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What is the Window Installation Process?

When you’ve finished looking on for a company to install windows for your home, the next step is of course the most important – the installation process. But what exactly goes into window glass installation in a home? This article will attempt to answer that question.

Make Sure You’re Hiring the Best

First of all, when hiring a contractor to install a window, make sure that they meet the highest standards in the industry. The American Architectural Manufacturers Association (AAMA) runs a training and certification program for installers of windows and exterior glass doors. It’s called the InstallationMasters program. More than 12,000 contractors currently carry the InstallationMasters certificate. The program aims to teach window and door installers the best practices and installation techniques based on established industry standards. It assures consumers that the installer has been trained and has passed a written test proving his knowledge of the subject area.

Measure the Window

After you’ve selected a qualified contractor, the next critical step in window installation is getting precise measurements of the openings for the windows in your home. Because nearly all replacement windows are manufactured to the exact specifications of the customer, it’s important for the company doing the installation to get this step right. Proper measurements will ensure that the windows will fit exactly in the opening. That, in turn, ensures a weather-tight, long-lasting seal and protection from the elements.

The width of the rough opening should be measured at the top, middle and bottom. The height of the opening should be measured in the middle and at both sides.

To ensure a good fit, the outside dimensions of the window should be at least ¾ of an inch thinner and a ½-inch shorter than the smallest width and height measurements, says This Old House general contractor Tom Silva.

Usually the contractor will schedule an appointment to visit your home and take these measurements.

Remove the Old Window

OK, the measurements have been taken, the order for new windows has been placed, and the replacement windows have arrived at the jobsite. Now it’s time to get to work.

If necessary, the installation company will probably be removing the older windows before replacing them. When they begin the work, they should take care at this step to make sure that they don’t cut too far into the original weather barrier or house wrap, which usually consists of sheets of specially coated material that’s designed to keep water out of the walls. This is important, because they want to make sure that the new window can be integrated into the older weather barrier.

At this early stage, it’s also important for the contractor to remove all traces of the sealants that held the old window in place so that the new sealants will adhere properly to the opening.

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Weatherproof the Opening

This might be the most important step of the whole window-installation process – and it’s one that is frequently done incorrectly. That can lead to expensive repairs and replacements.

Brendan Welch of Parksite, a company that serves the building products industry, says about 60 percent of builders don’t understand the proper installation techniques for this process,  called flashing. (Flashing is both a noun and a verb; it can refer to the materials used for weatherproofing a window, as well as the act of installing that material.)

One of the most important techniques for installing flashing is to put it on in “weatherboard fashion.” It means putting the flashing around a window from the bottom up. That way, when water hits it, it runs off the lower portion of your flashing. Overlapping the  existing flashing pieces from the bottom going up directs water off of it instead of behind it.

Flashing carefully around the top and bottom of a window opening is important as well. Missteps at this point in the job can create a lot of problems.

David Delcoma of MFM Building Products, which manufactures flashing materials, says it’s vital to waterproof the sill before putting the window in. He says inexperienced installers will put a window in and then use flashing tape on all four sides. That doesn’t give the water anywhere to go.

Another issue is flashing the header or the top of the opening. Tony Reis of MFM Building Products says the installer must cut back the house wrap and put the tape on the substrate. A common mistake he sees is installers going over the house wrap. When they do that, they’re basically creating a funnel. Any moisture that’s coming in behind the house wrap will go right into the window.

Installing the Window 

Silva says installers should use care to fold out the windows nailing fins before lifting the window to the opening. Then, they should set the window’s sill into the bottom part of the rough opening. Next, they’ll gradually push the frame in until all the nailing fins are flush against the wall.

Once the window is in place, the installer should use a level to make sure it’s properly aligned with the opening.

After that, the window installer will use nails or screws to secure it to the opening. He’ll apply more flashing to the exterior around the window. Finally, sealants are used to make sure that the window keeps air out. Foam sealants are often a popular choice for this part of the job.

Ready to Replace?

One of the best ways to find qualified window installers is through The website’s window dealer locator will help you find a qualified company to do the work. We have partnered with affiliated businesses all across the United States who have the skills needed to replace your windows expertly when you’re ready. Visit our site today to learn more. attempts to provide accurate information but cannot be held liable for any information provided or omitted.  You should always work with a licensed, insured and reputable glass shop that can assess your specific needs and local building codes and offer professional services. Never attempt to cut, install, or otherwise work with glass yourself. All content is provided on an informational basis only.

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  • This was a really helpful article to read for someone who is interested in getting a window installation. After my fiance and I moved into our new home, we decided that this would be our best option. Now we know that there will be a point in the process when sealants, nails, and screws will make sure it stays in place.

  • That is good to know that after selecting a contractor, the next step in installing windows is to get the exact measurements of the openings for the windows. My husband and I built our house a while ago and we have odd window sizes. I’m glad that all replacement windows are manufactured to the exact size that the customer needs.

  • Thanks for helping me learn how windows are installed. I actually didn’t know that you should first measure the old window and get the exact measurements for the window to fit right in. I’m interested to learn if there are some cases where you should remove the old window first to get accurate measurements.

  • My husband and I are finally building the home we want and are looking forward to the process. I found it very interesting to learn about the window installation process, especially that the installers pay special attention to weatherproofing the window area in a “weatherboard fashion.” It’s good to know that this step is not overlooked in the installation process.

  • Should I need to get new windows installed on my property, I’ll be sure to ask around for contractors with InstallationMasters certificates. It sure is nice to know that your contractor has gone through the proper training for the job. You hired professionals for a reason, after all. Thanks for this really helpful guide about the window installation process.

  • It is good that you mentioned that the next step in the window installation after getting the right contractor is the taking of the exact measurement of the openings for the windows at home. That’s a good reminder as my mother is set to hire a contractor for the replacement of the window that she wants to be done this month in time for the holiday season. I might as well advise her to take that measurement before seeing the contractor to give that person an idea of how large the window will be and also decide at once what type of window to use as a replacement.

  • I appreciate your tip about how when hiring a professional contractor to install a window, you should make sure they meet high standards so you can be assured they are trained and knowledgeable of the process. My husband and I are looking for a way to fix the cracks on our kitchen windows, but we aren’t sure how to make sure we’re hiring someone trustworthy. We’ll have to find a window glass replacement company in our area that is qualified and highly trained and that can help us get the glass we need for our kitchen.

  • Thanks for explaining that a level will be used after the window is in place to make sure it’s aligned with the opening properly. My husband and I want to use his recent bonus to have new, custom windows installed in our home. I wanted to better understand the process before we hire a contractor, so thanks for explaining how a level is used in the process!

  • Thanks for these tips on how to replace your window. It would be smart to find someoen who is qualified to help out with this as well. My husband and I are looking for a window installer, so we’ll have to follow your advice and measure the windows first.

    • Kenneth,
      Thank you for the question. This depends on many factors. It can vary depending on part availability from manufacturers, work backlogs, weather, employees, the size of the job, and more. In the current market, it is not uncommon to experience long wait times due to supply chain backups and a shortage of skilled labor.

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