11 Most Common Window Installation Mistakes

1. Wrong size rough opening

The rough opening requirements for a 2’x 3’ double hung window will vary depending on the manufacturer and the style of the window. A window can be built to fit almost any rough opening size, but many manufacturers build “standard” sizes which cost less than custom built windows.

2. Wrong size jambs

When ordering windows with the jambs attached, be sure to add to the width of the jambs if the finished surface of the interior walls is going to be plaster, tongue and groove, or any other material thicker than ½ inch drywall. You’ll also need to add to the width of a jamb when installing windows onto sheathing that is thicker than 7/16 of an inch.

4. Poor flashing adherence

Brush away dust and debris and wipe off moisture from window sills and the WRB (Weather Resistant Barrier) to improve the adherence of flashing tapes and membranes. Some tapes and membranes won’t adhere in cold weather, so try to find a product that does or be sure to follow the manufacture’s specific cold-weather guidelines. Some cold-weather solutions will include adding primers prior to installing the flashing, so read the instructions beforehand so you’ll have all the necessary materials come installation day.

5. Securing nailing fins without repeated inspections

A bent or angled nail can push or pull a window out of place. As you fasten the fins, check and recheck that the window continues to operate properly and that it remains centered, level, plumb, and square.

7. No back dam

A back dam on the sill will help prevent unwelcome water from reaching the wall cavity and the interior of the building. A back dam can be created with a strip of wood under the sill flashing, incorporated into a rigid sill pan, or could be created after the window is installed with a thick bead of sealant.

9. Misuse of spray foam

Filling the cavity around the entire perimeter of the window with expanding spray foam could prevent unwelcomed water from escaping back to the outdoors. Also, dispensing too much foam or the use of high-expanding foam could bow wood jams or even distort a window frame.

10. Dirty hands

Try to avoid smearing foam and sealant on any of the finished parts of the window. It’s especially difficult to remove stains from unfinished wood. Keep an extra pair of gloves on hand for applying foam and sealant.

11. Not following the instructions

While all windows share similar installation procedures, most of them have unique requirements based on extensive laboratory and field testing. If you want to get the most out of every window you install and avoid expensive callbacks, it’s important to follow each of the manufacturer’s instructions down to the smallest detail.