Building Materials: Window Glass and Glazing Systems
July 13, 2017 Leave a comment
Building Materials: Window Glass and Glazing Systems
Glazing sealants cannot exclude all water, so providing internal drainage is critical; see the discussion in the relevant window, skylight, curtain wall and door portions of the design guide.
Wet-glazed systems generally prevent water entry around the glass and into the glazing pocket much better than dry-glazed systems. Replacement intervals for dry gaskets and the cap bead of wet-glazed systems are about equal. Table 1 lists advantages and disadvantages of both systems. For maximum watertightness, a wet-glazed system, consisting of pre-shimmed butyl tape glazing tape and silicone cap bead, should be specified.
| Table 1—Wet vs. Dry Glazing | ||
|---|---|---|
| Glazing System | Advantages | Disadvantages |
| Wet Glazing (Gunable wet seal over back-up rod or glazing tape) |
||
| Improved resistance to water penetration | Requires exterior access for installation, maintenance and glass removal | |
| Protects i.g. unit edges and laminated glass from water and premature deterioration | Highly workmanship dependent (surface preparation, weather, etc.) | |
| Reduces glass movement (“walk”) | Costs more than dry glazing | |
| Dry Glazing (pre-formed rubber gasket) |
||
| Can be done from interior | Not as watertight | |
| Less dependent on field workmanship and weather | Gaskets can shrink, creating openings for water penetration | |
| Generally less costly than wet glazing | Gaskets can roll into pocket and place uneven stress on glass | |
| Glass can “walk” | ||
Improving Insulating Glass Durability and Thermal Performance
The durability of ig units is dependent on the quality of the hermetic seal and the level of protection from water afforded by the glazing seal and the window frame system. The following design requirements are critical to ig unit durability:
Dual seals (butyl-based primary seal and silicone secondary seal) are more reliable and durable than single-seal systems. The continuity and uniformity of both primary and secondary seals is critical, and continuous seals should be stipulated in the specifications. The spacer should be filled with desiccant and constructed with bent, welded, or soldered corners rather than corner keys. The units should carry a CBA rating per ASTM E774 to ensure comparable units have reasonable durability.
Other critical glazing features that should be specified to reduce the risk of i.g. unit seal failure include properly sized setting blocks (min. 1/4 inch thick) to raise the edge of the ig unit glass above water level in the glazing pocket. The setting blocks must be wide enough to support the entire ig unit cross section and be notched to allow water to drain toward the weep holes. Setting block material must be chemically compatible with the i.g. unit secondary seal. The frame design must promote water drainage away from i.g. unit (i.e. sloped glazing pockets, large (3/8 in. diameter) weep holes, and drainage within each glazing opening (i.e. do not use vertical mullions as “downspouts”); see the discussion in the window, sloped glazing and curtain wall sections.
Glass manufacturers publish center-of-glass U-values. The perimeter of insulating glass typically has a higher U-value due to heat transmission through the spacer. Fenestration framing will also have a different U-value. Therefore, window and curtain wall manufacturers publish total U-values based on specific glass products glazed into their systems. Heat loss and condensation problems almost always occur near the glazing perimeter. Thermal analysis of the entire window or curtain wall system, including all perimeter conditions, is required for high-humidity applications or buildings where condensation is a concern.
Improving Laminated Glass Durability
Similar to i.g. unit failure, failure of laminated glass by delamination is frequently caused by long-term exposure of the glass edge to moisture. Design recommendations to limit the risk of laminated glass failure include the following:
- Protect the edges of laminated glass from exposure to water to limit the risk of delamination. In general, glazing installation details that promote good waterproofing performance and i.g. unit durability (see paragraphs Improving Waterproofing Performance and Improving Insulating Glass Durability and Thermal Permformance above), will also result in improved laminated glass durability.
- Some materials used for laminated glass interlayers, such as polyvinyl-butyral (PVB) are not compatible with many building sealants, so some delamination will occur with butt-glazed joints where the sealant is in contact with the interlayer.
- Check the track record of laminated glass products that have several added plastic interlayers for increased impact resistance as some combinations of interlayer products adhere poorly and can cause delamination.
Designing for Fracture Resistance
Design recommendations to limit the risk of glass fracture include the following:
- Avoid glass-to-frame contact. Provide setting blocks and anti-walk pads to separate the glass edge from the metal. Follow GANA glazing guidelines.
- Use heat-strengthened glass for high temperature applications, such as spandrel glass, and where greater resistance to bending an thermal stresses, compared to annealed glass, is required. Limit the residual surface compressive stress to 7,500 psi to reduce the risk of breakage due to Nickel Sulfide (NiS) impurities. Producing heat-strengthened glass within these limits is difficult and requires tight control of the production process to avoid exceeding the upper limit for residual surface compressive stress and introducing the potential for NiS fracture.
- Use fully-tempered (FT) glass where required by code, but avoid use in areas where breakage poses a risk to safety due to the potential for spontaneous breakage from NiS impurities. Where the use of FT glass is unavoidable, and where its breakage poses a threat to people or property, heat-soak the FT glass to reduce the risk of spontaneous breakage due to nickel sulfide inclusions. This additional processing step adds cost and time, but is warranted where the consequences of glass fracture are significant. Alternatively, use laminated glass for safety glazing and fall-out protection. In many applications where FT glass is used, heat-strengthened glass is adequate to meet strength demands and reduces the risk of spontaneous fracture.
- For all glass types avoid edge and surface damage. Such damage concentrates stress from normal wind or thermal loads, especially for tinted glass or spandrel glass enclosing un-vented spaces.
Determining Glass Thickness
Use ASTM Standard E1300—”Standard Load Practice for Determining Load Resistance of Glass in Buildings” to select appropriate glass thickness to resist service loads.
Special Considerations for Overhead (Sloped) Glazing
The design and selection of glass for overhead glazing requires special attention to the following considerations; see Sloped Glazing for additional information:
The high degree of solar exposure and stratification of warm air beneath the glass results in higher temperatures, increased thermal movement and stresses. The increased strength demand generally requires heat-strengthened glass. The inboard lite of sloped glazing should be laminated for fall-out protection.
Dead loads, snow loads, seismic loads, live loads and wind loads must be analyzed in combinations as required by codes and good engineering practice. Service loads typically include maintenance workers walking on the glass and framing. Unlike wind loads, snow loads are long-duration loads. Glass strength diminishes with duration of loading so time of loading must be included in the analysis. The structural strength of glass is time-dependent and decreases with the length of load application.
Designing for UV Protection
Ultraviolet radiation can cause material deterioration. Methods to provide UV protection, e.g. for libraries or museums, include providing laminated glazing (the PVB interlayer absorbs UV), certain applied films, or curtains and shades. Depending on the thickness of the PVB interlayer, laminated glass can filter out more than 99% of the UV radiation. Applied films are easily scratched and eventually experience color changes, so they are less durable than laminated glazing.
Design Considerations for Safety Glazing
The International Building Code (IBC) Section 2406, state and local building codes, and federal safety standard CPSC 16 CFR Part 1201—Safety Standard for Architectural Glazing Materials, require safety glazing in specified hazardous applications, including:
- Glass in interior and exterior doors and sidelites
- Glass within 36 inches (horizontal) of walking surfaces, with bottom edge less than 18 inches and top edge greater than 36 inches above walking surface, and glass area greater than 9 sf.
- Glass in guards and railings
These code provisions are minimum requirements. Prudent design practice may dictate the use of safety glazing for other applications.
Available manufacturing techniques may limit combinations and choices for glass assemblies; e.g. bent, heat-strengthened or fully tempered, and laminated glass is difficult to achieve because the heat treatment warps the glass and makes mating glass surfaces during laminating difficult).
Logistical and Construction Administration Issues
Inspection and maintenance of exterior glazing sealants requires access to the exterior of windows and curtain walls. Provisions for this access (e.g. suspended scaffolding tie-off anchors) must be made during the design.
Primary and secondary seal continuity and uniformity of minimum width requirements for i.g. units is critical to durability and should be spot-checked on a representative number of actual production units, not just mock-up samples, prior to installation. Mock-up or sample installation often save time and money in the long run as it highlights potential production, lead time, coordination, and performance problems. The production quality of the glazing components and their proper configuration must be checked on a statistically relevant sample of production units.
Glass must be handled carefully during transportation and installation to avoid edge damage and reduce the risk of later glass fracture. Refer to GANA manual for appropriate handling techniques and PPG’s Technical Bulletin TD112—Handling Do’s and Don’ts to Reduce Glass Breakage.
Details
The following details can be downloaded in DWG format or viewed online in DWF™ (Design Web Format™) or Adobe Acrobat PDF by clicking on the appropriate format to the right of the drawing title.
Schematic of Poor Glazing System (Detail 3.1-1) DWG | DWF | PDF
This detail illustrates commonly found poor design and installation features that contrast with the good design features of the next detail, Schematic of Good Glazing System.
- The lack of weep holes in the glazing pocket allows water accumulation and promotes i.g. unit and laminated glass failure.
- Without anti-walk pads, the i.g. unit may “walk” and contact metal frame edges. Glass-to-metal contact may lead to glass edge damage and fracture.
- Fastener penetrations through the glazing pocket allow leakage into the wall cavity below. If sill flashing is present, fasteners set through the glazing pocket will puncture the flashing and cause leakage.
- The flat sill allows water to pond and increases the risk of leakage. Missing sill flashing allows water leakage into the wall cavity.
- Without an integral return edge, the frame provides inadequate bonding substrate for the perimeter sealant. A poorly configured perimeter seal will not be durable and will promote leakage past the window jamb.
Schematic of Better Glazing System (Detail 3.1-2) DWG | DWF | PDF
The most important features illustrated in this schematic detail are glazing pocket weep holes, sloped-to-drain sill glazing pocket and sill flashing.
- The glazing pocket weep holes drain water that penetrates the glazing seals. A well-drained glazing pocket prolongs the service life of the insulating glass (by reducing the exposure of edge seals to water), and reduces interior leakage.
- The sill flashing is sloped to the exterior to promote drainage. The window sill frame is attached through the back to a structural clip angle, to avoid fastener penetration of the horizontal portion of the sill flashing.
- The wet glazing seal provides better water penetration resistance than dry glazing (gaskets).
- The anti-walk pad at the window jamb prevents the glass from “walking” in the glazing pocket and contacting the metal frame.
- The perimeter of the window frame includes substantial return legs that provide adequate bonding surfaces for a properly configured sealant joint at the window perimeter.
Emerging Issues
Self-cleaning or easy-to-clean glass was recently developed and uses titanium dioxide coatings as a catalyst to break up organic deposits. It requires direct sunlight to sustain the chemical reaction and rainwater to wash off the residue. Anorganic deposits are not affected by the coatings.
Photochromic coatings incorporate organic photochromic dyes to produce self-shading glass. Originally developed for sunglasses, these coatings are self-adjusting to ambient light and reduce visible light transmission through the glass. In architectural glass they are typically used to provide shading.
Glass with electrochromic coatings utilizes a small electrical voltage, adjusted with dimmable ballasts, to adjust the shading coefficient and visible light transmission. Like photochromic coatings, they are intended to attain lighting energy savings.
Point-supported glazing is sometimes used in wall systems that are all glass. These systems utilize mechanical anchors at discrete locations near the glass edge, rather than continuous edge supports. Edge-supported glass is typically sized according to glass load resistance charts; see ASTM E1300. These charts do not apply to point-supported glazing, which requires specific structural engineering analysis.
Proper Choice of Glass and perfect positioning of elements cause the overall effect of architecture delight.
Ian Jay Bantilan
Try how we can make your House “Sweeter”.
Archian Designs Architect Studios is a Collaboration of Architects, Urban Planners, Interior Designers, Landscape Designers and Engineers in the Philippines. To see the contact information, email to archiandesigns@yahoo.com, or call 09778278415.
