E08: High Thermal Performance Glass

Requirement Summary

This measure can be claimed if the glazing is multi-paned (double or triple) and has a superior thermal performance.

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.

Intention

By selecting double or triple glazing, which has an improved thermal performance as well as a coating (tinted glass or Low-E) the heat transfer is reduced further than with low-E coating alone, and an even lower SHGC can be achieved.

Approach/Methodologies

Double or triple glazing or coating reduces the Solar Heat Gain Coefficient (SHGC) and thermal conductivity (UValue) 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 given 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 solar gains is required.

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 that the building is in an area without a heating season.

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:

Where:

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.

Potential Technologies/Strategies

Table 25 shows a range of U-values and SHGC values for different types of glazing. It is provided as 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. This can be converted using the following equation:

𝑆𝐻𝐺𝐶 = 𝑆𝐶 𝑋 0.87

Table 25: Approximate SHGC and U-values for different glazing types

Relationship to Other Measures

High performance glass 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. If the Higher Performance Glass measure is claimed, then this measure will not contribute to the calculation of savings.

Particular care must be taken in cold climates; although a Low-E glass with a very low U-value appears to be a good choice, it may actually have worse performance if it also has low SHGC solar heat gain that blocks the warmth of the sun and increases heating requirements. In those cases, a double or triple layer glass with a high solar heat gain coefficient is the right selection.

Assumptions

The Key Assumptions for the Base Case in the Design section show the base case values for the U-value and SHGC of the window. The default values for the improved case for high thermal performance windows are a U-value of 1.95 W/m² K and an SHGC of 0.28.

Compliance Guidance

When a project has multiple types of glazing with multiple U-values and SHGC, a weighted average U-value and SHGC can be entered into 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:  Manufacturer’s data sheets showing the seasonal average U-value for the window (including glass and frame) and the solar heat gain coefficient (SHGC) of the glass and frame; and  A list of different types of windows included in the design (window schedule).	At the post-construction stage, the following must be used to demonstrate compliance: Photographs of the glazing units installed;  Purchase receipts and delivery notes for the glazing; and  Manufacturer’s data sheets showing the seasonal average U-value for the glazing (including glass and frame) and the solar heat gain coefficient (SHGC) of the glass and frame.

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