Innovative Energy Engineering



Glazing

Humans are adapted to natural sunlight and daylight is beneficial for the human psyche. Sunlight contains UV light (< 400 nm wave length) visible light (400 nm - 700 nm wave length), and infrared (IR) light (> 700 nm wave length). UV light has higher frequency (more energy) and therefore can cause damage to molecule bonds. This causes aging of material (i.e. fading), body tissue and cancer. The good news is that high-energy UV radiation doesn't transmit well through regular glass.

The best and most expensive windows still perform worse than old uninsulated walls. Therefore glazing areas should be limited as much as possible. The U-value (lower is better) is the inverse of the R-value (higher is better). U-values of glazing units range from 1-5 [W/(m^2-K)] (0.18 -1.7 Btu/(hr-ft^2). Walls typically have U-values of 0.18 - 1.7 [W/(m^2-K)] (0.03 0.3 Btu/(hr-ft^2).

Visible light is the purpose of glazing areas. Ideally visible Transmittance (VT) is very high. Multiple panes, large framing areas and often high SHGC coefficients typically reduce VT. Useful lighting energy gained is limited to the amount that people actually need and is offset by the added cooling load and higher winter heat losses. Glazing areas of more than 15-20% of the exposed above-grade wall area will become net-energy losers in regards to lighting energy. Tinting should not be employed since it doesn't prevent glare and defeats the purpose of having glazing. A daylighting simulation can determine actually beneficial glazing area. To avoid glare and allow deep penetration of the space, daylight glazing should be installed close to the ceiling with inside light shelves and reflective ceilings. Glazing in occupant's head height can cause glare and should have adjustable shades.

Solar Heat Gain

All above properties are straight forward and it is easy to see that in all cases lower U-values and higher VT-values are desirable. For the Solar Heat Gain Coefficient (SHGC) it is not as straight froward since in many cases less heat gain is desired, but in some cases it may be beneficial. One needs to understand the energy balance around a glazing unit:

Better Glazing Systems

Limit glazing areas as much as possible. Glazing is more expensive than even a very expensive wall, and will perform worse than even a bad wall. East, North and West side should receive minimum glazing. Spaces that don't require much daylight should be located there (mechanical rooms, storage, bathrooms).

Modern glazing techniques include double and triple pane glazing with insulating gases (i.e. Argon or even Krypton) in between glazing layers. The framing of the component typically has worse U-value than the glass. Reduce framing and increase glazing areas of an assembly as much as possible. This increases lighting gain, while reducing thermal bridges in frames. The framing has structural functions and is exposed to water, which limits the choice of material. Fiberglass and vinyl are choices for smaller units. Larger commercial windows, storefront and curtain wall systems require higher structural integrity and aluminum is a very durable choice. One inherent problem with aluminum, and all metals, is it conduct heat very well. Cheap aluminum glazing systems have very high U-values (even with good glazing!) and cause condensation, ice formation etc. To improve the thermal properties the aluminum frames get thermally broken multiple times. This increases cost, but improves U-value significantly.

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