Why Windows Leak

A scary tale about what’s in your walls from Your Houston Home Inspector.

Many Houston Homeowners are Plagued with Leaking Windows

Many homeowners in the Houston market area encounter leaky windows. It’s a common defect that we see on many of our Houston home inspections. Unfortunately, window leaks are much easier to avoid than to correct and leaking windows cause progressive damage to the structure. Some windows leak due to manufacturing defects, field assembly issues at the mullions and debris accumulation at the water channel and weep holes. However, the majority of window leaks that we encounter are due to faulty installation, which often may require removing the window to correct. This article is specific to flanged window installation (the great majority of windows in Houston homes) in new construction. Replacement windows have different requirements and challenges.

Substandard Window Installation is the Norm

Many builders fail to establish quality standards, adequately supervise contractors and enforce those standards. Some contractors routinely install windows with little or no regard to well established and documented standards, virtually ignoring the manufacturer’s installation instructions which are typically affixed to every window. A window opening necessitates a hole through the exterior wall to the interior. To accommodate the window an opening must be made through the framing, wall sheathing, water-resistive barrier and whatever siding or veneer is installed. Failure to faithfully adhere to clearly documented installation standards is an invitation for water to enter the building envelope where at a minimum it is a nuisance and in many cases will cause extensive damage to the structure over time.

When we perform pre-cover/frame inspections on new construction, window installation is our number one area of focus and, regrettably, it’s the most common area for major defects. To make matters worse, it’s usually too late to do much about some of the deficiencies. Many of the mistakes made during window installation cannot be corrected without removing the windows and other materials from the structure. This is highly disruptive, likely to damage the windows and most builders are simply unwilling to correct the defects. Municipal code compliance inspectors seem to turn a blind eye to window installation deficiencies. This often leaves the buyer with a take-it-or-leave-it decision with a builder who may be reluctant to return the buyer’s deposit, despite substandard construction. If you’re getting the impression that this is a pet peeve of ours, you’re correct. Window installation defects can be easily avoided by simply taking care, supervising contractors, ensuring that installers are properly trained and following established practices (i.e., follow the installation instructions plastered right on the window!).  There is no excuse for a  professional construction superintendent and contractor to be confused when they are told that the window installation does not conform to established standards and is vulnerable to leaking.

There are Well Established Standards for Window Installation

In addition to the manufacturer’s installation instructions, there are well documented international standards which address both the manufacture and installation of windows. Because the scope of the standard covers both manufacture and installation, you will find the manufacturer’s installation instructions to be consistent with these standards. These instructions typically don’t vary greatly from manufacturer to manufacturer. The applicable standards include ASTM E2112 – Standard Practice for Installation of Exterior Windows, Doors and Skylights which provides the basic foundational standard for window and door installations; AAMA 2400 – Standard Practice for Installation of Windows with a Mounting Flange in Stud Frame Construction focuses specifically on installation of windows with a mounting flange in new stud frame construction; FMA/AAMA 100 Standard Practice for the Installation of Windows with Flanges or Mounting Fins in Wood Frame Construction for Extreme Wind/Water Conditions addresses installation of windows with a mounting flange in new stud frame construction, utilizing a membrane type drainage system (e.g., a house wrap) in areas subject to extreme weather conditions (these methods are tested to the equivalent of a 69mph wind force). Because this is not an instructional article, we won’t go into the specific variations between the standards. While some of the particulars and material assumptions vary from standard to standard, they all share a common set of core installation principles which apply to all flanged window installations in new stud frame construction, which is the focus of this article.

The standards include a couple of accepted variations for the installation: Method A (Flashing over the face of the mounting flange); Method B (Flashing behind the face of the mounting flange). Each has a type 1 variant depending on whether the weather-resistive barrier is going to be installed before or after the window. So the methods are A, A1, B, B1. The A1 method is by far the most common installation we encounter during Houston home inspections. Because these standards are copyrighted and available for purchase, we cannot publish them within this article. However, we have established a library of window installation labels from a variety of manufacturers that are available for download. You will find links to them at the bottom of this article.

The folks at TLS Laboratories have put together an outstanding video showing a textbook example of both the A1 and B1 installation methods.

Where Window Installation Goes Wrong

We’ve assembled a list of some of the more common and often serious deficiencies that we encounter with window installation in new construction today. We would like to thank DuPont™ Tyvek® Building Solutions for granting us permission to utilize their images to depict proper methods for the installation details below. While these images are specific to DuPont™ Tyvek® Building Solutions products the methods are common to other membrane type drainage plane systems as well.

Improper Window Opening

window installation framing rough opening

Rough openings for windows should be level, plumb and square, typically 1/2-3/4 inch larger than the window and the window should be centered in the opening. There should be an even gap between the window and the rough opening of 1/4-3/8 inch.

Most window manufacturer installation instructions specify that the opening should be level, plumb and square and that the opening should be 1/2 inch larger than the window itself in both width and height. This leaves approximately a 1/4 inch gap at each side once the window is shimmed to be level and centered in the opening. Too often, the opening is not level or square and is framed much larger than specified.  If the opening is not level, square and plumb, then the window will either be out of level or must be cocked inside the opening. The nailing flanges are designed to extend past the edge of the framed opening so that the window may be properly sealed and secured to the structure. When an excessive gap exists on one or more sides of the window, it can’t be sealed, the flange is not as well supported as it should be and the fasteners may not get enough “bite” into the framing in order to hold the window secure during high winds. This defect is also a contributing cause of several other problems that we will cover individually below. These include lack of caulking at the flange, damaged flanges, inadequate fastening, etc. If the window opening isn’t right, several other aspects of the installation will likely be affected. This condition is typically only detectable by your home inspector during frame/pre-cover inspections, but can sometimes be seen where windows are installed in unfinished attics during a pre-close inspection.

Examples of improper rough openings:

No Caulk at the Nailing Flange

window installation caulking the flange to the rough opening

Caulking the window flange to the water resistive barrier. The calk can be applied either to the rough opening or the window flange but must be applied before the window is set into the opening. Image: DuPont™ Tyvek® Building Solutions, used with permission.

Every standard that we know of and almost every window manufacturer’s installation instructions call for a continuous bead of caulk be placed between the window flange and the mounting surface on at least three sides (top and both sides) before the window is set into the opening. Sadly, we estimate that about 90% of the window installation that we inspect skip this critical step. To make matters worse, with few exceptions, this defect cannot be repaired without the complete removal of the window from the structure. You cannot caulk between the window and the structure with the window in-place. Most builders are unwilling to do this because of the amount of work/cost involved and the high probability of damaging fragile window mounting flanges. We only see a very small percentage of windows with this defect get repaired, even when it’s identified at the framing/pre-cover inspection stage. This is one of those things that it’s worthwhile to sit down with the builder and review installation standards and your requirements ahead of time. Once we identify this critical defect during the framing/pre-cover inspection, it’s usually too late to be corrected. When this defect is combined with other installation deficiencies, such as improper integration with the water-resistive barrier/drainage plane, the potential for the window to leak increases dramatically. This condition is typically only detectable by your home inspector during frame/pre-cover inspections, but can sometimes be seen where windows are installed in unfinished attics during a pre-close inspection.

Examples of failure to caulk window flanges:

No Caulk at Mitered Window Corners/Mechanically Joined Frames

window installation caulking mechanical joints

Caulking mechanically joined windows at the corner

Commonly, metal windows have frames that are mechanically joined at the corners and may have miter or straight seams at these corners (vinyl windows are typically welded at the corners with no gaps). Large windows that are created by joining multiple smaller windows together are typically field joined and have joints at the mullions (this applies to both metal and vinyl framed windows). The seams of these joints present a potential water penetration point so they must be properly sealed, during assembly for mullions and prior to installation for corner joints.  Failure to seal these joints invites water penetration. In the case of an improperly sealed mullion, such a leak will often present itself as a puddle on the inside sill directly below the center seam between two joined window units. This condition is typically not directly detectable by your home inspector during phase or final inspections.

Example of failure to caulk a mechanically joined window:

Windows not Centered and Level/Nailing Flanges not Supported

When windows are not properly centered in the opening, the opening is oversized or out of square, one or more of the flanges will often be left with an oversized gap between the window and the framing. When the gap is oversized the window mounting flange may not fully span the gaps, will not be properly supported and susceptible to damage and may not be able to be secured with fasteners. An unsecured window can flex excessively and even fail during high winds. Arched windows are often installed with no support or backing at all in the arched section. Often framing pieces will be nailed up along the contour of the arch, creating the illusion that the arch is supported, however, these pieces are placed there to support the sheetrock, not the window and don’t extend to the outer edge of the sheathing where they need to be in order to actually support the window. This leaves the window vulnerable to movement and water penetration as the big gaps adjacent the arch are simply bridged with adhesive backed flashing tape. This condition is typically only detectable by your home inspector during frame/pre-cover inspections, but can sometimes be seen where windows are installed in unfinished attics during a pre-close inspection.

Examples of windows not properly centered and leveled and/or nailing flanges inadequately supported:

Improper Nailing/Fastening

All windows need to be secured in the opening. Depending on your location, the fasteners will likely be nails, or screws (especially in high wind areas). Every window will have its own fastening schedule defining the size, type, material, spacing and placement of the fasteners.  Most installation instructions will specify corrosion resistant fasteners. If you are near salt water, they may specify stainless steel. Most fastener deficiencies take one of several forms: wrong location, too few/far apart, too close or far from corners, wrong size/type/material, overdriven through the flanges, etc. Often, if the gap between the window and framed opening is too large, the fasteners will miss the framing altogether or be too shallow to secure the window properly. This condition is typically only detectable by your home inspector during frame/pre-cover inspections, but can sometimes be seen where windows are installed in unfinished attics during a pre-close inspection.

Examples of improper nailing/fastening of windows to the structure:

Improper Opening Preparation

window installation sill flashing

Preparation and flashing of the rough sill for window installation. Image: DuPont™ Tyvek® Building Solutions, used with permission.

The lower sill of the rough window opening should be protected with a waterproof sill pan flashing. These can be pre-made or site made but should cover the entire bottom sill and extend up the sides of the opening. The front edge should extend out past and down the front of the exterior sheathing. The bottom flap of the water resistive barrier which may be folded inside the opening is NOT sill pan flashing.  Most commonly, what we will see is a piece of black plastic stapled across the opening and sloppily bunched at the corners, representing the bare minimum of what they can get away with. The sill pan flashing should be watertight, so it should be free of unsealed holes, penetrations, tears, etc. When an adhesive backed, applied flashing is used, it should be rolled to ensure full contact and adhesion. This sill pan flashing is supposed to guide any water that should get past the window back out of the building envelope. This condition is typically only detectable by your home inspector during frame/pre-cover inspections, but can sometimes be seen where windows are installed in unfinished attics during a pre-close inspection.

Examples of improper flashing of the rough sill prior to installing the window:

Improper Integration with the Water Resistive Barrier

Most residential construction, whether the exterior wall surface will be brick, stucco, stone or siding will incorporate one or more types of water-resistive barrier (WRB) to prevent water from getting into the interior of the wall. The WRB is an integral component of the wall assembly and may take any of a number of forms. Housewraps, building papers (e.g., Tyvek, Greenguard, impregnated paper, etc.) and engineered sheathing products (e.g., Thermoply, Blue board, ZIP system, etc.) can all perform the WRB function depending on the designer/builder’s choice. Regardless of the product or material used, the WRB must meet certain criteria.  It must be resistant to water penetration. It must have continuity without gaps or breaks, including at penetrations. Horizontal transitions must lap in shingle fashion to guide water outward as it migrates down the drainage plane. It must provide a means for water to escape when it reached the bottom of the drainage plane. It’s vital that windows be properly integrated into this drainage plane to prevent water penetration into the structure. When membrane or paper type products are used, they are cut where they span the framed window opening. The flap at the top of the window must be installed on the outside of the top flange of the window to direct water outward. If they are left behind the window flange, water may seep behind the flange at the window header and enter the house. If you combine this with failure to caulk the window flange prior to installing the window, there is a very high potential for the window to leak.  Adhesively applied flashings must be installed in the proper sequence and layered shingle fashion to direct water outward. Metal flashings installed at window and trim headers should NOT be caulked to the veneer/siding above in order to provide a means for water to exit the wall system when it reaches the barrier formed by the metal flashing at the top of the opening. Most of these conditions are typically only detectable by your home inspector during frame/pre-cover inspections, but can sometimes be seen where windows are installed in unfinished attics during a pre-close inspection.

Proper window integraton with the water-resistive-barrier for a rectangular window:

Proper window integraton with the water-resistive-barrier for an arched window: 

Examples of improper window integraton with the water-resistive-barrier:

Installing Windows Over Furring Strips

This issue really falls under the integration with the water-resistive barrier, however, it requires special attention, so we’ll cover it separately. Often times the builder will space the windows out from the sheathing to bridge the gap between the sheathing and a brick or stone veneer. Bumping the window out over furring strips or other spacers can create severe vulnerabilities to water penetration. We rarely see evidence that any consideration at all is given to maintaining the integrity of the drainage plane/WRB when this is done. Now, instead of a single seam between the window flange and the sheathing, there is a seam between the furring and the sheathing and another between the window and the furring. Usually, neither of these are sealed.  The furring strips are usually installed outside of the WRB, preventing the window from being integrated into the WRB. These spacers are typically sloppily cut with large open gaps at the corners, which is an open invitation for water entry. Instead of the window flange being flush to the sheathing, there is a pronounced step at the spacer.  Adhesively applied flashings are often not installed such that they are able to maintain a continuous seal from the WRB to the window flange over this step. This condition is typically only detectable by your home inspector during frame/pre-cover inspections.

Installing Windows After the Siding is Installed

When you see the contractors install siding before the windows, you’re in for trouble. Some builders/contractors will install siding products, prior to installing windows, leaving a gap the width of the window trim to accommodate the flange. It’s impossible to properly integrate the window into the WRB or properly flash the flange through this narrow gap. About the only thing you have to keep water from entering through the window opening is the trim piece around the window and the caulk they apply between it and the siding. This condition is typically only detectable by your home inspector during frame/pre-cover inspections and sometimes not even then if the outside trim is already installed and there is no gap large enough to see inside. We are suspicious any time we see siding around windows when performing a pre-cover inspection.

Damage to Nailing Flanges

The nailing fins surrounding the windows (especially vinyl windows) are fragile and susceptible to damage. Since these flanges are what holds the window in the opening and are integral or creating a water-resistant seal around the window penetration, damage to the flanges can be very detrimental to window weather performance. Damage to window flanges is almost always due to careless and improper handling of the window before or during installation. We often see cracked flanges due to aggressive nailing and curled lower corners, which cannot be sealed, due to dragging and maneuvering the window around on the slab while standing it on its corner. Windows with damaged flanges should be repaired by a manufacturer authorized repair contractor (this is not the installation contractor) or replaced. This condition is typically only detectable by your home inspector during frame/pre-cover inspections.

Examples of windows with damaged nailing flanges:

Improper Flashing

window installation applied flashing layering

Proper application and layering of adhesively applied flashings at windows. Image: DuPont™ Tyvek® Building Solutions, used with permission.

To prevent water penetration at window openings windows must be properly flashed. To be effective, the flashing must be of the right type for the application, applied at the correct time, in the correct sequence, using proper methods. We have seen all manner of things used in lieu of flashing at windows and other penetrations, including sheathing seam tapes; duct tapes; fiber sheathing itself and even duct mastic, which is water soluble; and sometimes nothing at all.

Flashing for windows gets installed in three stages: the window opening/sill, prior to installation of the window – usually plastic or adhered membranes; the window at the water-resistive barrier as the window is installed – usually adhered membranes; the window at the exterior cladding at the cladding is installed – corrosion resistant metal at the head of the window and trim. As mentioned earlier, flashings should always be installed in shingle fashion with the upper edge of the flashing covered by the flashing or water-resistive material above it in order to continually direct water outward as it migrates down the drainage plane.

window installation correct flashing detail

Window flashing integration at lower corners. Image: DuPont™ Tyvek® Building Solutions, used with permission.

Applied flashing must adequately sized and be compatible with the sheathing and water-resistive barrier material (e.g., house wrap) and rolled into place to ensure full adhesion and sealing to the underlying materials.

This condition is typically only detectable by your home inspector during frame/pre-cover inspections.

Examples of improper flashing of windows:

Protect Your Investment

As you can see, there are numerable ways that windows are frequently installed incorrectly, which leads to increased potential for water penetration, few of which are directly visible in a completed home, except for the water that comes inside as a result. Your Houston home inspector routinely finds multiple deficiencies in most homes that we inspect during the framing stage. It’s actually somewhat rare not to find significant deficiencies during a framing/pre-cover inspection. We literally have thousands of photos of window installation deficiencies from inspections.

Because many of these deficiencies cannot be properly remediated short of removing the windows and starting over, even at the framing stage, it’s wise to take steps to ensure that they get installed correctly from the outset. If you are having a home built, we think it’s wise to meet with your builder up-front, review the installation standards for the windows that are being installed and obtain a commitment that they will be installed in compliance with the standards before work starts. You might also ask your builder what steps they have taken to ensure that their window installation contractors follow established standards for window installations. We know one custom builder in the area that consistently gets windows right. When we mentioned it to them, they told us that they had their window installation contractors trained, they have established quality standards and they hold their contractors to those standards. While you might expect this to be the norm, it, unfortunately, seems to be the exception.

We strongly advise anyone building a home to hire a private home inspector of your choice, who is knowledgeable in proper window installation to perform your framing/pre-cover inspection.

Additional Resources

Window Manufacturer Installation Labels

(pdf downloads from our site open in a new tab)

Window installation label for BFS vinyl windows
Window installation label for BFS aluminum windows
Window installation label for Jeld-Wen integral fin windows
Window installation label for Krestmark windows
Window installation label for MI windows
Window installation label for PlyGem windows
Window installation label for RAM windows
Window installation label for Trophy windows

Window Manufacturer Installation Guides (links to the manufacturer sites open in a new tab)

Jeld-Wen INSTALLATION INSTRUCTIONS for Vinyl Windows with Integral Nailing Fin
Jeld-Wen INSTALLATION INSTRUCTIONS for Aluminum Windows with Nail Fin
Krestmark window installation instruction page
MI Installation Recommendations for Mounting Fin Windows
Ply-Gem window installation instructions page
RAM Industries Window Installation: Nail Fin Frames

Other Related Product Manufacturer Installation Guides (link to the manufacturer sites open in a new tab)

DuPont™ Flashing Systems Installation Guidelines for Windows and Doors
DuPont™ Building Envelope Solutions – How To Install index page
Fortifiber Building Systems Group Install Guides
THERMO-PLY® Structural Sheathing Installation Instructions
ZIP System® Roof & Wall Sheathing INSTALLATION MANUAL

General Guides and Videos

Journal of Light Construction-Installing Flanged Windows: Two Strategies Compared (Part 1)
Journal of Light Construction-Installing Flanged Windows: Two Strategies Compared (Part 2)

https://www.youtube.com/watch?v=yZ7Cr-7ILN8

References

ASTM E2112-07 (2016), Standard Practice for Installation of Exterior Windows, Doors, and Skylights. 2016, ASTM International, Conshohocken, PA.

AAMA 2400-02, Standard Practice for Installation of Windows with a Mounting Flange in Stud Frame Construction. American Architectural
Manufacturers Association, Schaumburg, IL.

FMA/AAMA 100-07, Standard Practice for the Installation of Windows with Flanges or Mounting Fins in Wood Frame Construction.  Fenestration Manufacturers Association and American Architectural Manufacturers Association.

FMA/AAMA 100-12, Standard Practice for the Installation of Windows with Flanges or Mounting Fins in Wood Frame Construction for Extreme Wind/Water Conditions.  Fenestration Manufacturers Association and American Architectural Manufacturers Association.

By | 2017-12-07T18:10:45+00:00 December 2nd, 2017|Home Construction, Home Inspection, How to|

About the Author:

Chuck Evans is a Texas Licensed Professional Inspector (7657), Certified Master Inspector and Level-III Thermographer. He has been performing Home Inspections in the greater Houston area since 2004.

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