This measure can be claimed if Low Emissivity (Low-E) coated glazing is used.
Even if the U-value of the actual window in the building is worse (higher) than the base case value, the measure must be selected, and the U-value entered when the measure is required (marked with an asterisk). For example, this could happen in countries where double glazing is the norm for office buildings, making the base case values quite good. The same principle is applicable to SHGC, i.e. if the SHGC is different from the base case assumption, the measure must be selected and the actual SHGC must be entered.
The addition of a Low-E coating to glazing reduces the transference of heat from one side to the other by reflecting thermal energy. Low-E coatings are microscopically thin metal or metallic oxide layers that are deposited on a glass surface to help keep heat on the same side of the glass from which it originated. In warm climates the intention is to reduce heat gain, and in cold climates the intention is to reflect interior warmth back indoors.
Low-E coating reduces the Solar Heat Gain Coefficient (SHGC) and thermal conductivity (U-Value) of the glazing. These concepts are explained as follows:
The SHGC is expressed as a number between 0 and 1 and indicates the fraction of incident solar radiation admitted through a window, both directly transmitted and absorbed and subsequently released inward. A solar heat gain coefficient indicates lesser solar heat transmitted.
All Low-E glass will have a reduced U-Value compared to plain glass; however, the product’s solar heat gain performance determines its appropriateness for a particular climate. For warm climates, Low-E glass with a low SHGC helps reduce unwanted solar gains but in cold climates, Low-E glazing that has minimal impact on SHGC is more desirable.
In both warm and cold climates, the lower U-Value of Low-E glazing is an advantage. Manufacturers often provide separate U-Values for summer and winter (or the heating and cooling seasons). A simple approach is to calculate the average of these two values. If an alternative approach is used to calculate the seasonal average, then this must be justified. For example, an acceptable justification is if the building is in an area that lacks a heating season. In cases where multiple glass types are used, a weighted average must be applied, which can be calculated using the built-in calculator in EDGE.
Note: EDGE uses the U-value and SHGC of the Window, which includes the glass and frame. For factory manufactured windows, the window manufacturer typically provides these values for the whole window. If they are not available, the project team must calculate them. Window U-value is the area-weighted average of the U-value of the glass and frame.
Simple method of calculating the U-value and SHGC of a window:
Ug = U-value of glass
Ag = Area of glass in elevation view
Uf = U-value of frame
Af = Area of frame in elevation view
Similarly, the SHGC of the Window is the area-weighted average of the SHGC of the glass and frame. For cases where the exact value may not be known, typical values can be referenced from the ASHRAE Handbook of Fundamentals.
Low-E coating is applied to different sides of the glazing depending on the climate. In single-pane windows, the coating may be applied inside or outside depending on the coating. For double pane windows, the coating is usually applied on the outer surface of the inner pane in cold climates to allow useful solar radiation to pass through to passively heat the interior, and to reduce the ability for infrared radiation to reflect back out. In warm climates, the coating is usually applied on the inner surface of the outer pane, as this helps to reflect the solar radiation back outside before it enters the air cavity.
Figure 8. Recommended position of the low-e coating for double-pane glass
Two types of Low-E coating are available: hard coat and soft coat. Only hard coat (pyrolytic coating) should be used in single-glazed units as it is more durable than soft coat (sputter coating).
- Hard Coat Low-E: Hard coat Low-E, or pyrolytic coating, is a coating applied at high temperatures and is sprayed onto the glass surface during the float glass process. The coating process, known as Chemical Vapor Deposition (CVD), uses a variety of chemicals including silicon, silicon oxides, titanium dioxide, aluminum, tungsten, and many others. The vapor is directed at the glass surface and forms a covalent bond with the glass, so the result is hard wearing.
- Soft Coat Low-E: Soft coat Low-E, or sputter coating, is applied in multiple layers of optically transparent silver sandwiched between layers of metal oxide in a vacuum chamber. This process provides the highest level of performance and a nearly invisible coating. However, it is highly susceptible to damage from handling (recommended in double glazing units).
Table 24 shows a range of U-Values and SHGC values for different types of glazing, and provides guidance for the selection of glazing. However, this data varies from manufacturer to manufacturer; for certification purposes actual values from the manufacturer must be provided. In addition, many manufacturers' literature indicates the Solar Coefficient (SC) instead of the SHGC, with the conversion equation as follows:
𝑆𝐻𝐺𝐶 = 𝑆𝐶 𝑋 0.87
Table 24: Approximate SHGC and U-values for different glazing types
Relationship to Other Measures
Applying a Low-E coating either reduces the heat load by reducing the heat loss through the glazing, or reduces the cooling load by reducing the solar heat gain. As with other measures which relate to the improvement of the building fabric, addressing and optimizing performance is cheaper to do before sizing/selecting heating, ventilation, and the air-conditioning plant.
Care must be taken in cold climates, because as the U-Value is reduced, the SHGC is reduced even further for many coatings. Therefore, although a Low-E glass with a very low U-Value may appear to be a good choice, the performance may actually be worse if it has low SHGC that reduces heat gain from the sun and increases heating requirements. In those cases, a window with low U-value but with a higher solar heat gain coefficient (SHGC) is the right selection.
Note: If the Higher Performance Glass measure is also claimed, then this measure will not contribute to the calculation of savings.
The base case values for the U-Value and SHGC of the window are included in the Key Assumptions for the Base Case in the Design section. These can vary based on several factors such as location and building type. The default values for the improved case assumptions for a window with Low-E coated glass are a U-value of 3 W/m² K and an SHGC of 0.45.
When the project has multiple types of glazing with multiple U-values and SHGC, a weighted average U-value and SHGC must be entered in the user entry fields. The following information must be provided to show compliance at the design and post-construction stages:
|Design Stage||Post-Construction Stage|
At the design stage, the following must be used to demonstrate compliance:
At the post-construction stage, the following must be used to demonstrate compliance:
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