Window installation and water resistive barriers (WRBs)

I’ve heard building scientists claim that rainfall that contacts the cladding can, during wind-driven rainstorms, end up between the cladding and WRB. This same moisture has the potential to make its way around a window, door, or other planned penetration. Properly installing a window or door involves proper integration with the WRB, so let’s explore what you need to know.

Need to back up a step? Gain an understanding of the importance of the water control layer and an introduction to the four control layers in my piece, Understanding control layers.

A WRB is a material or finish that helps reduce water intrusion. It’s intended to help keep a building element or assembly dry. It’s a term that’s often used interchangeably with housewrap, which is a specific type of WRB, but not the only type. Residential building codes require the use of a WRB. The code language is found in the 2021 IRC, R703.2 Water-resistive barrier.

Not fewer than one layer of water-resistive barrier shall be applied over studs or sheathing of all exterior walls with
flashing as indicated in Section R703.4, in such a manner as to provide a continuous water-resistive barrier behind the exterior wall veneer … 

But of course, the WRB is not just a code requirement, it’s an element that’s integral to the integrity of the building. And since windows and doors are too, it’s important to understand how to properly integrate them with your chosen WRB.

Let’s explore the four main types of WRBs currently on the market. 

1. Mechanically attached products, usually called housewraps, are synthetic sheets that are typically fastened to the structure using a cap fastener or cap nail. Example brands include Tyvek and Typar. In addition, there’s an upgraded option within this category that includes a drainage plane that’s integrated into the wrap. Drainable housewraps are typically installed like a traditional housewrap. Benjamin Obdyke and DuPont both make these types of products.
2. Fully or factory-adhered products are sheathing panels that have a factory-applied barrier. The sheathing is nailed to the wall framing, and all seams and penetrations are sealed using tapes and/or sealants. Examples include Huber’s Zip System and LP’s Weather Logic products.
3. Self-adhered products are basically giant pieces of tape that are stuck to the wall sheathing. All seams are lapped by the adjacent self-adhered WRB and, much like the fully or factory-adhered options, penetrations are sealed using tapes or sealants. Henry Blueskin and Siga’s Majvest SA are two options.
4. Fluid- or liquid-applied products are rolled on or sprayed to the wall sheathing, usually before any penetrations, such as windows and doors, are installed. Tapes or sealants are used to connect the planned opening and penetrations to the WRB. PROSOCO Cat 5 and Henry Air-Bloc All Weather STPE are a couple examples. Be sure to check the permeance, or perm rating, of fluid- or liquid-applied WRBs, as some are formulated for specific climates. 

How we connect the WRB to a window or door will depend on many factors, including the following:

• The type of WRB — mechanically attached, fully or factory-adhered, self-adhered, or fluid- or liquid-applied.
• The type of flashing used to connect the window to the WRB — self-adhered tapes, mechanically attached flexible membranes, or a fluid-applied membrane.
• How the window or door is fastened in the opening, i.e., with or without a flange. For flangeless applications, the opening prep is crucial in preparing the opening to install the window or door and to connect it with the drainage plane.   
• Where the window is installed in reference to the insulation, in wall assemblies with continuous insulation. Make sure you follow the window manufacturer’s requirements for framing to support the window or door.   
• In addition, you must consider the rainscreen. A rainscreen is a small planned space between the cladding and WRB that promotes drainage and drying. Integrating the window or door to the rainscreen can be done before or after the rainscreen is applied, just to be sure to do the proper rough opening prepartion and to shim, fasten, and flash appropriately. 

Because all of these factors must be taken into consideration in order to properly integrate a window or door with the WRB, installation methods will vary. However, my high-level advice is to think like a water droplet, which usually flows down with gravity and is directed out and away from the wall with flashing. Ask yourself: Where will water flow when it gets in between the cladding and the WRB? You want to make sure it will find a pathway down the WRB to the ground or cladding and, from there, down to the ground. Keep the water moving and provide avenues for exit, because any water that gets stuck has the potential to damage the structure.