Who doesn't love a home with tons of natural light? While windows can add charm and elegance to a house, they are in fact, large openings in the wall that if not waterproofed correctly can lead to leaks and potential damage.
So when your builder says the window installation is done, here is exactly what you should look for to ensure it has been done correctly. Better yet, we'll run through what you should discuss with your builder before installation even begins to make sure everyone's expectations are perfectly aligned.
Weather Resistant Barrier + Flashing Tape = Weatherproof System
The building code requires all new homes to have a weather resistant barrier (WRB) to "provide the building with a weather-resistant exterior wall envelope". How the waterproof flashing tape for the windows ties into this barrier is critical to ensure a water-tight window installation. There is no shortage of weather resistant barrier options these days, as well as the flashing tapes and sealants that compliment the entire system. We will be discussing waterproofing best practices for both WRB house wraps (like Tyvek) and "built in" barriers (like Zip System). Making sure your builder is following these details is a great way to ensure you this important step of your home building is being done the correct way, and will not be vulnerable to leaks or water damage in the future.
"Built In" WRB System
Installation steps and details:
The sill is protected from standing water, in this case with Huber's Stretch Tape for their Zip System, stretched tight in the corners, and continuing up the jack studs for about 8"
The windows are installed with a good sealant on the backside of side and top flanges, not the bottom (here is a great video from This Old House showing the full installation process of a window).
Zip flashing tape is then added to the sides first, and then to the top, with about a 3" overhang on the top piece. Similar to the sealant, the bottom is left untaped allowing any water that may have gotten inside an easy path back outside.
Metal "Z Flashing" is then added to the top and taped again.
This is Huber's stretch tape for their Zip system. There are also liquid flashings that work very well. Regardless of what you and your builder use, just make sure there is something protecting the sill.
Note there is no flashing tape on the bottom window flange.
WRB House Wrap
Installation steps and details:
The house wrap is cut at the rough window opening with the sides and bottom folding in. The top is cut to create a piece that is folded up and later overlapped onto the top of the flashing tape.
Like the Zip system, a waterproof tape is used to protect the sill.
The windows are installed with a good sealant on the backside of side and top flanges, not the bottom (here is a great video from This Old House showing the full installation process of a window).
The sides are flashed with a waterproof tape over top of the WRB.
The top of the window flange is flashed onto the wall sheathing, then the WRB piece is overlapped or "shingled" onto that tape. Those seams are then taped, as well.
As you can see in this photo, the WRB has been wrapped into the window bay and stapled, and a flexible tape has been added to protect the sill. The wrap above the window has been lifted up and away so the top window flashing can be adhered directly onto the plywood sheathing. Dupont now suggests this as the correct way to install and flash windows when using their Tyvek house wrap.
Again, note that the bottom window flange has not been taped to allow for any water that may have gotten inside a path to run back outside.
Be One With The Raindrop
It's a shame that the homeowner is responsible for the quality assurance on the build of their own home, but alas, that's often the case. This post's goal is to give you the specifics and technical information needed to discuss with your builder how, exactly, the windows will be installed and waterproofed in your new home. The above steps are designed to not only keep water out, but to also allow for a worst-case scenario: water has made its way into the home, so lets make sure it has a clear path back out. There are plenty of ways to address what we discussed: different brands of tape, different sizes, etc. Simply put, imagine if you were to pour a bottle of water above the window and watch it cascade down the wall. Is there a gap in the system where it could enter your home? Seal that. Then, if water did in fact make its way in, (1) is the sill protected and (2) is there a clear path for that water to exit?
It goes without saying, following the exact requirements of your WRB system is strongly recommended (when different brand products are used, it gives manufacturers an easy out on warranty claims). So long as your builder is doing some form of these best practices detailed above (we're being reasonable, here) you'll be just fine.