Hurricane Window Damage Assessment

Hurricane Window Damage Assessment

Brian G. Johnson, M.S., P.E.

One common assignment forensic engineers receive following catastrophic weather events is to determine the cause of water intrusion into a building. Frequently, it is alleged by the building owner or representative that wind forces damaged the windows of the building, resulting in leaks and damage to the interior finishes. The proposed scope of repair is replacement of all windows and repair of moisture damaged interior finishes. However, moisture damage to the interior finishes surrounding a window is not necessarily indicative of wind related damage to the window components.

In storm events with heavy rainfall and elevated wind speeds (such as when Hurricane Irma passed through Florida), window openings upon windward facing exterior wall elevations are susceptible to moisture intrusion from wind driven rain. Such wind driven rain can infiltrate at the interface of windows and abutted cladding systems, or through the window system itself. Factors which can exacerbate such moisture infiltration mechanisms include deteriorated sealants or weather-stripping components, construction deficiencies such as ineffective flashing provisions, and/or window manufacturing deficiencies. Windows are designed to allow water into the system.  Such water (under normal storm conditions) is designed to drain to the exterior of the structure through weep holes in the window system. Just because extreme storm conditions resulted in water infiltration does not mean the window system (the structural components supporting the glazing) were damaged by the storm.

The first step in assessing leaks at window openings is to confirm the moisture source is wind driven rain infiltration through the window system. Sometimes, rain can infiltrate a building at locations such as patios, vents, or cladding cracks, migrating through the exterior wall envelope and manifesting itself as moisture damage to the interior finishes surrounding windows. Also, it’s common to see instances of moisture damage surrounding windows which is consistent with condensation formation as opposed to moisture infiltration, in particular with single pane window systems. As such, it’s critical to document moisture stain paths, moisture content levels, and exterior conditions, along with interviewing the building owner/representative, to confirm the leaks originated at the windows.

Windows are generally classified based upon their (a) performance class and (b) performance grade. The window performance classes (R, LC, CW, and AW) are as follows per the NAFS 2011:

Window Performance Class Description
R Commonly used in one- and two-family dwellings.
LC Commonly used in low-rise and mid-rise multi-family dwellings and other buildings where larger sizes and higher loading requirements are expected.
CW Commonly used in low-rise and mid-rise buildings where larger sizes, higher loading requirements, limits on deflection, and heavy use are expected.
AW Commonly used in high-rise and mid-rise buildings to meet increased loading requirements and limits on deflection, and in buildings where frequent and extreme use of the fenestration products is expected.

The window performance grade is the allowable structural design pressure of the window in pounds per square feet (psf). For instance, an R15 window would be installed in one and two-family dwellings and be rated for a structural design pressure of 15 psf; whereas an LC25 window would be installed in a low/mid-rise multifamily dwelling with a structural design pressure of 25 psf. Windows are typically water tested at a pressure of approximately 15%-20% of the structural design pressure. The following table provides sample minimum water penetration resistance test pressures per NAFS 2011.

Minimum Structural Design Pressure Minimum Water Penetration Test Pressure Approximate Wind Speed Equivalent to Water Test Pressure
15 PSF 2.92 PSF 33 mph
25 PSF 3.76 PSF 38 mph
30 PSF 4.59 PSF 42 mph
40 PSF 6.00 PSF 49 mph
50 PSF 7.50 PSF 54 mph

As such, water intrusion through a window opening from wind driven rain would happen well before any structural damage to the window system would be expected to occur. For example, when Hurricane Irma passed through Miami, its peak wind gust speeds were estimated at 100 mph, with 8-10 inches of rain. Buildings in the Miami area have typically been designed to withstand wind speeds of roughly 150 mph (which can vary depending upon the applicable building code and building type). For a location with design wind speeds of 150 mph, windows would be expected to reach their maximum water resistance capacity upon being exposed to roughly 58 mph wind driven rain. As such, some water infiltration of window openings would be normal and expected upon exposure to 100 mph winds and 8-10 inches of rain when Hurricane Irma passed through the area, absent of any structural damage to the window system.

In summary, claims that wind forces damaged building windows and caused resultant leaks and moisture damage should be thoroughly investigated. First, it’s important to confirm the leaks originated at the windows. Second, is there any evidence of wind related damage to the window system, such as visible displacement of the window framing or glazing fractures from windborne debris? If not, it’s likely that wind driven rain overwhelmed the window assembly or associated flashing/sealant components, resulting in leaks, absent of damage to the window system. Establishing whether windows have been damaged during hurricane events is critical so correct coverage decisions can be made. Windows damaged by wind or debris during a hurricane event may necessitate repairs to interior finishes, replacement of the window system and/or glazing components, and localized removal and replacement of cladding components to access windows. Alternatively, wind driven rain infiltration of a window system, absent of structural damage to the window, may necessitate interior finish repairs only.


  1. North American Fenestration Standard/Specification for windows, doors, and skylights – 2011 Edition (NAFS 2011).

Should you have any questions about this article or any other claim matter, please email Brian  Johnson  at or call (404-313-5402).

This newsletter is a publication of Southern Loss Association, Inc., P.O. Box 421564, Atlanta, GA 30342. The articles written in the newsletter are in a general format and are not intended to be legal advice applicable to any specific circumstances. Legal opinions may vary when based on subtle factual differences. All rights reserved.  Published 03-06-18