Don’t ignore a leaking window—it might seem inconsequential, but even a small leak can lead to big problems down the road. Fortunately, you don’t have to be a contractor to find the source of the leak—you just have to be willing to deconstruct your wall assembly enough to figure out how the water is finding its way in.

We documented a recent experience investigating a leak in one of our team member’s Seattle-area homes. Below, we share what we found, what we did to address it, and some best practices for repairing leaky windows and putting things back together.

Notice moisture, leaking, or water damage

If a leak exists, it’ll likely show itself during or after heavy rainfall. Our team member realized their kitchen window was leaking when they noticed minor water damage and signs of water inside, particularly after rainy days. Their home had been remodeled by the previous owner, and over time they’ve learned those updates weren’t always done properly. Since leaks are often a result of improper window installation, this clue became the starting point of our investigation.

Check window installation

Checking to ensure your window was properly installed is a good place to start, whether you suspect faulty work or not. First, remove the exterior trim to see the flashing and weather-resistant barrier (WRB) used. If caulking products were part of the original installation, remove those carefully so you don’t cause any unwanted damage to your siding material or the window frame itself. If you don’t find any (WRB) or flashing products—or you find products that aren’t designed to work together—that’s a sign of improper window installation. Not sure what that means or what that looks like? These two posts can help get you started:

In our case, when we removed the exterior trim, we noticed the window was installed improperly. They did not use any flashing tape products and the layering details of the WRB product they used were inadequate. We could also tell that water had unfortunately gone behind the weather-resistant barrier, coming in contact with the wood underneath, but we weren't sure how deep the problem went.

Search for water damage

At this point, simply covering up water damage by replacing some of the flashing or fixing the exterior trim can lead to serious structural issues, such as rot, dry rot, or moisture seeping into the house. Continue your investigation until you understand the extent of the damage—that means being prepared to remove the siding until you can see where the water damage has stopped. 

Note that, because of gravity, once you locate water damage, there's a good chance it will migrate further down your wall assembly. Also, if the water has gotten past your WRB and sheathing, it often means that it's come in contact with your wall framing.  

After finding that our team member’s window wasn’t properly waterproofed, we selectively removed the siding around the window and carefully peeled back the WRB to see if water had damaged the sheathing or wall framing. In our case, a large section of the wall sheathing was totally rotted and some of the two-by-four framing under the window was compromised. We also noticed a small patch made by the previous owner, where they had tried to fix the leak in the past, but obviously hadn’t addressed the issues (further up the wall) that were the source of the issue.

Evaluate repairs based on your skill and comfort level

Now that you know the extent of the situation, it's time to decide what you need to repair, and if you want to do the work yourself or hire a professional.

Here's what we did:

We fixed the wall assembly by cutting out the damaged section of sheathing, removing the damaged framing lumber, and replacing both with new material. At this stage, we also had to answer the age-old question in this type of remodeling work: Where do you stop? 

This house was built in 1910, and we could see where other remodels and repairs had been attempted in the past. We tapped all surrounding wood to check for softness and found everything to be pretty solid—giving us confidence our solution would go a long way towards solving the problem. If we had two full weeks available and a little more budget to commit to the project, we likely would have done more to clean up the framing in the old wall assembly.

After repairing the wall assembly, we did our best to integrate new Tyvek® DrainWrap™ with the existing WRB. We then used Tyvek® FlexWrap™ on the window (to the extent we could in this scenario) and reinstalled the window. Our last step was to install the Tyvek® flashing tape on the sides and top of the window, along with one piece of stick flashing above the window.

Again, you can see how to properly install windows and appropriate WRBs in this tutorial. 

Regarding the “where do you stop” conversation, we chose not to remove all of the siding above the window. This decision meant reusing an existing piece of flashing that was protecting the exterior trim above the window. The risk here was not knowing the height of the back leg of the flashing or if it was integrated improperly with the WRB. 

We all agreed that if the window continued to leak in winter, we would commit to another day of work next spring to remove the rest of the siding up to the belly band and fully evaluate the flashing and WRB behind it.

Gather more advice on the Dunn Solutions blog

The lesson learned here? Don't wait to fix a leaky window! Like we mentioned above, even a seemingly small leak—especially if left unattended—can cause significant damage, requiring more time, work, and money to fix. It's also crucial to follow best practices when fixing or installing anything in your home—both to protect the home's structural integrity and to preserve its value.  

Waited too long to check what's causing your leaky window? If you’ve caught your leak too late and discover damage that’s spread throughout the wall, it may be a good idea to call in the help of a trusted contractor or structural engineer. 

Looking for other window-related help and resources? Check out this post on how to repaired decayed window trim or other Dunn Solutions posts and videos. You can also head to one of our four window showrooms and speak with a Dunn window specialist.