Analysis of thermal performance of different glazing performance specifications

Components of an insulated glass unit (IGU)

Schematic design is a good time to begin exploring the impact of glazing performance. In most buildings, windows are major contributors to heating and cooling loads, and they can also affect electric lighting demand. BEM is an ideal tool to evaluate these complex interactions.

Impact

Glazing performance has several important impacts that can be evaluated directly using BEM.

Energy consumption and energy demand for comfort conditioning.
Sizing of HVAC equipment.
Electricity consumed for lighting.

As also covered in the discussion of Window-to-wall ratio parametric studies, there are other important potential impacts including energy code compliance, daylighting performance, visual comfort, and thermal comfort.

Alternatives

From an energy perspective, the primary metrics used to describe window performance are U-factor, solar heat gain coefficient and visible transmittance, as described on this page: Fenestration performance. There are several strategies used in window construction to affect thermal and optical performance.

Insulated glass unit

The most common glazing configuration is a dual-pane insulated glass unit (IGU) with either clear or tinted glass as the outer pane, clear glass as the inner pane and a low-e coating on one of the interior surfaces. However, as shown in the following tables, IGUs with this basic configuration can provide a wide range of performance depending on the specific choices of glass types, coating characteristics, gap thickness and type of gas in the gap.

SHGC and VT for four different 1-inch IGUs, each with clear glass but with different coatings, providing an example of the wide range of performance available from the same basic glazing configuration
SHGC	VT	Light to solar gain ratio (LSG)	Solar transmission	Visible light transmission
0.38	0.70	1.84	moderate	high
0.38	0.47	1.24	moderate	moderate
0.23	0.49	2.13	low	moderate
0.14	0.08	0.57	very low	very low

Example U-factor data for 1-inch IGU with different gas fills and frame types, all with a low-emissivity coating
Frame type	Gas fill	U-factor (center of glass)	U-factor (whole window, fixed)	U-factor (whole window, operable)
aluminum	air	0.30	0.47	0.61
aluminum	argon	0.25	0.43	0.57
aluminum with thermal break	air	0.30	0.41	0.45
aluminum with thermal break	argon	0.25	0.36	0.41
vinyl	air	0.30	0.35	0.38
vinyl	argon	0.25	0.31	0.34

Tinted (heat absorbing) glass

Tinted glass, also called heat absorbing glass, contains pigments that absorb solar radiation. A portion of the absorbed heat is re radiated and convected to the outdoors, reducing solar heat gain into the building. The choice of pigment affects which wavelengths of solar radiation are absorbed, which determines the solar heat gain properties of the glass as well as the perceived color of the glass.

The following chart shows three examples of three different glass types with their transmittance profile superimposed on the solar radiation spectrum. Clear glass has high transmittance in both the visible and infrared portions of the spectrum. Gray glass reduces transmission across the full solar spectrum and retains a neutral color appearance. Green glass allows a higher portion of visible light to pass through and absorbs a greater portion of infrared light, resulting in a higher visible light transmittance with roughly equal solar heat gain compared to the gray glass. For this reason, green and blue tints are sometimes called spectrally selective tints.

Transmittance as a function of wavelength for clear glass and two examples of tinted (heat absorbing) glass, superimposed on the solar radiation spectrum

Glass coatings

Coatings affect solar heat gain by reflecting solar radiation rather than absorbing it. The design of the coating affects which portions of the solar spectrum are transmitted and which are reflected. The following chart shows examples of three different coatings applied to clear glass in an insulated glass unit. Each coating is designed to have peak transmittance in the visible portion of the spectrum but their transmission differs significantly in the infrared portion.

Transmittance as a function of wavelength for an uncoated double-pane clear IGU together with examples of three different coatings applied to clear glass in an insulated glass unit, superimposed on the solar radiation spectrum

Gas fill

The gas in the gap between panes of an IGU is either air, argon or krypton. The latter two offer lower conductivity and improve (reduce) the U-factor of the glazing.

Frame

For nonresidential buildings, aluminum is the most common frame material. Due to its high thermal conductivity, these metal frames can have a negative impact on performance. Therefore, frames with a thermal break, which is typically a plastic component that connects the inside and outside portions of the frame, are often used to reduce the thermal bridging penalty.

Glass with ceramic frit, San Francisco Federal Building

Other glazing alternatives

There are additional, less common, options that affect glazing thermal performance:

Additional panes, either glass or plastic film, to create triple or quadruple pane units.
Insulating spacers to reduce heat conduction at the edges of an IGU.
Dynamic glass. Transmission can be varied, typically with an electrical signal.
Laminated glass, which includes a plastic layer sandwiched between two panes of glass, is usually employed where breakage or security is a concern, but plastic layers with spectrally selective properties can be used.
A frit, a ceramic pattern applied to glass, is sometimes used to reduce light transmission and solar heat gain and is sometimes used to help prevent bird collisions.
Operable windows can be used to provide potential for natural ventilation. See Natural ventilation design features.

Finding glazing performance values

NRFC report. The ideal source is the performance data from the manufacturer tested according to standards from the National Fenestration Rating Council. However, at the schematic design phase, data for specific actual products may not yet be available. In that case, there are a number of potential information sources

Product data from manufacturer websites. This information is typically only center-of-glass performance for IGUs, and other sources will be needed for U-factor data.
ASHRAE Handbook Fundamentals includes tables of typical U-factors for a wide range of window types as well as tables of common SHGC and VT values.
Energy codes define maximum allowed values for U-factor and SHGC, which often vary by climate zone. These prescriptive requirements provide a good performance baseline.
WINDOW software developed by Lawrence Berkeley National Lab can be used to build a window construction from glass layers and includes libraries with a wide range of glass types. See also calculated properties.

Guidance on modeling approach

General modeling guidance in the discussion of window-to-wall ratio parametrics studies also applies here for inputs such as thermal zoning, internal loads, daylighting controls, HVAC systems, thermal comfort and the impact of adjacent buildings.

Window performance inputs

BEM tools often provide more than one option for describing window performance, which typically include a simplified method and a detailed method. The detailed method generally provides more accurate heat transfer calculations and is preferred if data are available. Otherwise, at the schematic design phase the simplified method may be acceptable in order to provide timely design feedback.

Regardless of the method used to model windows, ensure that the impact of framing is included in the U-factor input.

Window area

The window area might be based on the latest version of the schematic design if it is available. Otherwise, typical WWR values are listed on this page: Define fenestration (glazed constructions).

Window shading

If plans include window shading such as overhangs, then they should be included so that the model reflects their impact. See also Shading feature design analysis and Define shading features.

Checking inputs

To ensure model quality, check that the intended window performance is reflected in the model inputs. Simulation tools often include a summary report that lists window U-factor, solar heat gain coefficient and visible transmittance for windows in the model. See Review and analysis to verify model quality.

Guidance on presenting results

In addition to energy results, consider presenting results for peak electricity demand and HVAC system sizing because both can affect project construction cost.

General BEM results that are typically found useful are described in this page: Analyzing Model Outputs. Other important considerations:

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