Flashing a Window: Debating Best Practices
One of the first things I learned about building houses is that for any given process there are about ten ways to go about doing something. Five of those ways are probably wrong and will bite your behind if you stick around long enough. Of the other five, two will take too long, two are just about right, and one is too much of a good thing like an extra spoonful of sour cream on a taco.
For remodel the rule is the same, just double everything add some rain and a leaky roof and two old dead mice for good measure.
Which brings us to the Build Like a Pro video series on replacing a window with Scott Grice that we just posted. In the series there are a couple of details about best practices that I have been fielding questions about since we posted up the videos.
Namely: Should there be flashing from the edge of the window jamb (the face flush with the wall sheathing) over to the framing? Should the custom stool be attached to the window prior to installation rather than the window framing?
And, is putting fiberglass insulation into a 2-in. gap above the window a gross and ineffective attempt at weatherization?
My former boss and mentor, Kevin Ireton, once famously wrote in his letters to the editor column something to the effect that Fine Homebuilding was a place for conversations to begin rather than end.
With that in mind I asked Scott to respond to the issues and I ask those who have watched the videos to weigh in as well. Please write your thoughts in the comment section below.
Scott Grice replies:
Concerning flashing the 1/2-in gap between the window jamb and the framing, the material to make the window jamb has been treated. Loewen treats their windows (this may be brand specific) so the likelihood that the window itself will rot is extremely low. Besides, when was the last time a vertical wooden member rotted? Gravity works, so the potential rot, if the wood is untreated, is down low. This is why the tape is run over the top of the sill. If it hits this surface and runs inside and stains the drywall, all the better because the alarm bell has been sounded before a really expensive fix is needed. But if this is a concern it would be easy enough to place a a piece of bevel siding down before installing the window thereby directing the water out. For this installation we left that off because it would have complicated attaching the custom stool, see below.
Methodology- Flashing tape is great but for it to span the gap and create a watertight seal on the jamb requires absolute flatness and that the adhesion between tape and jamb be 100% all along the jamb. Possible but definitely not certain. I am more comfortable with back caulking the trim. If we had gotten the window with jambs already applied we would not be able to span the gap between framing and window with flashing. The factory applies those jambs exactly the way I do. In this scenario the trim does the work that any flashing would do.
I was trying to imagine water migration behind the siding- which is how water could get into the opening; our method protects from direct penetration from top and sides- and granted that somehow water went sideways to the gap- which it can’t because we back caulked the trim to the flashing tape- the water would run down the trimmer and hit the flashing over the framing and run out. Most likely it would not run in because of the urethane foam we added from the inside. Again, if it does run in it’s an indication of a bigger problem and I’d be glad to know it.
To review- I think everything the flashing is going to do, the trim does because we back caulked everything. If this isn’t good enough then how would one install windows with factory applied jambs?
Concerning attaching the stool to the framing, it would have been difficult and uncertain to secure our custom stool to the bottom of the window jamb prior to installation. Also the piece was fashioned from a tropical hardwood. The screw holes were caulked from the top and from bottom. If the house were left to rot unattended for two hundred years, the chimney and that piece of wood would be that only things left.
Finally, concerning the fiberglass insulation, the fiberglass is doing as much good as it is doing in the walls. The gap is big enough to fill with fiberglass without compacting the material. If this is an indictment of fiberglass, I agree. If it is an indictment of only this particular use of fiberglass, not so much.
Hopes this helps to clarify — Scott
Scott Grice responds to feedback on his techniques for replacing a wooden window.
News flash. Join the debate over "best practices" of window flashing from our video series on replacing a wooden window.