Flexible peel and stick flashing membranes have made rigid pre-formed pan flashings a thing of the past.

Today's Sneak Inspection: A mixed single-family and townhouse development in southern Maryland. These photos are taken in one of the single-family homes. They are middle upscale—nice, if not unattainable.

Installing windows well, as this builder illustrates, has gotten a lot easier with new products like flexible flashing tapes—tapes that can bend around corners for 3D seals.

Ten years ago hardly anyone used pan flashing on window sills. On some high-end custom jobs, or jobs with leading edge building science geeks, you'd see coil stock bent into the window and door openings, but it was NOT a widespread practice on jobsites in most parts of the country.

There is a pretty good history of best practices, just bad adoption

Those building science geeks had been reading things like JLC's 1995 article Making Walls Watertight by Paul Fisette. In the article, Fisette specified felt paper splines, properly lapped from sill to head, but no pan flashing, per se. [Paul: I am not knocking you. I am pointing out that even the best and the brightest, almost 20 years ago, were not specing metal pan flashing.]

As water problems in houses trickled their way to the forefront (through big construction defect events and small localized problems), sloppy window details flowed into the topics of conversation. Felt paper splines became aluminum coil stock pan flashings, and pre-formed rigid fan flashing products, before peel and stick roofing membranes made their way into window openings.

Repurposing roofing membranes to do windows was pretty much the pinnacle of best practices until the membrane makers noticed all of the troubles that carpenters were having making the membranes seal the corners of the sill pan. And along came flexible flashing tapes that bend around corners. A single piece of flexible pan flashing is much faster and more reliable than using three pieces of nonflexible pan flashing.

Why does this matter?

Pan flashing is the last line of defense against water that leaks into walls. The pan flashing is supposed to protect and directprotect against bulk water and direct it outside (better than directing it inside for obvious reasons).

The rubberized, vulcanized, or 'butuminized' tape protects the wood from water; directing the water is accomplished either with slope, a back dam, or both. There is no slope in this opening, so I suspect that a back dam will be used. There are many ways to do this—from stapling a length of backer rod to the sill plate to specialized air barrier gasket tape. Perhaps the biggest back-dam forehead-slapper is to tape a length of wallpaper corner protectors into the opening.

Beyond the sill, here is a look at how to detail the rest of the window opening:

Step 1 in great window installation

(Sorry about the funky angle on the photo—this was the only window not yet installed on the house and was the only opportunity to show how right they got it.)

A top flap is cut in the housewrap so that it can fold over the top nailing flange of the window. In effect, the window is integrated shingle-style into the wall by being outside the housewrap at the bottom and inside the housewrap at the top. This is almost exactly like the drawings that Building Science Corp. puts in books.

The flexible flashing tape seals the housewrap to the framing and wall sheathing at the bottom of the hole. The sides and top are sealed with flashing tape after the window is installed:

How great window installs look from the outside

The side flanges are sealed to the housewrap and the top flange is sealed to the wall sheathing. All of the tapes are layered shingle-style, too: sides go over the bottom, the top goes over the sides, the housewrap goes over the top, and patches go over all of it.

That wall ain't gonna leak, Bubba.

Problem-solving products and practical practitioners rock

What warms the heart of the Mystery Inspector about this photo is that not only are great products available to solve very real problems, but a production builder is incorporating best practices on a job. This means that the walls will probably not turn to mush, builders in general may enjoy a slightly better social standing (can we finally leap-frog lawyers and auto mechanics?), and I can write about things that are good rather than things that are bad.

At least I thought so until I drove by this window install in western Connecticut:

Bad dog, no biscuit!

Attaboy, first window installers; bad dog, no biscuit, second ones.

Another couple of attaboys to give out to Marylanders:

Attaboy for taping the housewrap so well. Well-detailed housewrap can play a significant role in the air barrier system of a house. It saves energy, protects against mold and rot, and makes a home more comfortable to live in.

Attaboy for foaming the AC lines going through the wall. If this window were poorly installed, those AC lines would represent another hole in the wall for water to creep into in its never-ending quest to destroy your reputation as a quality home builder.

See something that we missed? Make a note of it in the comments.

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