Our Lessons in Window Flashing

Weeks before our windows arrived, we had discussions with our lead contractor about how to flash the windows on this air tight house and in-law suite. We were already committed to using two inches of exterior insulation, along with plywood boxes extending all window and door frames by a corresponding 2″ on the outside. However, rather than the XPS foam suggested by our house plans, we opted to use a new product produced in Canada known as Roxul Comfortboard IS. (More on this insulation choice here.)

This switch meant reworking the flashing details around our windows. Because Comfortboard is made from fuzzy mineral wool, it’s difficult to attach tape or peel-and-stick to its surface. For this reason, builder and architect advised us to consider a small amount of 2” XPS foam around each window, just enough to nail the window flanges to, seal the rough opening with peel and stick membrane, and screw in the cedar trim. (We could also have opted for wood, though that would have provided less insulation value.)

With this new plan in place, flashing our windows involved the following steps:

1. Typar was cut with an X over windows, folded into the window opening, and cut and taped to the inside of the plywood window boxes. A bead of caulking was put all around the window box where it meets the wall to help seal foam in next step to the wall.
XPS foam in 3.5” strips was installed in a frame around the plywood window boxes.

2. At the bottom of the window, a peel-and-stick membrane sealed the face of the foam to the bottom of the window opening (still allowing any moisture from the rough opening to drain out). The top and side window flanges were sealed to foam with peel and stick membrane (after priming foam with foam-safe primer).

3. The site-milled cedar trim (1.5″ X 3.5″) was installed around the windows. The head trim was dadoed so that it laps over the window flange and side trim. The bottom piece was assembled with a downward sloping sill and below that, a dadoed piece that laps over the siding.

4. The Typar was cut above the window to properly lap the flashing. The bottom edge was taped with Tyvek tape.

5. Caulking was applied to the top piece of foam where it meets the wall and flashing installed over the foam and top window trim and then the top flange of the flashing was tucked into the slot cut into the Typar. The top of the flashing was sealed to the plywood sheathing with peel and stick.

6. The top edge of the Typar slot was taped to the flashing with Tyvek tape.

7. The Roxul Comfortboard IS was installed (with bug screen over the bottom edge), 1×4 strapping was attached (with bug screen over the bottom edge), leaving a 1” gap or so over the window to allow the rain screen assembly to ventilate and drain.

 

8. Fiber cement board siding was installed.

Now that the process is over, the window frames look deceivingly simple. Having witnessed the flashing details, however, we know otherwise. We feel quite certain none of our climate’s heavy rainfall will make it into our house. We also have a new appreciation for the term ”air tight”.