Mark Meshulam’s inside scoop on window glazing and window glazing leaks.
The word “glazing” has multiple meanings where windows are concerned. Glazing can be a verb, meaning the act of installing glass into a frame. Glazing can refer to the portion of the assembly which contains the glass or other glazed-in panels. Glazing can also refer more specifically to the means by which the glass is installed, and especially sealed to the frame. This third definition is today’s topic.
Seasoned window professionals have been heard to say – usually over a beer after a hard day of windowing, “Glass doesn’t leak. Aluminum doesn’t leak. It’s the stuff between them that leaks.” Very true. Glass and aluminum are non-porous, water-impermeable materials. The only way water can penetrate windows, then, is through the materials, or lack of materials, which are intended to seal aluminum and glass to one another.
A typical glazing system will consist of the following elements:
- A “glazing leg” on the frame against which the glass rests.
- A spacer tape or sealant tape which is applied to the glazing leg.
- An optional glazing sealant which is applied to the glazing leg next to the spacer tape. This is sometimes called the “toe-bead”
- The glass itself, which is placed and compressed against the spacer tape and glazing sealant.
- In addition to the above, some glazing systems have an additional line of sealant between glass and frame at the inboard plane of the glass. This is called the “heel-bead”.
- Occasionally one might see a line of sealant applied to the exterior of the glass where it meets the glazing leg. This is called a “cap-bead”.
Best designs for glazing systems will include two lines of glazing sealant lines, at the outer face of the glass and at the inner face. The heel bead, if it exists, makes two important contributions:
- The heel bead defends the exterior sealant line from differential pressures which might draw water through the glazing system, reducing the potential for water penetration.
- If water does penetrate the exterior line, the heel bead can prevent the water from coming into the building. With a two-line system it is important to have “weep holes” beneath the glass to allow any water which might penetrate the exterior line to drain harmlessly to the exterior.
The most common occurrences of glazing leaks involve holes, adhesion loss or discontinuities in either of the sealant lines. The second most common occurrences will involve a lack of, or clogging of the weep holes beneath the glass.
If water is allowed to be present beneath an insulated glass unit without adequate drainage, the water can degrade the insulated glass seals and cause premature seal failure, where water vapor enters the glass airspace and “fogs” the glass unit. A fogged unit will need replacement, which is expensive.
If water is allowed to be present beneath the glass without adequate drainage, it can also prematurely degrade the joinery sealant at the frame corners.
Another frequently seen glazing problem involves blockage of the drainage path. The glass edge sits on hard rubber “setting blocks” at the bottom of the glass and sometimes at sides and top also. If these setting blocks are fitted too tightly into the space around the glass, especially at front and back edges which might be embedded in the glazing sealant, the water drainage path can become blocked entrapping water within the system.
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