Leaky Windows — Are We Blaming the Wrong Thing?

One area of the building envelope that catches a lot of grief for being a “leak” problem is the window.  However, most quality windows of the 21st Century are extremely well built and probably aren’t the cause of window area water and air leaks.  If windows are properly installed, the majority of the leaks blamed on windows will probably disappear.  We need to focus on the window rough opening to solve the problem.

MTI recently wrote an article on this issue.  Here are some of the highlights:

  • Exterior building envelope construction systems (roofs and walls) often fail in the detailing of openings, projections and transitions.
  • 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.
  • 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.
  • The bottom of the Window RO needs to be covered with waterproofing material that runs a minimum of 8″ up the sides; there needs to be a back dam; and the sill needs slope-to-drain to the outside.
  • There needs to be a drainage plane at the bottom of the window RO.
  • There needs to be a moisture diverter above the window RO that moves moisture away from the RO into the building envelope drainage plane.

MTI is not the only entity to have made these points recently.  Brett Newkirk, P.E. has authored an article in the Winter 2010 issue of Applicator magazine entitled “Forestall Sill-Flashing Failure.  His premise is that many of the techniques we are currently using to protect against moisture failure in and around the Window RO are actually causing the problems we are trying to avoid.

Below are some MTI drawings of the procedure for properly protecting the Window RO.  If you would like a copy of the MTI article by our CEO, John Koester, you can download it at http://www.mtidry.com/news/windowRO.pdf.


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