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Moisture Management in Window Rough Openings
Innovative systems eliminate moisture,
The phrase “leaky, drafty windows” is common in the construction industry and, unfortunately, it seems too often true. However, the perception may not be the reality. There may be leaks and drafts in close proximity to the window, but the window units themselves may not have anything to do with the problem. In many cases it is the wall system that the window unit is installed in that may be the actual source of the leaks and drafts. We need to focus our attention on the rough opening to conquer some of the air and moisture problems blamed on windows.
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, tradespeople) 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.
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 managing 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 task, and materials used to complete that task, impact the final result. This may sound impossible, but it’s not. It may be difficult, but it can be accomplished, and if the parties involved in the construction process are truly committed to sustainable building, it must be accomplished!
Start by understanding that perfection is the ultimate goal. The reality of imperfection is the risk of some type of failure. The real task is to modify risk in order to minimize failure. 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 identified 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).
Risk zones of a window rough opening
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; 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
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 rain screen 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 voids at the top, sides, and bottom need to have predictable pathways to drain moisture that may enter these voids or that may 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!
Moisture Diverter provides drainage at the top of the window (see Figure C.). To provide drainage on the side, install Gravity 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 Gravity Cavity 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 (Photos 1 and 2). The Gravity Cavity will provide the drainage required for the side voids of the rough opening; the Moisture Diverter will provide drainage for the the top. The foam insulation will seal off the sides and top of the detail.
Use Window Drainage Plane for drainage at the bottom of the rough opening (see Figure E.). The Window Drainage 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).
Materials, technology and methods are completely different than they were just a decade or two ago. Society’s needs have also changed and the push is on to be responsible, to limit our use of energy and to conserve other resources. We have the knowledge and the materials to conquer the “leaky, drafty window” problem. We just need to employ the will to get the job done!
To see a video of the installation process, click on http://www.youtube.com/watch?v=WtQDRiousYc.
|Last Updated on Thursday, 10 February 2011 15:36|