How Do You Install Window Flashing?

Window FlashingHow to install window flashing is a fairly common question asked by homeowners. Many home builders tend to overestimate the effectiveness of siding at keeping moisture out of the house and consequently do not pay nearly enough attention to the window flashing. But, the siding alone is rarely enough to prevent moisture buildup in your house. Lack of window flashing or improperly done flashing not only affects the durability and warranty of your windows, but also exposes the wall cavity to moisture problems and encourages growth of mold. Additionally, poorly done window flashing can drastically lower the energy efficiency of even the best windows. Read on to find out how to property install window flashing.

Read the Manufacturer’s Guidelines

Window manufacturers recognize the full effects of improperly installed windows including significant energy loss. It’s important to strictly follow the manufacturer’s instructions when installing your windows so as to keep the wall cavity moisture-free and fulfil the window manufacturer’s warranty.

In some cases, you might find that the guidelines provided by the window manufacturer conflicts with the instructions given by the flashing-tape manufacturer or the housewrap manufacturer. In such cases, the experts advise that it’s best to go with the most-restrictive protocol. In some cases, the window manufacturer provides no flashing details so you would have to go with guidelines from the flashing tape or housewrap manufacturer.

If you are ever in doubt, then it’s best to follow the window manufacturer’s guidelines. The International Residential Code (R613.1), found at the International Code Council, provides for windows and flashing to be done according to the manufacturer’s instructions.

Prepare the Rough Opening with Pan Flashing and Housewrap

Some people make an X-cut into the housewrap and staple the tabs into the opening prior to installing windows. But, doing this allows water to find a way behind the window’s top flange. Instead of doing this, cut a top tab that will cover the top flange once the window is installed. Make a pan flashing on the site with beveled siding and flashing tape using a wide flashing flex. This allows you to form a seamless pan that contours nicely to the sill. A beveled sliding creates a sloped sill.

After wrapping the opening, nail a piece of siding along the rough sill using 13/4 inch roofing nails. Now apply a self-adhesive flashing tape. You can apply spray adhesive to improve the tape’s adhesion.

Caulk, Shim, then Nail the Window in Place

Once the rough opening is set, now it’s time to install your windows. Most window manufacturers specify that the side and top of the flanges to be caulked to the perimeter of the opening. Adding the caulk to the building instead of the flanges makes for a cleaner operation. Make sure there is a good seal around the flange. Nail your windows before the caulk dries off.

Make sure the bottom stays caulk free. A sealed bottom flange can easily trap water in the pan flashing. Use a window and door silicone caulk with an ASTM C-920 rating to comply with most window manufacturer warranties.

To eliminate drafts, use spray foam instead of stuffing fiberglass insulation around the windows. You can use backer rod to fill the spaces in deep walls before applying the spray foam.

Lap the Tape Shingle Style

Flashing tape usually falls into two categories: butyl-based and asphalt-based. The problem with using asphalt-based tape is that it can easily damage certain vinyl windows, so check the window specs to ensure it’s safe to use. Butyl tape is usually easier to work with. Butyl tape does not have a powerful initial tack, but its adhesiveness gets better over time.

Mechanical Flanges Require Extra Attention

All flanged windows are not made the same. According to Dupont, some windows have an integral nailing flange. These types of windows are usually a good choice because there are no seams or joints through which water can sneak through. Hinged and non-hinged mechanical flanges, however, have a few vulnerable areas that require extra attention during installation.

Peel-And-Stick Window Flashing

Another effective way to install window flashing is to use peel-and-stick flashing instead of the normal caulking. The major benefit of this method is that it seals tightly around the nails and staples. Listed below are directions on how to install peel-and-stick window flashing:

  • To create a flap, cut the housewrap at the top of the header at a 45-degree angle.
  • Apply the peel-and-stick flashing to the sill then lap the flashing up on the jambs.
  • Now caulk all around the rough opening apart from the sill then install your window.
  • Apply the flashing to the window’s side jambs so that it overlaps the sill edges and extends about one inch further and 2 inches past the window head.
  • Direct water beyond the top of the opening by extending the header flashing piece 1 inch past the jambs. If the window has a drip cap, make sure you install it first before the flashing can cover it.
  • Now fold the housewrap flap cover back over and tape it down so that the housewrap overlaps with the header flashing.

Flashing windows using the peel-and-stick method provides a smooth and good drainage that minimizes chances of warping and rotting over time. It’s an easy yet effective method to install window flashing.

Related Resource: How to Make a Basement Window Air and Water Tight

You should always pay special attention to how well the window flashing for your home is done since improperly done installations can become a major source of problems. With these guidelines, you no longer need to ask how to install window flashing.