Understanding Glass Makeup
In architecture, understanding the science of building materials, particularly glass, and their thermal performance is vital for designing comfortable and energy-efficient environments. Glass plays a significant role in modern architecture, not only for its aesthetic appeal but also for its ability to influence energy consumption. By selecting the right type of glass, architects can enhance natural light while minimizing heat loss or gain, contributing to a more sustainable building design. This careful consideration of glass properties ensures that spaces remain inviting and comfortable, while also reducing the overall energy footprint of the structure.
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U - Value
What are U-Values
U-values, also known as thermal transmittance or heat transfer coefficients, quantify the rate of heat loss (or gain) through a material or assembly. Measured in watts per square meter per degree Celsius (W/m²K), U-values indicate how well a material conducts heat. In simpler terms, a lower U-value indicates better insulation and reduced heat transfer, while a higher U-value suggests poorer insulation and increased heat flow.
Why is U-Value important
U-values play a crucial role in determining the thermal performance of building envelopes, encompassing walls, roofs, floors, windows, and doors. By understanding the U-values of different building components, architects and designers can optimise energy efficiency, maintain thermal comfort, and comply with building regulations.
What effects U-Value
Material Properties: The thermal conductivity of materials, such as insulation, glass, and concrete, significantly impacts their U-values. Materials with low thermal conductivity, such as foam insulation, typically have lower U-values, indicating better insulation properties.
Thickness: Thicker building components provide greater resistance to heat transfer, resulting in lower U-values. Increasing the thickness of insulation, for example, can reduce heat loss through walls and roofs.
Construction Methods: The construction techniques used to assemble building components can affect their thermal performance. Proper installation and sealing are essential to minimise air leakage and thermal bridging, which can increase U-values and compromise energy efficiency.
Glazing Systems: In the case of windows and doors, the type of glazing system, such as single-pane, double-pane, or triple-pane glass, influences U-values. Multiple glazing layers with inert gas fills and low-emissivity coatings enhance insulation and reduce U-values.
Thermal Properties & Appearance
Visible light transmittance
The percentage of visible light that travels through glass is known as visible light transmission (VLT). VLT may enhance daylighting and, if intelligently planned, can help balance electric lighting and cooling demands. A greater VLT increases daylighting, whereas a lower one increases privacy. Glare can be avoided by controlling VLT.
Solar Heat Gain Coefficient (SHGC)
A measurement of how efficiently a product prevents sunlight-induced heat. The lower the SHGC, the less solar heat is transmitted, and the occupants are more comfortable inside. In cold areas, the right SHGC can assist in maintaining warm inside air, while in hot rooms, it helps retain costly conditioned air.
R-Value
R-value is used to test the performance of most other components of the building envelope, such as walls, floors, and roofs.
How to Calculate R-Value?
Divide 1 by the U-value to calculate the R-value. For example, a U-value of 0.20 equals an R-value of 5 (1 ÷ 0.20).
Heat strengthened VS Annealed
Heat-treated glass is a term used to describe glass that has been processed through a tempering furnace to alter its strength characteristics, provide greater resistance to thermal and mechanical stresses and achieve specific break patterns for safety glazing applications as compared to annealed glass.
Annealed (AN)
Raw glass that has not been heat treated is annealed glass. In a specification, the designation for annealed glass is AN.
Heat Strengthened (HS)
Heat-strengthened glass offers roughly twice the strength of annealed glass of the same type, thickness, and size. When it breaks, it tends to fracture into large pieces similar to annealed glass, which reduces the chance of the glass dislodging from the frame.
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For glass up to 1/4″ (6mm) thick, surface compression typically ranges between 4,000 and 7,000 psi. For thicker glass, such as 5/16″ (8mm) and 3/8″ (10mm), the surface compression usually falls between 5,000 and 8,000 psi. (Due to variations in measurement tools and procedures, a tolerance of ±1,000 psi is generally applied to surface compression readings).
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While heat-strengthened glass offers enhanced durability and improved resistance to thermal stress, it does not meet the safety glazing criteria set by the American National Standards Institute (ANSI Z97.1) or the Consumer Product Safety Commission (CPSC 16 CFR 1201). As such, it should not be used in applications requiring certified safety glazing.
In specifications, this glass is designated as "HS".
Fully Tempered (FT)
Fully tempered glass is typically four times stronger than annealed glass and twice as strong as heat-strengthened glass of the same thickness, size, and type. When it breaks, it shatters into many small fragments, significantly reducing the risk of injury. However, due to the small shard size, the glass is more likely to fall out of the opening upon breakage.
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The minimum surface compression for fully tempered glass is 10,000 psi. It meets the safety glazing standards established by the American National Standards Institute (ANSI Z97.1) and the Consumer Product Safety Commission (CPSC 16 CFR 1201). This makes fully tempered glass the industry standard for commercial glazing applications where safety glazing is required.
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In specifications, fully tempered glass is commonly identified with the abbreviation "FT".
Heat Soaking
Fully tempered glass can occasionally break without warning due to the presence of nickel sulfide (NiS) inclusions naturally found in float glass. These inclusions can expand over time, potentially leading to spontaneous breakage.
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Although spontaneous breakage from NiS inclusions is relatively rare, increased awareness and publicity around such incidents have brought more attention to the issue. While some manufacturers have recommended limiting its use in specific commercial applications, fully tempered glass remains widely used—even in non-safety glazing areas—due to its enhanced strength and safer breakage pattern, which reduces risk to installers and the public.
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In situations where tempered glass is required to meet safety standards or provide added durability, a heat soak test can be performed to further reduce the likelihood of spontaneous breakage. This testing process provides added assurance by helping identify and eliminate glass with unstable inclusions before installation.
Low-Emissivity (Low-E) Glass
What is Low-E Glass?
Low-E (low-emissivity) glass is designed to improve energy efficiency by reducing the amount of infrared and ultraviolet light that passes through a pane of glass without compromising the amount of visible light transmitted. It features a microscopically thin, transparent coating—usually made of metal oxide—that reflects heat while still allowing light to pass through.
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How Does It Work?
In colder months, Low-E coatings reflect indoor heat back into the building, helping to retain warmth. In warmer months, they reflect exterior heat away, keeping the interior cooler. This smart control of thermal energy leads to better insulation and lower energy bills year-round.
Types of Low-E
Passive Low-E Coatings (Hard-Coat)
Applied during the glass manufacturing process while the glass is still hot, hard-coat Low-E is more durable and allows some solar heat gain. It's ideal for colder climates where maximizing solar warmth is beneficial.
Solar Control Low-E Coatings (Soft-Coat)
Applied after the glass has cooled in a vacuum chamber, soft-coat Low-E offers superior thermal performance. It reflects more heat and UV rays, making it perfect for hot climates or buildings with high energy-efficiency goals.
Benefits of Low-E Glass
Energy Savings
Reduces heat transfer, helping maintain indoor comfort and lowering heating and cooling costs.
UV Protection
Blocks a significant portion of UV rays, protecting furniture, flooring, and artwork from fading.
Enhanced Comfort
Helps prevent hot and cold spots near windows by stabilizing interior temperatures.
Versatility
Available in different configurations and tints to suit both residential and commercial applications.