Window Installation Gone Wrong:
A Guide on What Not To Do

Common Window Installation Mistakes and the Many Signs of a Bad Window Installation

Good window installation is essential to the performance of your new windows. But sometimes, contractors can cut corners or make costly mistakes.

Poor window installation can lead to many problems, such as water damage and high energy bills. The best way to avoid these problems altogether is to only opt of professional window installations from reputable window experts.

But if you’ve already had a window installation and you suspect it wasn’t done right, you don’t have to live with the consequences. Instead, you can look for the signs of poorly installed windows and get the actual pros to help fix the mistakes.

Here’s a look at the most common window installation mistakes, along with the outcomes of these mistakes and the signs to determine if your windows were installed incorrectly.


Unfortunately, poorly installed windows happen, either through inexperience or a lack of attention to detail.

Inaccurate Measurements

One of the most common mistakes that is so easily avoidable is not taking careful measurements of the windows and window openings before installation. The wrong measurements can lead to air and water leaks, poor operation, and an uneven appearance.

Messy Caulking

Caulking on the outer perimeter of your new windows is your windows’ first line of defence against the outdoor elements. But if the caulking seal isn’t even and continuous, there will likely be gaps that will allow in moisture.

Caulking requires a clean surface to ensure a good bond. The surface should be cleaned with isopropyl alcohol and a wet rag/dry rag first. If the surface is dusty and dirty, the caulking won’t stick to the surface, and it will allow moisture to seep in.

Caulking also needs to be tooled with a caulking spatula to stick properly. With the right tooling, the caulking will have a continuous, smooth, and concave surface. In contrast, poor caulking will be bumpy and uneven and it won’t last long.

Caulking takes about a month to dry and settle, so make sure to inspect the caulking after this time.

Stains & Smudges

The glues, caulking, and cleaning agents window installers use can end up leaving stains on your window glass. So look for these stains during your post-window installation inspection so the installers can address them immediately.

Missing Connection between Window and Weather Resistive Barrier (WRB)

A continuous WRB (flashing) is essential to the building envelope since it prevents air drafts and water leaks. But sometimes, window installers don’t seal the WRB to the window frame on all sides. And as a result, the windows have drafts and leaks.

Missing/Unsealed End Dams

Windows require gutters, flashings, or sub sills beneath the window openings when there are multiple windows, or there are trim pieces/receptors that could let water into the window system. These gutters, flashings, or sub sills also need well-sealed end dams to prevent water from running into the wall cavity.

Poor Shimming

Shims are plastic or wood spacers used between the window frame and its surrounding structure. Fasteners, like screws, pass near or through the shims to anchor windows in place.

Shims should be evenly spaced at the top, bottom, and sides of windows. Shims must be used on the sides of windows to keep the frame square. And the shims at the bottom of the window should be spaced according to manufacturer instructions to avoid having the frame distort under the weight of the window.

As a result of poor shimming, window frames move out of position over time and can lead to serious performance issues.


Keep an eye out for these signs of poor window installation and address these as soon as possible to avoid further damage and costs.

visible gap in window
Source: Brennan Enterprises

Visible Gaps

If there are noticeable gaps between the window frame and the wall or sill, then the windows don’t fit properly, and the installer likely did not take careful measurements before installing the window.

These gaps allow air and moisture to seep into your home, causing water damage and a spike in your energy bills. They will also make your home less comfortable and outdoor noises louder.

sloppy caulking window
Source: BOMA

Sloppy Caulking

If the caulking around your windows is noticeably messy, then the contractor didn’t take the time to caulk the windows correctly. And a bad caulking job will eventually lead to leaks, drafts, water damage, and higher energy bills.

drafty window blowing out a match
Source: Binswanger Glass

Drafts & Higher Energy Bills

A sure sign of poor window installation is drafty windows. If you hear wind whistling or feel air coming in through your closed windows, then there is an air leak. And you’ll likely notice your energy bills going up as well. Also, if you can hear outdoor noises clearly from inside your home, the window is not well insulated.

You will need to address these drafts to ensure your windows are more energy-efficient.

damaged window from condensation
Source: Feldco

Water Leaks & Damage

Water leaks will cause noticeable water damage. Signs of water damage will be found along the window sill, frame, and surrounding walls, and include:

  • Peeling paint
  • Powdery black spots (mould)
  • Rotting wood
  • Puffy or sagging walls
condensation between window panes
Source: Feldco

Condensation Between Window Panes

Condensation between the glass panes of your windows indicates improper installation and poor seals since moisture is getting inside of the windows.

window painted shut
Source: London Sash

Difficulty Opening or Closing the Window Sash

While older windows might be difficult to open and close, new windows should operate easily and smoothly. If your windows stick, creak, and require effort to open, then the sash is likely the wrong size for the rest of the window.

Along with being a problem to open, the wrong sized sash can lead to drafts, water leaks, and mold and mildew growth.

unlevel windows
Source: Brennan Enterprises

Windows are Not Level

If your windows appear crooked when you look at them, then you likely had a bad replacement window installation. Windows should look great and be levelled.


Skilled window installation experts are careful and attentive during window installation and repairs. And they avoid making these costly installation mistakes by doing it right the first time.

The pros will even have excellent warranties on both the window products and labour, so you can invest in new windows worry-free.

Windows are an investment in your home’s appearance, comfort, and energy-efficiency, so only leave window installations to the reliable pros.

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