Masonry Magazine February 2011 Page. 43
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
Moisture risk at the top of a window detail can come from a number of sources.
* 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 entrance down to the top of the window detail with an open waterproofing system.
Window Rough Opening
Back Dam
Exterior Sheathing
Sill Sloped to Drain
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!
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February 2011
MASONRY 33