Window Wise: Window R-value Calculator
High efficiency windows are extremely important to have a high performance, low energy building. Often times, the rated R-values of glass can be undermined (lowered) by mullions that break up insulating glass and/or conductive metal frames that are not thermally broken. These breaks in the glass with highly conductive (thermally) materials can make windows less efficient than design professionals think. The reality is, any given window has an effective R-value that captures the true effect of how the window assembly performs thermally.
In many cases, for off-the-shelf products, this effective R-value is the rated R-value from the manufacturer. However, for custom made window assemblies on both residential and commercial projects, there may not be an effective R-value for a product (since it is a custom assembly). This tool is specifically made for those cases to help gauge the performance of any given assembly.
Effectively, this tool is designed to help Architects, Facilities Managers doing retrofits, and savvy homeowners to understand the effects of frames on window performance by automating this average R-value calculation.
- Center of Glass = portion of the glass from the center of the window out to a certain distance along the perimeter.
- Edge of Glass = portion of the glass 2.5” away from the frame along the perimeter of the center of glass.
- Frame = material holding the glass in place. This is also used interchangeably within this tool for “mullion” for simplicity.
There are variety of Inputs to include within this tool:
- Window Dimensions (Represents Total Area of the Opening)
- Window Length (ft)
- Window Width (ft)
- Mullion and Frame Dimensions
- Mullion Horizontal Spacing (ft)
- Mullion Vertical Spacing (ft)
- Frame Profile Width (in) - this is the width of the frame (elevation view)
- Thermal Properties
- Center of Glass R-Value (from manufacturer - rated window)
- if you do not have a specific product in mind, see below for some typical R-values for varying assemblies. (Source: CEE 176A Energy Efficient Buildings Lecture Slides)
- Frame Material (obtained from Manufacturer)
- If you do not have a specific product in mind, the below are typical R-values for frame materials
- Aluminum - R-0.5
- Wood - R-0.7
- Vinyl - R-2.4
- Edge of Glass R-value (can be obtained from thermal readings on a glass assembly).
- This could be difficult to obtain. Typically, this value ranges from 0.5 to 3 depending on the COG R-value and the Frame R-value.
- If the COG R-value and Frame R-value increase, then the EOG R-value will be on the higher end. If they are relatively low, then the EOG R-value will be on the lower end.
- Effective R-value of the Window
- This is the value to be used when understanding how your window assembly will perform thermally.
- Area Percentage Breakdown
- Center of Glass
- Edge of Glass
- Feedback (Conditional Output Message)
- If the Average R-value is greater than 3 (threshold based on CA Title 24 prescriptive energy code), then the message shows “Great job achieving a high Average R-value!”
- If the Average R-value is less than 3, then the message shows "The average R-value is below Title 24 spec. try choosing a more insulating frame, increasing the mullion spacing, or choosing a more insulating glass (for COG R-value).”
- Revit visual of your mullion separation as glass panels.