Flashing

Flashing windows correctly

I recently saw a quote that said:

there are two types of windows: Those that leak, and those that will leak¹


This sums up my paranoid view of windows pretty well. We invest all this effort in making shelter with really good walls, and the we go cut big gaping holes, called windows, in those walls.

Because of our supposed "semi-arid" climate in the Yukon, the building industry has gotten very complacent in many of the water management details of our buildings. Yet, I've seen bulk water coming through windows (e.g. windows dripping water inside) even in some well built, R2000 buildings build by some of our better builders. I look around town at buildings being built, and immediate I can tell by the window detailing if the building is doomed to failure. This has made me very paranoid about water management, especially if we are trying to build "sustainable" homes.

In our SuperGreen duplex we are building, we have done the windows detailing right. However, this is far beyond "common" practice here in Whitehorse, and yet flashing windows correctly is not hard to do and does not cost much more (especially when you look at the total cost of the building!)

What we did was follow the window water management and flashing detailed prescribed by building scientist Joe Lstibruek (most succinctly illustrated in Figure 10 of his article "Drainage, Holes and Moderation ). I've reproduced those window flashing details below, without permission:
As is a chronic problem, you can't get all of these materials in Whitehorse. So, for example, noone can seem to supply flexible flashing. So, instead we used Blueskin which is carried by both Kilrich and Igloo. Here is the instructions for building a window sill backdam with Blueskin:
Make sure you use the Blueskin primer - which is basically a contact cement. This make the Blueskin stuff stick really well. Below are pictures of our pan flashing installed. Note the corner gussets. We also made the backdam (the small "ridge" at the inside edge of the sill flashing) with 3/4" wide neoprene gasket (the stuff for window and door sealing) installed before the Blueskin waterproof membrane. This gasket material made the backdam compressible in case we needed to squeeze or shim the window in, but still creates a ridge to prevent any water flowing towards the inside:


Here is another view of the sill flashing from the inside:


Not only are the windows flashed, but the door openings too:



And then, install the windows. Once the windows are on, use flashing tape to seal the window flanges. Most builders simply use the red Tuck tape, but really this tape is not rated for this application. So, we used proper flashing tape - you can get actually Typar RA in a variety of widths locally from Home Hardware.



This stuff is a rubberized asphalt tape that turns to a gooey mess that seals the window flange much better than a simple building tape. It's pretty easy to install, so no excuse for the builders.

Finally, we installed a metal trip edge flashing over the window trim, which also was sealed to the weather barrier with flashing tape. This makes sure that any water that gets behind the siding is directed outward and away from the window before it even gets to the window: a true belts and braces approach to protecting the building from its #1 enemy: water.

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¹BSI-039: Five Things, Building Science Corporation. April 20, 2010.