How solar heat gain affects building design
The SHGC of glazing is a tiny number in a sea of values to consider, but it has broad impacts on HVAC systems, lighting design, the appearance of the building, façade costs and the health and comfort of occupants.
- Understand the direct and indirect impacts to building and occupant performance as a result of a specified SHGC value.
- Analyze glass and coating options to co-optimize performance for solar loads with building appearance and occupant health and comfort.
- Evaluate the benefits and trade-off between competing building and occupant performance that is controlled or indirectly influenced by the SHGC specification.
Solar heat gain is a tiny coefficient in a sea of values to consider for mechanical engineers. This number not only drives load calculations, systems selection and shaft sizes, it also can significantly change fenestration costs, procurement and most challenging, the appearance of windows on the building.
As if this number didn’t control enough decisions, it also has secondary impacts to occupants’ visual comfort, thermal comfort and circadian health. So, what’s the difference between 0.27 and 0.23? For solar heat gain coefficient, the answer is entangled with many issues requiring coordination and consensus from team members.
What is solar heat gain coefficient?
It is important to recall the definition of SHGC as distinct from the legacy shading coefficient. SHGC is defined in Equation 1 and includes two terms (see ANSI/NFRC 200-2020).
The first term describes the total directly transmitted solar radiation(τsol) while the second term describes the inward flowing fraction of absorbed and reradiated energy (). It is because of this second term, that even opacified fenestration assemblies such as spandrel glazing with frit still have a nonzero SHGC due to some small but nonnegligible inward flowing reradiated heat.
E(λ) = Source spectrum, ASTM-G173, AM1.5
τ(λ) = Glazing system transmission
The outdated SC is still often confused with SHGC. While SHGC describes the fraction of total direct and indirect solar energy transmission through a glazing system as compared to the incident solar energy, the SC is benchmarked to a single layer of 1/8-inch clear glass and describes the fraction of transmitted solar radiation as compared to that benchmark glass, which is given a value of 1.0. Note that SHGC and SC are not interchangeable and are approximately related in Equation 3 from ASHRAE Handbook of Fundamentals below.
How to tune SHGC values
What decisions control the SHGC of glazing? What levers are available to tune its value? High performance glass coatings that are a part of the insulated glazing unit are the most common way to achieve varying levels of SHGC performance. However, it’s important to keep track of other factors of fenestration design that impact the total SHGC performance but are controlled primarily by other design parameters such as structural, thermal or acoustic requirements. The number of glass and air gap layers in the IGU, the composition of the glass substrates, the substrate thickness, interlayer films used for laminates, as well as the window unit size and glass-to-frame ratio all additionally modify the total assembly SHGC.
Figure 1 shows three different IGU buildups using the same coating but the structural, thermal and acoustic requirements differ significantly for the different projects. The nominal glazing represents a standard IGU and performance often quoted by a manufacturer. Project No. 1 is for a high-rise office building with tall floor-to-ceiling glass and high wind loads. Project No. 2 is for a school in a mixed humid climate with large glass sizes, cold winters and more aggressive U-value targets. Project No. 3 is for a hotel very near an airport with stringent acoustic requirements particularly for low-end frequencies. While the same solar control coating is applied to each IGU, the center of glass SHGC significantly varies.
High performance solar control glass coatings made from thin–film sputter depositions of silver-dielectric stacks dominate the market. These coatings are offered in standardized product lines that react to prescriptive code requirements for SHGC and the desire for maximum visible light transmittance. These product lines are broadly categorized as single- (LSG<1.4), double- (1.4<LSG<2.1) and triple-silver (LSG>2.1) coatings with the light-to-solar gain ratio increasing with the number of silver layers.
The International Glazing Database maintained by Lawrence Berkeley National Laboratory collects measured and validated optical data of the many thousands of coatings available to the architectural market. Analysis of the database and processing through the LBL WINDOW software enables a high-level view of all the glass coatings available to architects and engineers. Figure 2 shows the distribution of coating products by their two primary performance characteristics, SHGC and visible light transmittance (Tvis).
There are a few general product categories to consider and it’s important to understand how a SHGC specification will fall into these classifications along with the resulting implications to appearance and daylighting. Single-silver coatings often do not have sufficiently low solar gain without also sacrificing visible light transmittance, so the nonresidential commercial market focus is on double– and triple-silver coatings. Group a. coatings are the most standard double-silver coatings with high visible light transmittance, good color neutrality and low exterior reflection while achieving a SHGC at or near 0.40. These coatings intend to be unnoticeable, with high transparency and excellent color neutrality. Group b. coatings are standard triple-silver coatings with high visible transmittance but with lower SHGC usually around 0.26. The improved SHGC leads to subtle sacrifices in transparency and color neutrality for triple silver coatings. Group c. coatings are darker double silver coatings that sacrifice visible transmittance to reduce SHGC and come with some more noticeable color shifts. Group d. coatings are a tight cluster of darker triple-silver coatings, again designed to meet increasingly stringent SHGC requirements but with a linear reduction in visible transmittance and more noticeable color shifts.
There are alternate trends to satisfy reduced SHGC specifications that rely on higher exterior reflection coatings that are more mirror-like or rely on applying the any of the group a-d. coatings to a body tinted substrate. Reflective coatings in group e. can be used to achieve good SHGC values around 0.25 to 0.35 and where view into the building is either not important or not desired. Consider also the hazards of higher exterior reflections (in the visible and solar spectrum) on neighboring people, properties and plants.
The second trend mixes coatings with body tinted substrates to again reduce SHGC with mostly linear reduction in visible transmittance. Body tinted substrates come in a number of blue, gray and green hues with a range of (uncoated) transmittance between 0.35 and 0.65. The transmittance of body tinted glass is dependent on glass thickness, so where structural requirements dictate thicker glass in the 3/8– to ½-inch range, there will be a reduction in SHGC and Tvis. Double-silver coatings on body tinted substrates fall in the broad range of group f, while triple-silver coatings on body tinted substates fall in the broad range of group g.
The standard double– and triple–silver coatings that provide the most transparency with good color neutrality have limited SHGC tunability. These coatings can achieve a SHGC around 0.4 or 0.26 in standard configurations. Tuning SHGC with coating selection will then almost always incur a sacrifice to visible light transmittance and color neutrality which have secondary impacts to daylighting, the appearance of the building and human health and comfort.
Impacts to glass procurement and cost
Before examining the implications of a specified SHGC on building and occupant performance, consider the increases to glazing costs and restrictions on procurement. The most basic performance jump from a double-silver to a triple-silver coating results in an average 18% increase in the glass material costs, assuming all other fabrication, heat-strengthening and markup costs are equal. Changing a clear glass substrate to a body tinted substrate results in an average 22% increase in the glass material costs. A change from a high light transmission double-silver or triple-silver to a low transmission or dark version of the coating is usually cost neutral for most glass suppliers.
There can be further restrictions on glass procurement when tuning SHGC to lower values. A vast majority of the many thousands of coatings on the market will not be available to every project due to local market conditions, variations in coating product lines and differences in manufacturer supply integration. Façade fabricators may have a preference for, or an exclusive agreement with, a glass and IGU manufacturer which may limit coating selections, particularly post-award.
Nearly all manufacturers offer a coating product that falls in Groups a-d. Most offer at least one reflective coating in Group e. However, some manufacturers have multiple reflectivity variations. Most suppliers can offer their coatings on a tinted substrate. However, color options can be limited for some and depends on whether the manufacturer’s downstream integration includes the float line itself. Vertical integration can also impact supply schedule and cost where manufacturers coat glass to order can satisfy non–stocked options faster and without a premium.
These cost and procurement limitations usually reduce the many thousands of coating options in the IGDB down to 3 to 5 options for a given project, in a specific local market and with a given façade contractor, which reduces the seemingly smooth gradient of SHGC possibilities into discrete options that are not always in alignment with the basis of design parameters.
Impacts to daylighting
Due to material limitations of silver oxide based solar control coatings, the optimization for maximum visible light transmission and minimum SHGC will always approach a pareto frontier that currently peaks at a light-to-solar gain ratio of 2.48. The drive for lower SHGC values to meet either prescriptive code requirements or beyond code performance targets therefore fundamentally challenges the daylighting potential of a project.
The double- and triple-silver coatings in Group a and b offer the highest visible light transmittance and the best potential to make use of daylight. Most double-silver coatings have a nominal Tvis around 0.72 while most triple-silver coatings have a nominal Tvis around 0.62. The dark shifted double- and triple- silver coatings in Groups c and d offer improved SHGC at the expense of Tvis and daylighting potential, each with a linear decrease from their transparent versions. These darker coating options of Group c and d, together with all other options for reducing SHGC in groups e-g will all sacrifice the maximum Tvis and hinder the daylighting potential. The trade-off between reduced loads from lower solar heat gain therefore requires careful coordination with daylighting goals.
The most common metric to evaluate a spatial and temporal average of the daylighting potential in a building is the spatial daylight autonomy metric used by LEED and other sustainability rating systems and is described in IES LM-83. Figure 3 shows a patient room at the Valleywise Health Medical Center, Phoenix, which has a perimeter bathroom and family area with a window to the left. Glass coating options to improve SHGC also decrease the visible light transmission with a resulting drop in spatial daylight autonomy which will limit the patient’s access to daylight.
Impacts to visual appearance
Beyond the impact to the daylighting potential of a project, the choice of glass and coating will change the building’s exterior appearance. This is one of the most challenging aspects to coordinate because the design intent is not quantitative and not always precisely articulated. The most common trend is for unnoticeable glass that is highly transparent and color neutral such the visual presence of the glazing disappears.
There are some tools available to bring the qualitative design intent of glass appearance in alignment with qualitative metrics of glass and coating properties. The validated, photometrically accurate and open source rendering engine RADIANCE together with GLAZE program offers the best nonproprietary option for simulating glass appearance and uses the IGDB measured spectral data.
A more fundamental calculation is often beneficial to assess the relative difference in color appearance when a desired baseline glass and coating configuration is known, as is often the case when the design intent is to match the glass of an existing building that has been deemed visually acceptable. The delta E metric described in ASTM D2244 is intended to quantify difference between two colors as perceived by the human eye and calculated from measured L*a*b color coordinates.
Color coordinate data is available from the spectral measurements in the IGDB enabling a comparison of specific IGU configurations. Figure 5 shows the calculated delta E color difference of glazing options for a pedestrian bridge at an Orlando hospital intended to match the appearance of the existing building to which it connects but has more stringent solar heat gain performance requirements.
Impacts to visual comfort
Perhaps most critically, the specification of SHGC will indirectly impact the visual comfort of occupants by way of visible light transmittance. For most programs from office buildings to hospitals to lab buildings, the increased interest in daylighting and views must also account for glare and visual discomfort of occupants to productively perform their tasks. Daylight glare probability is the commonly accepted metric developed for indoor lighting scenarios that include daylight from windows.
Figure 6 shows the view from a patient bed at the Valleywise Health Medical Center with false color images of the simulated scene luminance and the DGP rating for that viewpoint. The west facing patient room has challenges with low angle sun in the late afternoon and the visible light transmittance, indirectly controlled by the SHGC specification, impacts the probability a patient will experience visual discomfort when looking toward the family seating area.
It is difficult to satisfy visual comfort at all times with a fixed Tvis of the glazing given the large changes in exterior light levels. For most hours of the day, high Tvis is desired for daylighting; lower Tvis is beneficial when the sun comes into view of the window. When the direct sun is in view a very low Tvis, around 0.01 is preferred and often accomplished with interior window treatments.
An emerging consideration for daylighting and interior lighting design is for the relationship between the intensity, color, duration and diurnal timing of light in a space and the resulting impacts to occupants’ sleep-wake cycles. Rapidly progressing research has shown that human sleep-wake cycles are entrained through a nonvisual system of intrinsically photosensitive retinal ganglion cells (ipRGCs) with a sensitivity that peaks around 490 nm. These blue-sensitive retinal cells respond the intensity and color of light and suppress melatonin to entrain the body’s circadian clock. This means not only is the intensity of light as controlled by the glazing Tvis important for human health, but the color of the light as modified by the glass and solar control coatings can impact occupants’ sleep-wake cycles.
The SHGC is a deceptively small specification with broad impacts beyond HVAC systems selection, sizing, loads and operational cost. Driving toward lower SHGC targets can significantly change fenestration costs and procurement, the appearance of the building and the daylighting potential of perimeter spaces with further indirect impacts to occupant visual comfort, thermal comfort and circadian health. Because the decision entangles so many disparate issues, it is critical to carefully coordinate the SHGC specification and arrive at a consensus from all team members.