be effectively added to the wall system without creating leaks if careful steps are taken to mechanically integrate these structures into the drainage plane and cladding. Sequentially placed flashing materials shed water from around the window or door unit’s perimeter, and any rainwater that may make its way past the flashing material is captured by the drainage plane, from where it can flow down the plane’s surface without penetrating the wall.

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Window-related leaks have been so common in the past two decades that a number of guidance documents have been produced to help builders correctly design and install flashing around windows. In deciding how to properly flash windows and doors, first study the design of the exterior wall system (the type of cladding and drainage plane) and the design of the windows and doors you plan to use (e.g., the flanges provided for attaching the window to the rough opening). Flashing windows and doors requires the sequential layering of flashing materials, each successive layer overlapping the previous layer (roof-shingle style). It begins with cutting an opening in the drainage plane and folding the drainage plane material out of the way. The first flashing (pan flashing) is applied over the rough sill framing; its drainage performance will be improved if a piece of beveled siding is placed underneath it to aid the drainage of any water that collects in the "pan" where the window unit rests. A bead of caulk (check compatibility) is applied to the sides and top (but not along bottom) of the rough opening in the wall and the window is inserted and secured according to its manufacturer’s recommendations. To provide for drainage, do not apply caulk under the flange at the bottom of the window or to any drainage holes in the bottom of the unit.

Once the window or door is secured in the opening, apply the side flashing, lapping it over the pan flashing and continuing up each side of the unit. Next, apply the head flashing to cover the window’s head flange and overlap the side flashing. Finally, the drainage plane material at the top of the opening (that was folded up at the beginning of the process) is lapped over the head flashing and secured in place. The key to long-term performance is adhering to this layering of materials.

Some manufacturers’ flashing systems may rely solely on the adhesive qualities of the materials and may not require their sequential overlapping. They are applied where necessary to connect the window flange to the drainage plane. However, the sequential lapping of materials as previously described is always preferred for extended performance.

There are other factors to consider. Be aware of the incompatibility of flashing with certain building materials. For example, aluminum materials should not be in direct contact with masonry or concrete. And some peel-and-stick membrane flashing materials are not compatible with all types of housewraps; check with the manufacturer’s recommendations for each product.

Some window and door units are described by the manufacturer as “self-flashing”; this feature alone will not meet the requirements of Moisture Control Section 1.6. For the purposes of this specification, flashing means a durable material such as sheet metal, metal coil stock, or self-adhesive membrane material designed by manufacturers specifically for flashing window and door units.

Finally, do not rely on caulks to prevent the penetration of water around windows and doors.

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References/Additional Information

  • ASTM E2112.07, Standard Practice for Installation of Exterior Windows, Doors and Skylights. See www.astm.org/ exiting epa.
  • Durability By Design, A Guide for Residential Builders and Designers. U.S. Department of Housing and Urban Development, Office of Policy Development and Research, Partnership for Advancing Technology in Housing, PATH, May, 2002. See www.hud.gov/.
  • Best Practices Series: Volume 2 - Builders and Buyers Handbook for Improving New Home Efficiency, Comfort, and Durability in the Hot-Dry and Mixed-Dry Climates. U.S. Department of Energy, Building America Program, May, 2005, Version 1, 9, pp. TRD 7-8. (Fig 1.10b). See www.eere.energy.gov/buildings/building_america/.
  • Water Management Guide exiting epa, Lstiburek, Joseph, 2006, Building Science Press.

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