Preparing Window Opening
Making sure that a rough window opening is properly prepared and the window installation is properly sealed is the single most important step of any window installation. See instructions on how to properly prepare a rough window opening for window installation. See how to care for building wrap, how to prepare and place window flashing, and how to caulk and tape seal the window for a perfect weatherproof window installation.
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If building wrap has just been applied to the building, cut the wrap flush with top, bottom and sides of the rough window opening. If replacing a window, be careful to not damage the old building wrap, if damaged, replace by reapplying from the bottom up in a shingle like layered fashion well overlapping. What you're doing by this is making sure that if water runs down it, it will stay on the outside and not go behind the next layer.
On the top of the rough window opening, make a 3 inch cut on each of the 2 corners on a 45 degree angle away from the opening, then fold up and secure with tape (the tap is to temporarily keep the flap out of the way).
Cut the Flashing
Working with a roll of 6' wide self-adhesive window/door flashing, cut to length. Measure the rough window opening width. Cut the flashing to this dimension plus 6'. Cut a slit 3' in from each side of the flashing.
Place the Flashing
Pull the adhesive covering wrap off the back of the flashing and place the flashing into the rough opening as shown. Take care so that it does not wrinkle or fold back onto itself. Take your time and make sure that it's perfect, this will help keep the window sealed.
Caulk the Opening
Using window/door exterior sealing caulk, caulk a bead around the entire window perimeter 1/2' - 3/4' from the opening.
Tape the Window
After the window has been installed, use window sealing tape to seal the window (also known as building wrap tape). Start on the bottom and place the tape over the entire nailing flange/fin and overlap it onto the house wrap. Then tape the window sides the same way, overlapping onto the tape that was just applied on the bottom of the window. Then apply tap to the top of the window, overlapping onto the side tape. Make sure that the tape does not wrinkle or fold back onto itself. Take your time and make sure that it's perfect, this is the single most important step for keeping the window perfectly sealed from the elements.
Finish the Building Wrap and Corners
Release the building wrap that was folded up in step 1 and let it cover the wrap tape that was just applied. Secure the wrap to the tape by placing a bead of caulk under the wrap, and then tape the ends with the building wrap tape. Finish the sealing process by applying caulk to the top corners of the window with a 1/4' bead of sealant.