Why Windows Leak: Understanding the Real Cause of Water Intrusion
When homeowners discover water around a window, the first reaction is often to blame the window itself. While windows can fail over time, the window is rarely the primary cause of the leak. In many cases, the problem lies with the surrounding building enclosure and how the window was integrated into it.
At Building Enclosure Forensics, LLC (BEF), our investigations routinely reveal that replacing a window without addressing the underlying building enclosure deficiencies often results in the same leak returning.
Windows Are Designed to Manage Water
Modern windows are not intended to be completely waterproof. Instead, they are engineered to manage incidental water that reaches the window assembly.
A properly installed window functions as part of a complete building enclosure system that includes:
Weather-resistant barriers (WRBs)
Flashing systems
Drainage planes
Air barriers
Sealants
Exterior cladding
These components work together to direct water safely back to the exterior before it can reach the interior of the building.
When one part of this system fails, water often finds a path into the structure.
The Most Common Causes of Window Leaks
Improper Flashing Installation
Flashing is one of the most critical components of any window installation. Its purpose is to direct water away from vulnerable areas around the opening.
Common flashing deficiencies include:
Missing head flashing
Improper sill flashing
Incorrect sequencing of flashing materials
Reverse laps that trap water
Gaps at flashing transitions
These installation defects often allow water to bypass the intended drainage system and enter the wall assembly.
Failed Sealants
Caulk is frequently misunderstood.
Sealant is designed to accommodate movement and prevent water entry at specific joints—not to compensate for poor construction or missing flashing.
Over time, sealants can:
Crack
Shrink
Separate from adjacent materials
Lose adhesion due to weathering
Applying more caulk without correcting the underlying deficiency often provides only a temporary solution.
Inadequate Drainage
Water naturally enters many window systems during wind-driven rain. Quality windows are designed with internal drainage channels and weep holes that allow this water to escape.
Problems arise when these drainage paths become blocked by:
Dirt
Debris
Insects
Paint
Improper sealant application
Blocked drainage can cause water to accumulate inside the frame until it finds another path—often into the building.
Improper Integration with the Weather-Resistant Barrier
One of the most common deficiencies identified during forensic investigations is poor integration between the window and the surrounding weather-resistant barrier.
Even a high-quality window can leak if it is not properly tied into the building's drainage plane.
This is particularly common during replacement window projects where the original flashing and weather barrier are disturbed but not properly restored.
Building Movement
Buildings move.
Seasonal expansion and contraction, foundation settlement, and structural deflection can all create openings around windows that were once watertight.
As these openings develop, water and air can infiltrate the surrounding wall system.
Condensation Is Not Always a Leak
Not every wet window is leaking.
Interior condensation occurs when warm, humid indoor air contacts a cold window surface and reaches its dew point.
Contributing factors include:
High indoor humidity
Poor insulation
Air leakage
Thermal bridging
Inadequate ventilation
Condensation can cause damage that closely resembles water intrusion, making a proper diagnosis essential.
Why Replacing the Window May Not Solve the Problem
Many homeowners replace windows expecting the leaks to disappear.
Unfortunately, replacing the window alone often addresses the symptom rather than the cause.
If deficiencies remain in the:
flashing system,
drainage plane,
weather-resistant barrier,
siding,
roofing, or
adjacent building components,
the new window may begin leaking just as the old one did.
A successful replacement requires proper integration of the entire building enclosure system—not simply installing a new window.
How BEF Investigates Window Leaks
Every investigation begins with understanding how water moves through the building enclosure.
Depending on the project, our evaluation may include:
Comprehensive visual assessment
Moisture mapping
Infrared thermography
Building enclosure evaluation
Review of flashing details
Assessment of drainage systems
ASTM E1105 water penetration testing (when appropriate)
Evaluation of surrounding wall assemblies
Rather than focusing solely on the window itself, we examine how all building components interact to determine the true source of water intrusion.
The Value of an Independent Investigation
Building Enclosure Forensics, LLC does not sell replacement windows or perform repair work.
Our role is to provide independent, science-based evaluations that identify the actual cause of moisture intrusion. This allows property owners to make informed decisions and helps avoid unnecessary repairs that fail to resolve the underlying problem.
Need Help Diagnosing a Window Leak?
If you're experiencing recurring leaks, staining around windows, condensation concerns, or unexplained moisture intrusion, a comprehensive building enclosure assessment can help identify the source before additional damage occurs.
Building Enclosure Forensics, LLC provides independent forensic investigations, building enclosure assessments, and ASTM testing throughout Colorado to help homeowners, property managers, attorneys, and contractors understand why buildings leak—and how to correct the problem properly.
