E01: Reduced Window-to-Wall Ratio

Requirement Summary

Window-to-Wall Ratio (WWR) should be selected and the WWR value entered in the EDGE App in all cases, irrespective of the value. Savings can be achieved if the Window to Wall Ratio is lower than the local base case as set out in the Key Assumptions for the Base Case in the Design section. EDGE will calculate the impact of any improvement beyond the base case.

Intention

The sun is the most powerful light source but is also a source of significant heat gain. Therefore, it is important to balance lighting and ventilation benefits of glazing against the impacts of heat gain on cooling needs and/or passive heating. Finding the correct balance between the transparent (glass) and the opaque surfaces in the external façades helps to maximize daylight while minimizing unwanted heat transfer, resulting in reduced energy consumption. The design goal should be to meet minimum illumination levels without significantly exceeding the solar heat gains in temperate and warm climates, as well as to make the most of passive heating in cold climates in winter time.

Windows generally transmit heat into the building at a higher rate than walls do. In fact, windows are usually the weakest link in the building envelope as glass has much lower resistance to heat flow than other building materials. Heat flows out through a glazed window more than 10 times faster than it does through a well-insulated wall. While glazed areas are desirable to admit solar radiation in cold climates during the day, windows in warmer climates can significantly increase the building’s cooling loads.

Approach/Methodologies

This measure uses the Window to Wall Ratio (WWR), which is defined as the ratio of the total area of the window or other glazing area (including mullions and frames) divided by the gross exterior wall area. The WWR is calculated with the following equation:

Glazing area is the area of glass on all façades regardless of orientation. Gross exterior wall area is the sum of the area of the exterior façades in all orientations, which includes walls, windows and doors. To calculate the exterior wall area, the interior surface of the exterior wall must be used to determine the lengths.

The actual WWR for the design case must be entered in the system. While a higher WWR may have a negative impact on energy savings, it can be compensated for by other energy saving measures.

The improved case WWR must be calculated and entered for each façade separately, i.e. for the North Façade the % WWR of the North façade only should be entered. This will impact the solar gain in each façade and impact the cooling and heating load.

For projects with multiple sub-projects with multiple EDGE files, the recommended method is to calculate an average WWR for the whole building and use that in every subproject. Modeling each subproject with its own WWR is also acceptable, but unless a significant difference exists between the sub-projects with some containing double height spaces or very different glass areas, this approach is not recommended. For example, if the average WWR of a residential building is 35%, that will be used for all unit types regardless of their individual WWR. (However, individual window opening sizes will be considered for the natural ventilation measure).

Windows and walls facing internal courtyards or gaps between buildings (open to outside air) should be included in the WWR calculations.

Spandrel panels (opaque insulated glass panels) should be included as external walls in the WWR calculations.

The following examples should be excluded from the calculations of WWR:

•  Walls with windows/ventilation openings into interior shafts only (for example, as seen for bathrooms in residential projects in India) 
• Any external wall that is not directly exposed to the environment. For example, underground walls, earth-bermed walls or walls in direct contact with another building 
• Walls that do not enclose interior spaces. This includes walls that have more than 30% of the area as a permanent opening for ventilation. The next enclosing wall should be used instead. 
• Openings that are only ventilation openings (without glazing)

Potential Technologies/Strategies

A building with a higher WWR will transfer more heat than a building with a lesser WWR. If the WWR is higher than the default value, then other measures such as shading or a lower solar heat gain coefficient (SHGC) of the glass should be considered to offset the energy loss. In cold climates, when the WWR is higher than the default, the insulation of glass using double or triple glazing should be considered.

With regards to daylight, two basic strategies are available for using the sun for lighting while minimizing heat gain. The first is to use a small window opening (15% WWR) to illuminate a surface inside the space that then spreads the light out over a large area. The second is to use a moderately sized window (30% WWR) that “sees” an exterior reflective surface but is shaded from the direct sun. To increase the daylight availability, the selection of higher visible light transmittance (VLT>50) for the glass is also important.

Relationship to Other Measures

Envelope heat transfer is a function of the thermal resistance of the external materials, the area of the building façade, and the temperature difference between the exterior and interior of the building. The primary causes of heat transfer are infiltration and windows. The size, number and orientation of windows have a significant effect on the building’s energy use for thermal comfort purposes (heating or cooling).

In cold climates, direct solar radiation passes through the glass during the day, passively heating the interior. If sufficient thermal mass is used, this heat is then released, helping to keep the room comfortable later in the day. In this climate type, the glass placement that is most desirable is at the elevation with the greatest exposure to sunlight. However, in warm and temperate climates, the WWR should be lower as the reduction of glass leads to a reduction in the overall cooling load and reduced need for air conditioning.

It is important to consider that lighting and cooling energy use can be reduced by the use of daylighting. This should be balanced with the corresponding solar and convective heat gains.

Assumptions

The base case for the WWR is included in the Key Assumptions for the Base Case in the Design section. The base case varies by building type, and can also vary by location. The default assumptions for the improved case for the WWR may vary from country to country. If the actual WWR is different than the default, the actual WWR value for the improved case must be entered manually.

Compliance Guidance

At the post-construction stage, it is important to ensure that the WWR has been maintained to achieve the energy savings indicated in the EDGE results. Compliance is achieved when the design team can demonstrate that the WWR in all elevations is equal or lower than the claimed specification, using the formula explained in “Potential Technologies/Strategies” above.

Design Stage	Post-Construction Stage
At the design stage, the following must be used to demonstrate compliance:  Calculation of “Glazing Area” and “Gross Exterior Wall Area” for each façade of the building and the average area weighted WWR; and  All façade elevation drawings showing glazing dimensions and general building dimensions.	At the post-construction stage, the following must be used to demonstrate compliance:  Updated WWR calculations if necessary, or confirmation that the design WWR is still valid; and, either As-built façade drawings; or External and internal photographs of the building showing all the elevations

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