Terminology

• Standard unit: 3-7/8 in. thick glass block with the widest range of patterns and sizes
• Thin unit: 3-1/8 in. thick block made for small prefabricated window panels
• Hollow block: unit with a partial vacuum inside
• Solid block: solid glass unit with the highest breakage resistance, such as 3 in. solid units
• Panel anchor: metal strip embedded in mortar joints that ties a panel to its frame
• Channel frame: metal channel that holds the panel edges
• Expansion strip: compressible strip at the panel head and jambs

Standards

Sizes and performance

• Square units 6, 8, and 12 in.; rectangular units 4 by 8 in. and 6 by 8 in.; bullnose, corner, and angle units
• Light transmission: flat sheet glass 90 percent; solid block 80; standard and thin block 75; tightly ribbed 50; fibrous inserts and thick-faced block 50 to 55
• U-value: standard block 0.51, thin block 0.57, solid block 0.87
• STC: hollow block 35 to 40, thick-faced 48, solid 53

Fire

• One manufacturer guide states no glass block assembly qualifies as a fire-rated wall
• UL-listed fire-rated units meet at least a 45-minute window test; thicker-faced and solid units reach 60 and 90 minutes

Materials

• Hollow units in patterns and inserts: most walls and windows
• Solid and thick-faced units: higher breakage and sound resistance
• Mortar: Type S or N; Type S recommended for exterior work
• Joint reinforcement, panel anchors, and expansion strips: hold and release the panel

Detailing

• Space panel anchors at 16 in. on center maximum, and horizontal joint reinforcement at 16 in. maximum
• Provide 3/8 in. expansion strips at the top and sides; caulk exterior joints
• Size channel frames: for 4 in. block the opening is 4-1/4 to 4-1/2 in. wide and at least 1-3/8 in. deep, with a 1 in. minimum recess
• Design supports for the load or 200 lb per linear foot, whichever is greater
• Exterior panels: design guidance of 20 psf wind, 144 sq ft maximum, up to 20 ft high or 25 ft wide; larger panels need stiffeners, shelf angles, or expansion joints
• Interior panels: design for 5 psf; up to 250 sq ft
• Use integral waterproofer in exterior mortar and no antifreeze or accelerators

Uses

Specific uses

• Bathroom and basement windows
• Stair and corridor light walls
• Interior partitions and room dividers
• Exterior light panels and accent walls
• Curved walls and corners

Pros and cons

Pros

• Passes light while giving privacy
• Insulates better than single glazing
• Many patterns and sizes

Cons

• Not load-bearing and no fire-rated wall assembly
• Panel size limits and support needs
• Mortar joints need care to stay watertight

Manufacturing and cost

Construction

• Two glass halves are pressed and fused into hollow units, then laid in mortar with reinforcement or set in a channel frame with silicone or anchors

Main manufacturers and suppliers

• Seves Glass Block

Trade association: The Masonry Society.

Sources

• Pittsburgh Corning via BNP Media: designing with glass block, https://continuingeducation.bnpmedia.com/courses/pittsburgh-corning/designing-with-glass-block-abundant-applications-provide-practical-aesthetic-and-green-solutions/2
• Masonry Magazine: glass block makes a comeback, https://www.masonrymagazine.com/blog/2017/12/28/tech-talk-glass-block-makes-a-comeback
• GoCodeBook: California Building Code 2110 glass unit masonry, https://gocodebook.com/library/us/ca/ca-building-code/chapter-21-masonry/section-2110-glass-unit-masonry
• Unified Facilities Guide Specifications: 04 23 00 glass unit masonry, https://www.wbdg.org/FFC/DOD/UFGS/UFGS 04 23 00.pdf