Five Tips on How to Successfully Install Windows

This post originally appeared on the Andersen Windows and Doors blog. ProViews | Andersen Windows

I write a lot about the four control layers every building envelope has. There is an order of importance:

  1. Water control layer
  2. Air control layer
  3. Vapor control layer
  4. Thermal control layer

A window needs to be able to perform the functions of all four control layers in order to be successful. Window manufacturers design their units to be effective at controlling the movement of water, air, vapor, and heat through good product design and manufacturing techniques, but where they lose control is in the installation of the window. Often the success or failure of a window will come down to the installation, but there are ways to limit risk from water and avoid some of the window installation failures I’ve come across.

  1. Where possible, design the building to limit windows’ contact with water

If you design a roof with large overhangs, or use awnings above windows, the part of the window that’s most exposed, the window head, doesn’t get wet. If it doesn’t get wet, it can’t leak water, which is one big problem solved. Another common problem is when an adjacent roof overhang is directly above a window. This is where a kickout flashing can be a good solution. A kickout flashing is a manufactured or site-built way to divert water away from the wall surfaces (and any building component below) where the slope of the roofline meets an exterior wall.

A roof that dumps water onto a window below can be problematic. A roof kickout flashing could have saved this installation. Both these photos are of the same project. The mushroom growing out of the siding was the indicator that there was a problem.
  1. Always use metal head flashings

A missing window head flashing is one of the most overlooked details I see. The idea is to keep water moving so it can’t find a place to sit. If it sits, there’s a chance it can find a hole and end up inside the building. A simple Z-flashing with a slight pitch towards the front of the window is effective at keeping water moving.

  1. Shingle lap whenever possible

We’ve been good at keeping roofs from leaking for a long time. We do this by lapping roofing materials over the lower courses— simple and effective. This works around windows and doors as well. Lap the courses of water resistive barrier (WRB) above, and over the courses below, and lap over the top of the window. Nowadays, there are some newer sheeting products with integrated WRB that rely on tapes to prevent water from ending up someplace it shouldn’t. In those cases, the key is to follow the sheeting manufacturer’s instructions — i.e., roll the tape!

This self-adhered water resistive barrier was shingle-lapped over this window, creating an effective water-tight seal.
  1. Use the right tape or sealant for the right job

Both the sealants that come out of caulking tubes and the adhesive part of tapes have limitations as to what they will effectively adhere to. Most stick to a lot of different surfaces, but none stick to everything. Be sure to read the technical data sheets available from manufacturers. They will usually say what they are and are not compatible with.

  1. Sometimes it’s not what you are sealing out, but what you are sealing in

Sealants and tapes are good, until they aren’t. Here’s an example: You’ve just installed some sort of clapboard cladding around the windows. You want the installation to look finished, so you caulk between the window frame and cladding. Did you caulk the window head, the space between the metal flashing and cladding? If you did, you just potentially trapped any water that might end up behind the cladding at the top of the window. Again, you want to follow the manufacturer’s instructions. In this case, the cladding manufacturer’s instructions would tell you if you should leave the space above a window open for drainage.

This installation includes the metal head flashing but the contractor then caulked the flashing to the cladding, trapping any water that ends up behind the cladding. A small water leak above this door could be trouble in a few years.

My best advice is to read the manufacturer’s instructions — all of them. The window manufacturer’s instructions, the cladding manufacturer’s instructions, and don’t forget about instructions from manufacturers of weather resistive barrier, tape, and sealants. Read and understand all of them, and in my experience, your chances of having a successful and long-lasting window installation will improve.

Leave a Reply

Your email address will not be published. Required fields are marked *