Installing windows for continuous water control

Windows and doors require large holes in our environmental separator — aka the exterior walls of our buildings. Those exterior walls rely on four critical control layers: water, air, thermal, and vapor. A window or door itself provides the function of all four control layers, but it’s the installer’s job to make sure the unit connects to each of the control layers of the wall. Continuity of these control layers is what keeps the building durable. With this first blog in our series about the four control layers, we’ll look at how we can make the water control layer continuous between the wall and window.

1. Mechanically attached membranes 

This is the most common type of WRB. This includes asphalt-impregnated felt (tarpaper) and modern plastic house wraps. A mechanically attached membrane is typically vapor-open while still keeping bulk water out. These types of products are fastened with cap nails or staples. They’re usually inexpensive and easy to install but can be damaged by wind or by surfactants (soaps, detergents, certain wood extracts) that reduce surface tension and compromise water repellency.

2. Self-adhered membranes 

This type of WRB is essentially a large sheet of tape. A self-adhered membrane bonds directly to the sheathing, making them resistant to wind damage and improving air-sealing potential.

3. Fully- or factory-adhered systems 

This is a sheathing panel with the WRB bonded to it at the factory. ZIP System is an example of this type of combined sheathing and WRB product. This type of system creates a simple, high-performance WRB with consistent installation results.

4. Fluid-applied WRBs 

This is a liquid membrane that’s rolled, brushed, or sprayed onto the sheathing. A fluid-applied WRB conforms to irregular surfaces and produces a fully adhered, continuous barrier when installed correctly.

The drainage method

The thought behind the drainage method is that some moisture will, at some point, find its way into the window assembly. Either water will enter through the window itself, or between the window and rough opening. In that thinking, we protect both the window rough opening so that a wetting event does not affect the structures framing. We also provide a pathway for the water to escape, out the bottom or sill of the rough opening.

This method accepts reality. Windows and wall systems occasionally get wet. Our job is to manage that water, not pretend it will never appear.

The barrier method

The second window water management method is called the barrier method and is less common in residential construction. With the barrier method, the intent is to keep all water from ever entering the installation assembly. For this to work, the window needs to be meticulously sealed to the WRB, and that seal requires periodic maintenance. The homeowner would need to be trained in how to inspect and repair the system. 

This method demands a lot from homeowners. This is why it’s more common in commercial construction, where inspection and maintenance cycles are more common. 

Here’s a simplified sequence for connecting the window to a mechanically attached WRB: 

3. Apply sealant to the flange

Apply sealant to the sides and top edge of the window rough opening, where the window will fasten to the wall sheathing. Do not seal the bottom flange, leaving that bottom unsealed allows water to escape.

4. Set and fasten the window

Place the window in the opening and fasten it according to Andersen’s instructions.

6. Fold the WRB head flap down

Fold the WRB flap back into place and skip-tape it. This allows drainage behind the WRB if water ever gets between the sheathing and the house wrap.

Please visit www.andersenwindows.com/installation for full installation instructions and other installation details. 

This is the basic method for making the window and WRB work together as one continuous water-management system. The goal isn’t just to keep water out — it’s to make sure that if water does get in, it has a controlled path back out.

More complex assemblies — like walls with continuous exterior insulation, integrated WRB sheathing, or fluid-applied membranes — require slightly different detailing. Luckily, there are excellent resources and installation guides available to help build a resilient, durable connection between the window and the wall.