The areas where two or more different construction products, details and systems intersect are always places of potential risk.


Leaking windows are one of the most frequent callback issues in the building industry. They leave many contractors feeling like the old sailor in “The Rime of the Ancient Mariner” when he cried, “Water, water everywhere!” When wind and water leaks into the interior living space from the perimeter of a window, it’s all too easy to blame the window for the problem. However, the odds are great that it isn’t a faulty window but rather a faulty window rough opening that’s responsible for those leaks and drafts. We need to focus our attention on the window rough opening and the wall system itself to overcome these air and moisture infiltration issues.

Rough opening moisture management.

The areas where two or more different construction products, details and systems intersect are always places of potential risk. Incompatibility of products or designs and poor communication between the various people involved (architects, contractors, tradesmen) are some of the more common scenarios that result in failures. In many cases, the solution to the “leaky, drafty window” problem is to focus on the materials and methods used around the window rough opening.

Moisture should also be diverted away from construction details with drainage products as quickly as possible to minimize risk.

Detail Failures

Exterior building envelope construction systems (roofs and walls) often fail in the detailing of openings, projections and transitions. The detailing process is complicated because it involves more than a single individual or discipline. The challenge is uniting the various disciplines using a wide range of materials into a cohesive unit. The need for holistic building is imperative. Each party involved needs to know how their work and materials are used to complete that task and how they impact the final result.

The first and most important task of moisture management, as it relates to construction products and details, is keeping moisture off of them. If water can’t get to the detail, it can’t damage it. The second task is to isolate as many details as possible, identify risk zones and design details, and concentrate on their intrinsic weaknesses. Once these are identified, then create a moisture solution. When a potential problem isn’t addressed in one area, it often leads to failure in another. A poorly prepared rough opening develops leaks that then get blamed on the window. In reality, had the window detail been properly addressed in the first place, a failure in an adjacent detail may not have happened. Many wall failures, both structural and veneer-related, are a result of a moisture management failure on a window installation (specifically, problems with preparation of the rough opening).

A thin veneer with a predictable rain screen drainage plane affords an opportunity to apply moisture diverter technology on the inside of the void of the rainscreen drainage plane rather than by adding architectural details on the outside.

Risk Zones of a Window Rough Opening

Moisture risk at the top of a window detail can come from a number of sources, such as:

Condensation or frost accumulation from warm, moist air coming in contact with dew point temperature in the air or on surfaces of the interior of the window rough opening and on the surfaces of the window unit.

Openings or voids in the waterproofing material at the top of the window (installation flange and flashings).

Moisture that may have entered the wall system above the window and moved from the high point of the entrance down to the top of the window detail in an open waterproofing system.

When there are voids or chases in construction details of the exterior building envelope, the air that is in these voids or that can move into them must be controlled or conditioned. The control mechanism is usually some kind of vapor retarder membrane placed on the warm side of the detail and made as airtight as possible. When warm, moist air can’t meet a dew point temperature, there is no condensation and therefore, no frosting. These voids also need to be filled with some sort of insulation to interrupt temperature transfer-no dew point temperature, no condensation problem.

To a lesser degree, the voids at the sides of the window rough opening have the same condensation concerns as the top and bottom because the surfaces are vertical rather than horizontal. However, they are dependent on the top of the window being properly treated to manage moisture correctly so that it doesn’t allow moisture to move down into the voids at the side.

The bottom of the window area is of greater concern. It can be compared to the bottom of a bucket; everything runs downward. Any and all liquid that gets into the window’s rough opening will accumulate at the bottom of the window rough opening. The accumulation of moisture at a low point in a window rough opening leads to absorption and migration into the surrounding details resulting in one of the most common failure scenarios in the construction industry.

Cover the construction details that you don’t want to get wet with moisture-proof or moisture-resistant materials. Moisture should also be diverted away from construction details with drainage products as quickly as possible to minimize risk. The combination of a well-designed drainage system and a moisture-resistant or waterproofing system is the ultimate detail to manage moisture.

Historical Basis for Moisture Diversion

Diverting water away from window details is not a new idea nor is it a new technology. Designing a pattern in the veneer immediately above a window has a long and successful history. What is not commonly known or understood in today’s construction and design industry is that most of the patterns in older historical building veneers were there to manage moisture and to move it off of-and away from-sensitive details such as windows and doors.

This historical idea of diverting moisture away from sensitive details with architectural details can be applied today, but with a twist. A thin veneer with a predictable rain screen drainage plane affords an opportunity to apply this moisture diverter technology on the inside of the void of the rainscreen drainage plane rather than by adding architectural details on the outside. Here are two examples of this moisture diversion practice of creating a detail above the window to move moisture away from the top and out and around the sides, one from the past and MTI’s “inside the envelope” solution.

Rough Opening Moisture Management

The bottoms of chases in framed construction window rough openings should be addressed with the following details and materials:

Figure A Detail

The construction materials that make up the bottom of this detail must be covered with a waterproofing material that turns up the sides of the window rough opening a minimum of 8 inches.

The top surface of the bottom of the window rough opening must be sloped to drain to the exterior of the building.

The back edge of the bottom of the window rough opening must have an elevation change that is higher and creates a back dam.

Figure B Detail

A pathway must be provided for moisture to move out of and off this detail. This needs to be done for moisture that may enter at the sides as well as at the bottom.

A pathway must be provided for moisture to exit the wall detail once it has drained out of the window rough opening.

Figure C Detail

The next step involves moving water away from the top. Remember that moisture moves downward. If we follow the examples from history that created external details away from the top of windows and doors, much of the moisture problem can be eliminated from the window rough opening. The only difference is that the moisture diverting mechanism is inside the building envelope.

A moisture diverter is placed above the window rough opening. It should slope 1/4 inch per 1 foot and should extend at least 4 inches past the side of the rough opening.

Apply flashing tape to the top edge of the moisture diverter. The layers of construction paper should overlap the top of the moisture diverter and extend down into the trough.

Incorporating these practices into rough opening design gets moisture away from, off of and out of the window construction detail as quickly as possible. These practices constitute a well-designed rough opening. The voids at the top, sides, and bottom need to have predictable pathways to drain moisture that may enter these voids or condense in them. The voids also need to be insulated because they can allow air infiltration with negative results. The idea that moisture can’t get in, or that if the voids are filled with insulation there isn’t enough room for moisture, is just wishful thinking.

The moisture diverter provides drainage at the top of the window (see Figure C). To provide drainage on the side, install cavity strips on the sides of the rough opening (see Figure C) and on the sides of the window frame. It is critical that these strips are centered on the sides of the rough opening and the sides of the window frame, and that the edges of the strips are at least 3/4 inch in from each edge of the rough opening. (see Figure D).

Insulate the top and sides of the rough opening with foam (see photos 1 and 2). The strips will provide the drainage required for the side voids of the rough opening; the diverter will provide drainage for the top. The foam insulation will seal off the sides and top of the detail.

Use a window drainage plane for drainage at the bottom of the rough opening (see Figure E). The plane will also provide a pathway for moisture to enter the rainscreen drainage plane system.

The bottom should be insulated with fiberglass batt insulation cut to size; the addition of the insulation will seal off the bottom of the detail while still allowing drainage (see photos 3 and 4).

Conclusion

Placing blame is always easier than working for perfection. Most of us parrot the words “best construction,” “sustainable design” and “green building,” but do we truly try to attain them? If we honestly want to employ these practices, we need to use what works even though it may seem to infringe on our long-held beliefs about correct building methods.