The building science community talks a lot about control layers. I’ve written on the topic several times, basic overview of these principles which can be found at: A Crash Course on the Four Control Layers. Let’s take a deeper look at water resistive barriers (WRB) and how windows and doors need to be integrated into the WRB.
Definitions and code requirements
Before we get going, let’s get a couple of definitions and discuss what the code requirements are surrounding WRB’s.
Water resistive barrier (WRB)-A material or finish that helps reduce water intrusion, intended to help keep a building element or assembly dry. The WRB is usually part of the drainage plane.
Housewrap (house wrap or home wrap)-A modern synthetic material used to protect a structure from moisture, preventing the wetting of the wall assembly.
These two terms are used interchangeably, identifying a product or group of products that keep moisture (and sometimes air) from entering a wall assembly. Residential building codes require the use of water resistive barriers, 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…”
What’s interesting about the residential building code, the only product listed as a WRB is #15 felt, commonly called tar paper. The code does go on say that products tested and passing certain ASTM testing can also be used as a water resistive barrier.
And of course, windows and doors are part of the water resistive barrier, the code has requirements on how to deal with their integration as well.
R 703.4 Flashing installation at exterior window and door openings.
“Flashing at exterior window and door openings shall extend to the surface of the exterior wall finish or the water-resistive barrier complying with section 703.2 for subsequent drainage. Air sealing shall be installed around all window and door openings on the interior side of the rough opening gap. Mechanically attached flexible flashings shall comply with AAMA 712. Flashing at exterior window and door openings shall be installed in accordance with one or more of the following:
- The fenestration manufacturer’s installation and flashing instructions, or for applications not addressed in the fenestration manufacturer’s instructions, in accordance with the flashing manufacturer’s instructions. Where flashing instructions or details are not provided, pan flashing shall be installed at the sill of exterior window and door openings. Pan flashing shall be sealed or sloped in such a manner as to direct water to the surface of the exterior wall finish or to the water-resistive barrier for subsequent drainage. Openings using plan flashing shall incorporate flashing or protection at the head and sides.
- In accordance with the flashing design or method of a registered design professional
- In accordance with other approved methods.”
The codes are specific in how to ensure windows and doors are integrated into the water (and air) barriers. Follow the installation instructions from first, the window and/or door manufacturer when available, and if not, the WRB manufacturer instructions can be used. Let’s explore the different WRB options currently on the market.
Different types of WRB’s
We have four different types of water resistive barriers to choose from. The most common is the mechanically attached products such as Tyvek or Typar. This line of products is fastened to the structure, typically using a cap fastener or cap nail.
A second type is fully, or factory adhered, examples include Huber’s Zip System and LP’s Weather Logic products. These are sheathing panels that have a factory applied barrier applied. The sheathing is nailed to the wall framing, all seams and penetrations are sealed using tapes and/or sealants.
The third type is a self-adhered WRB, basically a giant piece of tape that is 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.
The last type is a fluid or liquid applied WRB. These products are roll-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.
One additional choice, if you are choosing to use a layer of continuous exterior insulation, the insulation itself may be able to be used as the WRB. Check with the manufacturer of the insulation for details.
Connecting windows and doors to the WRB
Any time we are trying to control water and air on the exterior of a building, the key is to have a plan on how to deal with water if or when it ends up in an area we don’t want it. I’ve heard building scientists claim that up to 10% of rainfall that contacts the cladding during wind-driven rainstorms, end up between the cladding and WRB. This same moisture has the potential to also make its way around a window, door, or other planned penetration through the WRB, where, over time it can cause damage to the structure.
How we tie our WRB to a window or door will depend on the style of WRB and type of window or door that is being installed. There are two main window and door installation types, flanged and flangeless. Flanged windows and doors are usually installed in a way that allows water to drain through the system should it find a path between the window and rough opening.
Often, the flangeless options are installed as a barrier system, meaning we are trying our best to keep water from entering between the window and rough opening. A barrier installation requires much more care and attention to detail and is less common in residential construction.
Here are a couple of tips to keep in mind when integrating a window into a WRB:
- A window (and door) is a planned opening in the building envelope and creates a break in the water resistive barrier. It’s important that there is continuity between the WRB and window. This continuity is provided by flashings, tapes and sealants. If air can find a pathway from outside to inside a structure, there’s a good chance that water can use that same pathway (along with bugs, dust and other undesirables we don’t want inside our homes).
- Many mechanically attached WRB products (the most commonly used WRB) will have two different installation instructions, one that is used when water control is the only concern, a second (and usually much more detailed) set of instructions are used when air control is also required.
- A WRB usually has a perm rating that will allow for some vapor movement through the product. Perm ratings of 20-50 are common for most applications. Under 20 might be desirable if the cladding system has the potential to store moisture (traditional stucco),
- A WRB is used for bulk water management, it is typically not a vapor retarder (under 10 perms, though it can be one, mostly only used in the hot and humid climate zones 1 & 2).
One final note with regards to the installation of Andersen Windows and Doors, did you know that Andersen has installation kits? The kit includes shims and fasteners, sealants, tapes, canned expanding foam, backer rod and the instructions on how to correctly install an Andersen window or door. You can find more information on the Andersen website.
This post originally appeared on the Andersen Blog, Proviews.