Why Windows Leak and How to Control Water Ingress Effectively — Part 1

Published On: March 23rd, 2022Categories: Building Science

Windows and the window-wall interface are significant contributors to water problems in residential buildings. But to eliminate the issue, the architectural community must move past the debate of whether a window will leak; as it almost always will. Instead, the focus should be on strategies to manage water leakage between the window and wall interface.

This is a two-part series. In this installment, Part 1, we’ll explore where windows leak and how to integrate various control layers to ensure water ingress does not occur. Then next month in Part 2 we’ll explore the Logix Brands solution to better control water ingress through a window assembly or a window-to-wall interface.

Read on to find out more!

Window Details

Why Traditional Installation Methods Cause Windows to Leak

Often enough, installers fit windows into the rough, untreated openings and simply spray-foam the perimeter. And while the spray foam does provide some thermal resistance, it does not provide:

  1. Leaves the sill particularly vulnerable to water ingress
  2. Can’t direct water out and away at the head of the window
  3. Doesn’t adequately air seal the window to the air control layer of the wall assembly
  4. Offers no drainage path for water to escape

These shortcomings mean that water can get in and stay in, leading to the formation of mold or deterioration of the wall assembly materials.

How and Where Does a Window Leak?

RDH Building Science consulting firm answered this question in their research report, “Water Penetration Resistance of Window,” created for the Canada Mortgage and Housing Corporation.

While testing various windows and window-to-wall interfaces, RDH Building Science considered 6 possible leakage paths to determine which of them caused the highest frequency of water penetration and the most deterioration to wall components. These were the 6 paths the company assessed:

Possible Window Leakage Paths 1

Possible Window Leakage Paths 1

Of these, RDH Building Science determined that paths 4 and 5 represented the highest frequency water entry and resulted in the most amount of water damage.

Leakage Paths – Risk of Consequential Damage 1

Leakage Paths – Risk of Consequential Damage 1

RDH Building Science also pin-pointed 6 risk factors surrounding window design and installation that allowed water leakage paths to occur in the first place. These are:

  • Sealants
  • Gaskets and tapes
  • Penetrations
  • Components
  • Window design and selection
  • Quality assurance/quality control

Below, let’s take a look at the control layers of a window-to-wall interface, as these are crucial in preventing and controlling leaks.

Critical Control Layers of the Window-to-Wall Interface

A control layer is deemed “critical” when it’s essential to the successful performance of the building enclosure. Some of these critical barriers are easier to achieve than others.

The section detail below highlights these critical control layers:

Critical Barriers at a Window to Wall Interface 1

Critical Barriers at a Window to Wall Interface 1

Water Shedding Barrier

The function of the water shedding barrier (aka water resistive barrier) is to deflect or drain water away from the building enclosure.

The water shedding barrier consists of:

  • Glazing
  • Glazing tape between the glass surface and window frame
  • Exterior surface of the window frame
  • Sealant between the window frame
  • Sill drip flashing
  • Exterior surface of the cladding

The NBCC refers to this barrier as the “first layer of protection.”

Exterior Moisture Barrier

Next comes the exterior moisture barrier, whose function is to handle some of the water without allowing damage to the interior finishes or wall assembly materials.

The exterior moisture barrier is provided by the:

  • Exterior glass surface
  • Seal between the glass and the window frame
  • Seal between the window frame and sub-sill membrane
  • Sub-sill membrane
  • Exterior sheathing membrane

The NBCC refers to the exterior moisture barrier as the second plane of protection.

Air Barrier

An air barrier prevents the infiltration and exfiltration of air — i.e. the flow of interior conditioned air to the exterior, and vice versa, respectively.

If air movement in either direction is not controlled, a significant amount of moisture can find its way into the building enclosure and lead to the formation of mold and deterioration of materials.

Air leakage also impacts the energy consumption and comfort of a building. To achieve a durable, low energy , and comfortable building air leakage must be minimized.

For an air barrier to be effective, it must extend throughout the entire building enclosure and connect to other components, such as windows. That’s why a seal between the window frame and the wall air control layer is essential — it connects the two air barrier components and forms an effective, continuous air barrier system. Often, spray foam insulation is used to form this seal, however, flexible caulking and a backer rod provide superior performance.

Vapor Retarder

Water vapor that tries to enter an assembly is restricted by the presence of a vapor retarder (codes reference barrier), which is typically provided by:

  1. Polyethylene sheet
  2. Window frame
  3. Interior face of the sheet of glass

In most climate zones in Canada and the USA, the vapor retarder is located on the warm side of the assembly.

How Windows are Sealed to the Exterior Moisture Barrier

Two ways are typically used to seal windows to the exterior moisture barrier.

Face Seal

With a “face seal,” a window is sealed to the exterior face of the exterior sheathing membrane.

A face seal configuration does not provide an adequate barrier when water infiltration occurs. Due to this, a face seal detail should only be used in areas of low rainfall, or where an overhang protects the window 2.


In a rainscreen configuration, a window is not sealed in line with the exterior sheathing membrane. Instead, the exterior moisture barrier wraps towards the interior of the rough window opening with an air gap that functions as a capillary break. This capillary break minimizes any water that bypasses the shedding surfaces, and also allows excess water to drain.

Water Penetration Control Strategy for Windows (Ricketts et al., 1984)

The Importance of Pan (Sill) Flashing

As we mentioned above, most water leaks occur through the window-to-wall interface and due to gravity end up at the base of the window . That’s why it’s critical to properly detail the sill pan flashing when designing a rainscreen connection between the rough opening and the exterior moisture barrier.

Pan flashing is a key component, which keeps water away from reaching sensitive materials, such as wood and gypsum board. The objective of pan flashing is to collect and redirect water towards the exterior of the building enclosure. The component is installed directly below a window, and may extend up the entire window jamb, or to a specified distance.

Pan Flashing Materials 2

Pan Flashing Materials 2

Dr. Joseph Lstiburek, from the Building Science Corporation, notes that there are 4 essential characteristics of pan flashing 2:

  1. The pan flashing surface is a durable, waterproof material that provides a continuous water barrier without holes, tears, or wrinkles that could retain water in the opening
  2. The pan flashing has a back dam or positive slope to direct water to the outside of the wall
  3. The pan flashing has end dams at the sides to prevent water from moving laterally into the wall
  4. The pan flashing laps over the drainage plane beneath the opening
Back Dam Options for Membrane Pan Flashing 2
Back Dam Options for Membrane Pan Flashing 2

Back Dam Options for Membrane Pan Flashing 2

Wrapping It Up

The information presented above should help the architectural community understand the following:

  • Typical window leakage paths
  • Critical control layers in a window-to-wall interface

In the next post, we will discuss the steps to implementing a durable window flashing detail with Halo® Exterra® and Logix ICF with Pro Buck. So, be sure to bookmark our blog and check back next month to learn more!


  1. Water Penetration Resistance of Windows – Study of Manufacturing , Building Design , Installation, 2002
  2. Pan Flashing for Exterior Wall Openings for All Climates (BSC Information Sheet 302), 2013.
Tyler Simpson, MBSc.

Manager of Technical Services & Building Science (Canada) for Logix Brands

[email protected]

Tyler is passionate in the design of building enclosure systems that are durable, safe, efficient, and engage material selections that limit advancement of carbon emissions. His educational background provides a solid foundation of field research, hygrothermal modeling, and forensic investigation of building failures.