Masonry Magazine February 2011 Page. 44
MOISTURE MANAGEMENT
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 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.
Then and Now
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 eight 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. The Window Drainage Plane material will move water from the rough opening into the wall drainage plane (such as Gravity Cavity or Sure Cavity).
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 (such as MTI's Moisture Diverter DS2858) is placed above the window rough opening. It should slope 1/4-inch per one foot and should extend at least four inches passed 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.