• Matt Weber

Window Replacement – DIY or Why Not?



Window replacement can be a difficult task for a DIY’er and should probably be left to the pros.


I know this because I’ve installed a few windows, and I’m not a pro window installer.


“But Matt, this blog post clearly shows photos of you installing a window, so if you can do it, then why shouldn’t I?”


I’m not going to make a blanket decree about who should and should not attempt window installation. The insulated Weather-Shield replacement unit shown here looks beautiful, works great, is sealed up tight to conserve energy—and I’m proud of the work I’ve done.


Nevertheless, I thought it would be helpful to point out some of the on-site problems an installer is likely to encounter—obstacles and difficulties that won’t be covered in a manufacturer’s installation manual—and which may help you decide whether you’re up to the task, or if you should call a contractor to do the job.

On this project, we installed a Weather Shield Premium Series Double-Hung Window, which is an aluminum-clad wood window. The extruded aluminum resists dents, dings and scratches and is backed by Weather Shield’s 20-year limited warranty. It’s finished with commercial-grade premium AAMA 2605 paint that resists fading and chalking.


Straight Talk about What to Expect

First and foremost, today’s double-pane insulated windows are heavy. The larger they are, the heavier they are, and the more difficult they are to handle. Furthermore, glass is fragile and requires great care when carrying and maneuvering a window through the home and into the rough opening. Installation often requires at least two people.


Ladder work may be required. Although replacement windows can usually be installed from inside the house, the old windows might have to be removed from outside the house, which can require dangerous work from atop a ladder.


Removal of the old window can be more difficult and time-consuming than you anticipate.

Preparation of the rough opening and installation of window casing may require more tools, materials and skills than you expect.


The replacement window’s step-by-step installation manual will be critical for any job, because window construction differs among models and manufacturers. However, the manuals generally only provide guidance when installing in “best case scenarios,” without providing solutions to the various construction difficulties that can be revealed once the old window is removed. The manuals tend to make the procedure seem simpler in theory than it actually is in practice, and that’s where pro-grade experience can pay off.


Here’s What Happened on This Project

I should mention that the project shown is not my first rodeo with windows. A few years ago, I replaced a few units in a brick home, but admittedly I was out of practice for this project. Back then, we replaced single-pane aluminum windows by first removing the sashes then prying the old frames from out of the surrounding brickwork. That was my plan for this project as well.


However, when the Big Day arrived, I found the glass sashes of the old window to be locked inside the metal window frame. I tried and tried, but could not figure how to remove the sashes without breaking the glass, which would be a huge, dangerous mess (at a house full of children).


To avoid breaking the glass, I opted to pry off the window trim and cut away the wood siding around the metal window frame. Note that I’m already reaching for a powered circular saw to remove the old window—and that’s a tool that is not mentioned in window-installation manuals. Plus, all this siding and trim demolition will require significant repair work.


After I chopped away the siding, I pried out the surrounding flange nails then lifted out the old window, sashes and all. Thankfully, this was a ground-level window between a kitchen and screened porch, otherwise I’d need to do all this extra work while clinging to a ladder.

Had I known that I would have to cut away the siding from around the window, I could have ordered a new-construction window (with a nailing flange) as a replacement rather than the new pocket-style window which I had on site. (A pro installer might have determined this in advance.) Having a flanged replacement could have simplified this job, because now that the old window was out, I discovered the rough opening was not level and plumb, as required for the pocket window. The sides of the opening flared out toward the bottom and required adjustment.


Keep in mind, the entire time the window is out, unconditioned air, dust and allergens are drafting throughout the house. You want to work as quickly as possible, and surprises such as an out-of-square opening don’t help the situation.


Prior to installation, I had to fasten exterior window stops (plus shims to square up the perimeter) at the top and sides of the rough opening to prepare for the new window. The “stops” are strips of exterior molding that frame the window and literally stop it from pushing all the way through the wall when you place it inside the opening.


I added flashing to the bottom of the window opening. Although the window is covered by a roof, I planned to use the exterior window stool as a beverage shelf, and the flashing adds moisture protection in case of a spill. Always use a roller to press down your flashing tape.

I ripped strips of stop molding on my table saw and brad-nailed them to the rough opening. Be sure the stops don't encroach more than 1/2" inside the window frame.


That wasn’t the only hurdle. When I test-fitted the window, I found that the bottom of the rough opening was flat, but the bottom of the pocket replacement window was sloped to fit a sloped sill (and shed water away from the house). This was a smart window design, but to support the window, I had to cut a sloped (wedge-shaped) shim to install beneath it and provide lower support. I bevel-cut the sill board on my table saw—which is another tool not mentioned in a window-installation manual and one which not every DIY homeowner will have at their disposal.


I also discovered another construction issue: The thickness of the wall was inconsistent from one side of the window to the other. This problem resulted in a 1/4-in. inconsistency with the “reveal,” which refers to the measurement of construction materials that will surround the installed window. In a nutshell, you want the surrounding wall and trim to look the same all the way around the window.

Here's the wedge-shaped shim block I added beneath the sloped window bottom.


My workaround solution: I installed the window flush with the interior, which simplified trim installation in the kitchen. The screen-porch side of the window was trickier.


The window did not sit flush on the exterior side of the wall. So, when repairing the butchered siding with a variety of trim boards (and shims), I was careful to conceal the problem with caulk and carpentry “magic.” I’m a good trim carpenter and a decent magician, but disguising this anomaly in the trim might drive some less experienced DIY’ers absolutely bonkers.


So yes, a DIY’er can feasibly install their own window, but most should call a pro. You’ve been warned.


Don’t Screw it Up

To those ambitious homeowners who might ignore our advice to call and pro and instead charge ahead with a DIY installation: Proceed with caution. Understand that if the window is installed improperly and is not plumb, level, and square, a number of problems might occur.


The double-hung or slider sash may be difficult or impossible to remove. A casement sash may not operate properly due to excessive drag on the sill. The sash pivot bar on double-hung windows could bind and cause the sash to become inoperative. Or, the weather-stripping may not seal properly— allowing air and water infiltration, even if the sash is locked. Remember that products vary by design, so all window installers—both professionals and DIY—should always consult the manufacturer’s guidelines for installing their specific units.


Installation Tips and Techniques

When installing the window, apply high-quality, exterior-grade sealant along the inner edge of the exterior stop molding. Liberally apply three beads of sealant to the window sill, and at the bottom corners of the rough opening.

Your replacement window has to be level, plumb, and square, otherwise it won’t open or close properly, and air and water could infiltrate around the edges.

Check both sides for plumb and the bottom for level.


To check for square, measure both window diagonals from corner to corner to make sure they match.


Adjustment can be made with shims, which should be installed at all anchor points and anywhere necessary to keep the unit correctly in place.

Inflatable Air Shims helped to adjust the window's fit during installation. Just slip them between the window and house frame, then pump them up or deflate them as needed to adjust the window's position.

Once you're satisfied with the window position, insert permanent wood shims at all the screw locations.

Once the window is plumb, level, and square, drive the installation screws (included with the window) into the prefabricated holes in the jamb. Don’t over-tighten, or the window might bow and not function properly. Cut the excess shim material flush with the wall.

Check the sash for proper operation once the screws are in place. If there are any gaps greater than 1/8 inch between the window and house frame, loosely pack insulation in the interior. Spray foam is acceptable if the product is designated for “windows and doors,” so it won’t expand to the point that it could prevent proper operation.


Reinstall new stop molding on the installation side, then caulk, trim and paint to your preference.

Here is the fully sealed Weather Shield window ready for new trim.


- M. Weber


Learn more about window replacement:

· How to Measure a Window for Replacement

· Selecting New Window Styles

· Reduce Condensation on Windows

· Installing Craftsman Window Trim

New Windows Save Energy

Select replacement windows with energy-efficient designs for year-round thermal performance. New windows with vacuum-sealed, double-pane glass can reduce a home’s power bill by reducing heat and cold transfer.


Low-E coatings reflect infrared light to keep the heat in during the winter and outside in the summer. The term “Low E” means low emissivity. Emissivity is a property of materials such as glass, which light can freely pass through. Low E is a coating of microscopic layers of silver sandwiched between layers of anti-reflective metal oxide coatings. Added to the surface of window and door glass, Low E provides greater energy efficiency, increased comfort and protection from damaging UV rays. By filtering out the part of the light spectrum that transmits heat, Low E reduces a window’s U-Value and increases its R-Value. These coatings can also reduce fading in carpets, artwork and photos by helping block the damaging ultraviolet rays. Learn more about window options at Weather Shield.

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